Periodically Poled Ridge Waveguides
and Photonic Wires in LiNbO3
for Efficient Nonlinear Interactions
Thesis
Submitted to the
Department of Physics, Faculty of Science
University of Paderborn, Germany
for the degree
Doctor der Naturwissenschaften (Ph.D / Dr. rer. nat.)
By
Li Gui
Reviewers:
1. Prof. Dr. W. Sohler
2. Prof. Dr. K. Lischka
Date of the submission: November 12, 2010
Date of the defence examination: December 20, 2010
1
Abstract
Periodically poled LiNbO3(PPLN) waveguides have been successfully used for effi-
cient nonlinear interactions using quasi phase matching (QPM) due to the fact that the
optical wave is confined in the waveguide with a high intensity. A further increase in
nonlinear conversion efficiency requires strongly reduced cross section dimensions which
can be only achieved in a waveguide of a high refractive index contrast. Such a wave-
guide not only facilitates efficient nonlinear interactions but also enables fabrication of
sub-micrometer periodical domain structures. Therefore, counter-propagating nonlinear
interactions can be realized.
The aim of this work is to develop PPLN waveguides of high refractive index con-
trast and small cross sectional dimensions, and then to investigate various nonlinear
interactions in such waveguides. Towards this goal, two different types of LiNbO3wave-
guides, i.e. ridge waveguides on X(Y)-cut substrates and LiNbO3-on-Insulator (LNOI)
photonic wires, are developed. The methods of fabricating periodical domain structures
in such waveguides are investigated to enable quasi-phase-matching (QPM) nonlinear
interactions.
First, ridge waveguides on X(Y)-cut LiNbO3substrates are fabricated using plasma
etching and a subsequent Ti in-diffusion. A local poling technique is developed to
fabricate periodical domain structures only in the body of the ridge guide. Various cha-
racterization methods have been used to evaluate the quality of the ridge guides as well
as the periodical domain structures. A reduced mode size compared to a conventional
Ti in-diffused channel waveguide is observed. The inverted domains inside the body of
the ridge are sufficiently deep (∼5µm) to overlap the transmitted optical modes. As a
result, a normalized SHG conversion efficiency of 16.5 % W−1cm−2is obtatined, which is
50 % higher than that in a conventional Ti in-diffused channel waveguide. Moreover, as
a promising feature, a strongly reduced sensitivity to photorefactive effects is observed.
This could be of strong interest for the nonlinear applications using high optical power.
Second, periodically poled LiNbO3-on-Insulator (PPLNOI) material platform is fab-
ricated by direct bonding of PPLN in collaboration with Hu. PPLNOI photonic wires
are then fabricated using Argon milling. 1st order SHG is demonstrated using a PPLNOI
photonic wire of 3.2 µm periodicity; a parabolic dependence of the generated SH power
vs. the fundamental power is observed. We also demonstrate the second approach
of fabricating PPLNOI by directly poling LiNbO3thin film. The promises as well as
challenges presented in our preliminary experiments are discussed in detail.
2
3
Contents
1 Introduction 14
1.1 Motivation................................... 14
1.2 Strategy for material development . . . . . . . . . . . . . . . . . . . . . . 15
1.3 Overview of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 Theoretical background of QPM nonlinear interaction 20
2.1 Nonlinear polarization and coupled-mode equations . . . . . . . . . . . . 20
2.2 Quasiphasematching ............................ 24
2.3 Second harmonic generation . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.4 Cascaded nonlinear interaction (SHG/DFG) . . . . . . . . . . . . . . . . 28
2.5 Nonlinear interactions in ridge-type waveguides . . . . . . . . . . . . . . 30
2.6 Summary ................................... 34
3 Fabrication of PPLN ridge waveguides 36
3.1 Ridge fabrication and Ti in-diffusion . . . . . . . . . . . . . . . . . . . . 36
3.2 Localperiodicpoling............................. 40
3.3 Summary ................................... 46
4 Characterization of PPLN ridge waveguides 48
4.1 Waveguideproperties............................. 48
4.1.1 Propogation losses . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1.2 Optical mode distribution . . . . . . . . . . . . . . . . . . . . . . 50
4.2 Periodic ferroelectric domains . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2.1 Domain visualization using selective chemical etching . . . . . . . 51
4.2.2 Domain visualization using CLSM . . . . . . . . . . . . . . . . . . 58
4.3 Summary ................................... 62
5 Nonlinear optical interactions 64
5.1 Second harmonic generation (SHG) . . . . . . . . . . . . . . . . . . . . . 64
5.2 Cascaded second harmonic generation and difference frequency generation
(cSHG/DFG) ................................. 69
5.3 Summary ................................... 72
4
6 Periodically poled LNOI material platform and photonic wires 74
6.1 PPLNOI material platform . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.1.1 Fabrication of LNOI . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.1.2 Direct bonding of PPLN . . . . . . . . . . . . . . . . . . . . . . . 77
6.1.3 PolingofLNOI ............................ 82
6.2 PPLNOI photonic wires . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.2.1 Fabrication and optical properties . . . . . . . . . . . . . . . . . . 86
6.2.2 Nonlinear interactions in PPLNOI photonic wires . . . . . . . . . 88
6.3 Summary ................................... 92
7 Conclusions and Outlook 94
7.1 Conclusions .................................. 94
7.2 Outlook .................................... 96
A Holographic lithography 100
5
List of Figures
1.1 A sketch of (a) a conventional channel waveguide, (b) a ridge waveguide
and (c) a LNOI photonic wire. . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2 Atomic structure of LiNbO3in (a) paraelectric phase and (b, c) ferroelec-
tricphases. .................................. 17
1.3 Sketch of a PPLN waveguide on (a) Z-cut and (b) X-cut substrate. Ar-
rows indicate the orientation of the Zaxis.................. 18
2.1 The growth of the SH power with propagation distance (divided by Lc)
for (i) quasi phase matched, (ii) birefringent phase matched and (iii) non-
phase matched interactions. . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Scheme of (a) a copropagating SHG and (b) a counter-propagating SHG. 26
2.3 Schematic sketch of SHG in a waveguide on a X-cut LiNbO3substrate.
Inset: sketch of the energy conservation of SHG. . . . . . . . . . . . . . . 27
2.4 Schematic sketch of cascaded SHG/DFG in a waveguide on a X-cut
LiNbO3substrate. Inset: sketch of frequency mixing. . . . . . . . . . . . 29
2.5 Simulated optical intensity profiles for TE modes at 1550 nm (red curves)
and 775 nm (blue curves) in (a) a ridge waveguide fabricated on X-cut
Y-propagating LiNbO3with the top width of 7 µm and the height of
3.5 µm and (b) a X-cut photonic wire with the width of 1 µm and the
height of 0.5 µm. The lines represent 10, 30, 50, 70 and 90 % of the peak
intensity for both modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.6 Comparison of Seff of ridge guides with different parameters. . . . . . . . 32
2.7 Normalized SH efficiency varies as a function of propagation loss and
interaction length for a ridge guide with Seff = 38.5µm2, given αf=αSH . 33
3.1 A flow chart of ridge waveguides fabrication. . . . . . . . . . . . . . . . . 37
3.2 A SEM micrograph of an etched ridge. . . . . . . . . . . . . . . . . . . . 38
3.3 Scheme of the flood exposure technique used to deposit Ti only on top of
theridge. ................................... 39
3.4 A SEM micrograph of a Ti in-diffused ridge waveguide. . . . . . . . . . . 40
3.5 Scheme of the poling configuration for a ridge on X-cut LN. . . . . . . . 40
3.6 Calculated Ez(MV/m) in a ridge of 4 µm height and 9 µm width, as-
suming 1 V applied to the left electrode with the right electrode grounded. 41
3.7 Top view of the electrodes of 16.6 µm period for a 9 µm wide ridge. . . . 42
3.8 Top view of the electrodes of 2 µm period for a 8 µm wide ridge. . . . . . 43
6
3.9 A sketch of the electric circuit for the local poling . . . . . . . . . . . . . 43
3.10 Poling characteristics of (a) an undoped ridge and (b) a doped ridge of 8
µm width and 3.5 µmheight. ........................ 44
3.11 The accumulated charge per rectangular poling pulse of 270 V as function
ofthepulsenumber. ............................. 45
3.12 Poling characteristics of poling an undoped ridge on X-cut LiNbO3using
a positive polarity electric field (red curve) and subsequently a negative
polarity electric field (green curve). Voltage waveform is given in black
curve. ..................................... 46
4.1 Schematic diagram of the loss measurement setup utilizing the low-finesse
Fabry-Perot contrast method. PD: photodiode, ECL: external cavity laser. 49
4.2 Schematic diagram of the setup to measure the mode profile. IR: infrared. 51
4.3 Transmitted optical mode from a Ti in-diffused ridge guide of top width
7.5 µm and height 3.5 µm, Ti thickness 70 nm in (a) TE polarization and
(b)TMpolarization. ............................. 51
4.4 Selectively etched, periodically poled undoped ridge on X-cut LiNbO3.
(a) Top view. (b) Side view. (c) Side view after cutting the ridge. (d)
Cuttingscheme................................. 53
4.5 Selectively etched, periodically poled undoped ridge on Y-cut LiNbO3.
(a) Top view. (b) Side view. (c) Side view after cutting the ridge. (d)
Cuttingscheme................................. 53
4.6 (a) Top view and (b) cross section of a selectively etched, periodically
poled Ti in-diffused ridge on X-cut LiNbO3. The white dashed line in (b)
indicate the hexagonal domain shape. . . . . . . . . . . . . . . . . . . . . 54
4.7 Side views of a selectively etched, periodically poled doped ridge on X-cut
LiNbO3. .................................... 55
4.8 Top view of a selectively etched, periodically poled ridge of periodicity 2
µm on X-cut LiNbO3. ............................ 55
4.9 Selectively etched doped ridge of top width 8 µm on X-cut LiNbO3poled
using a voltage pulse of (b) 500 Volts, 3 ms duration; (c) 400 Volts, 30
ms duration and (d) 500 Volts, 60 ms duration. (a) Cutting scheme, the
circledareaisimaged. ............................ 56
4.10 Selectively etched doped ridge of top width 7 µm on Y-cut LiNbO3poled
using a voltage pulse of 600 Volts and different duration (b) 20 ms, (c)
30 ms and (d) 50 ms respectively. (a) Cutting scheme, the circled area is
imaged. .................................... 56
4.11 Top views of selectively etched undoped ridge of top width 8 µm on Y-
cut LiNbO3poled using a 20 ms voltage pulse of (b) 300 Volts, (c) 500
Volts and (d) 700 Volts respectively. (a) Imaging scheme and +zand −z
indicate the crystallographic orientation of the side walls of the ridge. . . 57
7
4.12 Images of surface damage on periodically poled ridge guides on a X-cut
LiNbO3substrate revealed by chemical etching: (a) a top surface of a
ridge guide where the electrode tips are on the top surface, (b) ground
surface of the ridge guide; (c) AFM image of the scratch-like surface
damage on the substrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.13 Sketch of the experimental nonlinear CLSM. . . . . . . . . . . . . . . . . 59
4.14 CLSM images of the top surface of an undoped ridge of 9 µm width and
2µm height in (a) linear mode and (b) nonlinear mode. . . . . . . . . . . 60
4.15 Nonlinear CLSM image of the top surface of a periodically poled undoped
ridge on X-cut LiNbO3............................. 60
4.16 Nonlinear CLSM image of the top surface of a periodically poled undoped
ridge on Y-cut LiNbO3............................. 61
4.17 Charaterization of a periodically poled undoped ridge on X-cut LiNbO3us-
ing (a) nonlinear CLSM and (b) selective chemical etching. Graph (b)
represents the etched cross section whose position is indicated as a dashed
lineingraph(a). ............................... 61
4.18 Nonlinear CLSM image of a periodically poled undoped ridge on X-cut
LiNbO3atdifferentdepth........................... 62
4.19 (a) Depth resolved nonlinear CLSM image of a periodically poled undoped
ridge on X-cut LiNbO3scanned along the plane sketched in (b). . . . . . 62
5.1 Sketch of a setup for SHG characterization. ECL: external cavity laser.
PC: polarization controller. EDFA: erbium doped fiber amplifier. PD:
photodiode. .................................. 65
5.2 Generated SH and transmitted fundamental powers as a function of the
fundamental wavelength for a 14 mm long, periodically poled ridge wave-
guide on X-cut LN. Inset: Results around the maximum efficiency plotted
withhigherresolution. ............................ 65
5.3 Tuning characteristics at different coupled fundamental power levels: SH
power vs. fundamental wavelength. . . . . . . . . . . . . . . . . . . . . . 66
5.4 Power characteristics at (a) low fundamental power levels and (b) high
fundamental power levels: SH power vs. fundamental power. . . . . . . . 67
5.5 Generated SH power as a function of time at room tempeature. . . . . . 67
5.6 Tuning characteristics at different temperature. . . . . . . . . . . . . . . 68
5.7 SH generation in a broad fundamental wavelength range: SH power vs.
fundamental wavelength. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.8 Sketch of a setup for cascaded SHG/DFG characterization. ECL: ex-
ternal cavity laser. PC: polarization controller. EDFA: erbium doped
fiber amplifier. DFB: distributed feedback laser. OSA: optical spectrum
analyser. FBG: fiber bragg grating. . . . . . . . . . . . . . . . . . . . . 70
8
5.9 Calculated power evolution of fundamental, SH, signal and idler waves in
cSHG/DFG interaction assuming a coupled pump power of ∼200 mW:
fundamental power and SH power are in linear scale on the left vs. inter-
action length; signal power and idler power are in logarithm scale on the
right vs. interaction length. . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.10 Measured spectral power of fundamental, signal and idler waves in cSHGDFG
interaction at a coupled pump power of ∼200 mW (Resolution: 0.1 nm):
spectral power density vs. wavelength. . . . . . . . . . . . . . . . . . . . 71
5.11 Generated idler power vs. time at room temperature. . . . . . . . . . . 72
6.1 (a) Relation between maximum single mode core size and relative index
difference ∆. (b) Relation between minimum bending radius and relative
index difference ∆. ∆ = n12−n22
2n12. The red dots refer to LiNbO3as the
core material. The figures are taken from [1]. . . . . . . . . . . . . . . . . 75
6.2 The next generation of optical systems built in a photonic chip, taken
from the website of CUDOS [2]. . . . . . . . . . . . . . . . . . . . . . . . 75
6.3 Fabrication of a LNOI wafer: (a) Ion implantation of the LiNbO3wafer
A; (b) SiO2deposition on the LiNbO3wafer B; (c) Crystal bonding and
splitting; (d) Annealing and CMP polishing. . . . . . . . . . . . . . . . 76
6.4 An optical micrograph of a fabricated LNOI wafer of 3” diameter taken
from[3] .................................... 77
6.5 Sketch of a PPLN-SiO2-LNthinfilm..................... 78
6.6 Fabrication of a PPLN substrate: (a) spin-coating of photoresist; (b)
exposure and development of photoresist; (c) periodical poling. . . . . . . 78
6.7 Two poling schemes: (a) conventional poling (Q= 2 ×Ps×A); (b)
overpoling (Q > 2×Ps×A). The arrows represent the directions of the
spontaneous polarization. . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.8 A typical characteristic of an electric field over-poling experiment: voltage
(black curve), current (blue curve) and charge (red curve) vs. time. . . . 81
6.9 Optical micrographs of a PPLN substrate of a periodicity of 1.7 µm after
selective chemical etching: (a) the original −Z face and (b) the original
−Yface..................................... 81
6.10 Optical micrographs of two different regions on the original −Z face of a
PPLN substrate of a periodicity of 0.8 µm after selective chemical etching. 82
6.11 Optical micrographs of (a) a PPLN substrate of a periodicity 3.2 µm and
(b) a LNOI thin film of the same periodicity. . . . . . . . . . . . . . . . . 82
6.12 A sketch of a LN-Ti-SiO2-LN structure with a top electrode and poling
scheme. .................................... 83
6.13 The induced current as a function of the applied voltage. . . . . . . . . . 84
6.14 The poling characteristic during the voltage pulse of 26 V: the applied
voltage and measured current vs. time. . . . . . . . . . . . . . . . . . . . 85
6.15 Optical micrographs of the surfaces of poled LN thin films after selective
chemicaletching. ............................... 85
9
6.16 (a) Scheme of periodical poling of a LNOI thin film. (b) Optical micro-
graphs of the domain inverted area after selective chemical etching. The
circle in (a) indicates how the domain inverted region is corresponding to
theelectrodepattern.............................. 86
6.17 (a) SEM micrograph of a LNOI photonic wire of 1 µm top width taken
from [3]. (b) A sketch of the cross section of the photonic wire. . . . . . . 87
6.18 Optical micrographs of PPLNOI photonic wires of 1 µm width and a
periodicity of (a) 9 µm and (b) 3.2 µm.................... 87
6.19 Measured (a) and calculated (b) intensity distribution of the fundamental
TE mode in a photonic wire of 1 µm top width and 730 nm height, taken
from[3]..................................... 88
6.20 Calculated effective indices neff , calculated and measured group indices
ngfor the fundamental mode of TM polarization in a photonic wire of
1µm top width versus the wavelength. The refractive indices of bulk
LiNbO3and SiO2are displayed for comparison. This graph is taken from
[3]........................................ 88
6.21 Periodicity of the QPM structure for 1st order SHG in photonic wires of a
height of 730 nm and a width of 1 µm (red curve) and 7 µm (black curve)
as function of fundamental wavelength. Fundamental and SH waves are
inTMpolarization............................... 89
6.22 Calculated wavelength-tuning curves of SHG in a photonic wire of a height
of 730 nm and a width of 7µm: Generated SH power as function of
fundamental wavelength. Input pump power is 1 mW. The periodicity
and the interaction length of the photonic wire are 3.2 µm and 1.5 mm
(red curve), 3.3 µm and 1.5 mm (black curve), and 3.2 µm and 0.1 mm
(blue curve, 100 times of the actual values) respectively. . . . . . . . . . . 90
6.23 (a) Measured (dots) and fitted (line) SH power versus input fundamental
power (measured in front of the coupling lens) in a PPLN photonic wire
of 1 µm top width and 9 µm periodicity. Measured mode distributions
at (b) fundamental and (c) SH wavelengths. This graph is taken from [4]. 91
6.24 (Measured (dots) and fitted (line) SH power versus input fundamental
power (measured in front of the coupling lens) in a PPLN photonic wire
of 7 µm top width and 3.2 µm periodicity. Measured mode distributions
at (b) fundamental and (c) SH wavelengths. . . . . . . . . . . . . . . . . 91
7.1 (a) Schemes of an OPO : co-propagating and counter-propagating. β:
propagation constant; K: wave vector associated to periodical domain
structures; Λ: periodicity of the domain tructures. (b) Calculated tuning
characteristics for counter-propagating and conventional co-propagating
OPOs. The red and blue curves represent the backward coupled idler
wave and the forward coupled signal wave in the counter-propagating
OPO; The black dashed curve represents the signal/idler wave in the
co-propagatingOPO.............................. 98
10
A.1 Lloyd interferometric optical holography setup: (a) photograph of the
setupand(b)schetch. ............................101
A.2 Lloyd interferometer: (a) schematic diagram and (b)photograph of the
samplestage. .................................101
A.3 Scheme of fabricating photoresist grating with rectangular profile: (a)
interference of the beams; (b) the profile of the modulated illumination
intensity; (c) the photoresist grating after exposure and development. P:
Intensity of the illumination, Pth: the intensity threshold of the illumina-
tion, below which the photoresist can not be resolved in the developing
solvent, T: thickness of the photoresist after development. . . . . . . . . . 102
A.4 Photoresist graing of 720nm periodicity on a glass substrate. . . . . . . . 103
A.5 Photoresist graing of 420nm periodicity on a glass substrate. . . . . . . . 103
A.6 Photoresist graing of 720nm periodicity on a LiNbO3substrate. . . . . . 103
11
List of Tables
2.1 Mode size in horizontal and vertical directions and effective cross section
of a conventional Ti in-diffused channel waveguide, a Ti-diffused ridge
guide and a LNOI photonic wire. The orientation and geometric design
of the waveguides are described in the text above. . . . . . . . . . . . . 32
4.1 Average propagation loss of Ti in-diffused ridge guides on X-cut LiNbO3at
1550 nm in both TE and TM polarization. . . . . . . . . . . . . . . . . 50
4.2 Measured FWHM horizontal (Hor.) and Vertical (Vert.) mode sizes of
Ti in-diffused ridge guides on X-cut LiNbO3at 1550 nm in both TE and
TM polarization of ridge guides with different top width and Ti thickness
(th.). ..................................... 52
12
13
Chapter 1
Introduction
1.1 Motivation
The concept of integrated optics is using an optical waveguide as a basic structure to
implement compact and robust devices which perform complicated functions for specific
applications by integration. It was first introduced in the late 1960’s. Since then, a lot of
waveguide devices such as optical modulators, amplifiers and etc. have been developed
and implemented in areas such as optical memories, optical sensors, and most success-
fully optical communications. By applying waveguide technologies in nonlinear optics,
one can realize highly efficient nonlinear interactions. This is because that optical waves
can be confined in a small cross section and maintained with high optical intensity over
a long propagation length. Moreover, using a waveguide offers new possibilities in phase
matching and various nonlinear interactions. Therefore, it is possible to develop multi-
functional devices by integrating different nonlinear optical functions and/or peripheral
waveguide components performing passive and active functions. Such an integration is
essential for future applications.
LiNbO3, also often referred to as “the silicon of nonlinear optics”, is an excellent
candidate for this application because of the following reasons: First, LiNbO3of optical
quality is produced using well established techniques and available worldwide. Second,
LiNbO3is transparent from 350 nm to 4.5 µm, providing low loss for both the fundamen-
tal and harmonic for visible light generation. Third and most important, LiNbO3has
nonlinear coefficients among the largest of all inorganic materials. Last but not least, by
taking advantage of the established technologies in LiNbO3integrated optics, one can
make a complex optical circuit combining both linear and nonlinear functionalities on
one single chip. In the last few decades, LiNbO3waveguides and quasi phase matching
(QPM) technologies have been intensely investigated. However, the challenges always
exist in several aspects. One is to achieve high nonlinear efficiency and stability, so that
the high power light source based on nonlinear interaction can be realized. The other
aspect is to realize versatile interactions, some of which one can not normally achieve
using existing technologies, for example, counter propagating nonlinear interactions.
Attractive applications of such a counter propagating nonlinear interactions include for
example mirrorless optical parametric oscillator (OPO). The research presented in this
14
thesis was directed towards investigating new techniques of fabricating LiNbO3wave-
guides to achieve an enhanced nonlinear efficiency and lead a way in realizing counter
propagating nonlinear interactions in LiNbO3.
Developing a suitable material platform for the nonlinear interactions of interest is
the starting point of this research. The strategy for developing such a material platform
is briefly discussed in the next section. The central contributions of this research are
then listed, followed with an overview of this dissertation.
1.2 Strategy for material development
LiNbO3Waveguide
LiNbO3is a synthetic dielectric material that doesn’t exist in nature. It was first dis-
covered to be ferroelectric in 1949 [5]. LiNbO3was first synthesized in single crystal form
and investigated in detail at Bell Laboratories. This resulted in a cornerstone series of
publications in 1966 about the structure and properties of this material [6, 7, 8, 9, 10].
Fabricating LiNbO3waveguides is essential for its use in integrated optics. Design-
ing a waveguide for a specific application requires comprehensive consideration of the
crystallographic orientation of the wafer, the composition of the material, the required
refractive index contrast and most importantly the available fabrication techniques.
The key technique behind waveguide fabrication is to increase the refractive index
of the material in a certain region where the light can be confined inside. Many tech-
niques have been proposed over decades, some of them are well established such as
Ti in-diffusion and proton exchange. However, proton exchanging only increases the
extraordinary index, bringing a certain limitation for applications. In comparison, Ti
in-diffusion increases both ordinary and extraordinary indices, and has been demon-
strated with high performance of various functional devices.
The conventional LiNbO3waveguides are generally small channels under the surface
of the wafer as sketched in figure 1.1(a). The light confinement is achieved due to the
increase of the refractive index in the region of treatment (either Ti in-diffusion, proton
exchange or etc.), except for the interface between the air and LiNbO3where the air
has the refractive index of 1, much smaller than that of LiNbO3(∼2.2). Intuitively, one
might imagine to improve the light confinement greatly by using the interface between
LiNbO3and air. This gives rise to the concept of a ridge waveguide as sketched in
figure 1.1(b). Several works have been reported to fabricate LiNbO3ridge waveguides
[11, 12, 13]. Hu et.al. demonstrated that a Ti in-diffused ridge waveguide on a Z-cut
LiNbO3substrate fabricated by using wet-etching has a smaller mode size compared to
a conventional channel waveguide [13]. Similar work can be done also in X- or Y-cut
substrates but with a different technique [14]. Even though X- and Y-cut substrates
are less frequently mentioned in the literature, they are favored for many applications
such as a fully integrated optical parametric oscillators, where frequency conversion and
electro- and acoustooptic functions are to be integrated in the same substrate.
15
(a) (b) (c)
Figure 1.1: A sketch of (a) a conventional channel waveguide, (b) a ridge waveguide and
(c) a LNOI photonic wire.
Inspired by silicon on insulator, so called “SOI” in the semiconductor industry
[15, 16], the idea of LiNbO3-on-Insulator (LNOI) waveguides also called “photonic wires”
came naturally, but was not realized until the very recent years [17, 18, 19, 3]. A LNOI
thin film is fabricated by ion-slicing and crystal bonding. A photonic wire is then fab-
ricated on such a thin film as depicted in figure 1.1(c): the LiNbO3channel is sitting
on top of the low index material SiO2. Owing to the high contrast between the index
of refraction of LiNbO3and the surrounding material, the dimensions of the waveguide
can be as small as sub-micrometer for single-mode transmission. High index contrast
also means that small-radius devices can be made. Therefore, high density integration
becomes possible. Concerning the nonlinear interaction, the high index contrast and the
small cross section of the waveguide result in extremely small mode size and excellent
overlap of different modes, leading to an extremely high nonlinear efficiency. Moreover,
with the size of the LiNbO3waveguide so small, periodically inverted domain structures
(also referred to a “QPM” structure in this work) of a wavelength scale becomes achiev-
able. Therefore, a broader range of nonlinear interactions, such as counter propagating
nonlinear interactions, can be realized in LiNbO3.
Essentially, both ridge waveguide and LNOI photonic wire are ridge-type waveguides.
In this thesis, in order to avoid confusion, we refer “ridge waveguide” specifically to a
ridge guide on a bulk LiNbO3substrate, and refer “photonic wire” to a waveguide
fabricated in a LNOI thin film.
Periodical domain structures in LiNbO3waveguides
Periodical domain structures (QPM structures) are essential to obtain efficient non-
linear interactions in LiNbO3waveguides. LiNbO3’s crystal structure and the concept
of spontaneous polarization needs to be discussed first in order to understand this tech-
nique. As a typical ferroelectric material, LiNbO3has both paraelectric and ferroelec-
tric phases. Figure1.2 (a) shows LiNbO3’s crystal structure in its paraelectric phase at
temperatures above its ferroelectric Curie temperature (∼1200 ◦C). In the paraelectric
phase, the lithium ions sit in an oxygen plane and the niobium ions are centered between
two oxygen planes. These positions make the paraelectric phase non-polar [10]. When
the temperature decreases from the Curie temperature, the lithium ions and niobium
ions are forced to move into new positions. The positions of the lithium ions are equally
16
probable to be either above or below the oxygen plane, as shown in 1.2 figures (b) and
(c). The charge separation resulting from this shift of ions relative to the oxygen octahe-
dra causes LiNbO3to exhibit a spontaneous polarization. The spontaneous polarization
is reversed when the lithium ions are moved from above the oxygen plane to below the
oxygen plane (at the same time niobium ions are moved in the same direction with a
smaller displacement) under the influence of, for example, an external electrical field.
This process is called “domain inversion”. A QPM structure requires a periodically re-
versed polarization along the propagation direction as sketched in figure 1.3. Theoretical
analysis of a QPM nonlinear interaction is presented in chapter 2.
(a) (b) (c)
Figure 1.2: Atomic structure of LiNbO3in (a) paraelectric phase and (b, c) ferroelectric
phases.
To create QPM structures in LiNbO3, various approaches have been studied such
as stacking of alternately oriented thin plates, or growth of periodic domain structures
in ferroelectrics [20]. For waveguides where QPM is required only at the surface of the
crystal, periodic annihilation of the nonlinear coefficient and periodic domain inversion
by dopant in-diffusion in ferroelectrics [21, 22] have been employed. Years later, Ya-
mada, et al. first reported bulk periodic domain formation in ferroelectrics by applying
an electric field using lithographically defined periodic electrodes [23]. This last tech-
nique is referred to as electric field periodic poling. The electric field periodic poling
has been explored mostly in a Z-cut LiNbO3substrate; a resulted periodically poled
LiNbO3(PPLN) waveguide is sketched in figure 1.3 (a). The poling only succeeds when
the electric field applied between +Z face and −Z face is high enough to overcome the
coercive field Ec, generally ∼21 kV/mm for congruently melted LiNbO3.
The periodic poling of a X- or Y-cut substrate has to be done differently to a Z-cut
substrate because the direction of the spontaneous polarization is parallel to the surface
as sketched in figure 1.3 (b). Several approaches have been proposed [24, 25, 26, 27, 28]
to fabricate periodic domain structures on the planar substrate. Periodically poled ridge
guides can be fabricated by lapping, polishing and mechanical micro-machining periodi-
cally poled LiNbO3(PPLN) layers, glued or bonded, respectively, to a LiNbO3substrate
[12, 29]. The challenge of periodic poling in a X- and Y-cut substrate is that it is difficult
17
(a) (b)
Figure 1.3: Sketch of a PPLN waveguide on (a) Z-cut and (b) X-cut substrate. Arrows
indicate the orientation of the Zaxis.
to get the reversed domain deep enough to overlap the optical modes. Recently, a deep
domain inversion in an 84 µm wide ridge on X-cut LiNbO3with a proton exchanged
channel guide in its center was reported [30]. In the research presented in this thesis,
a technique of fabricating QPM structures only in the ridge waveguide (“local periodic
poling”) is developed. A 14 mm long PPLN ridge waveguide on a X-cut substrate is
fabricated and nonlinear interactions including SHG and cascaded interaction of SHG
and difference frequency generation (DFG) are demonstrated.
Another task of this research is to fabricate QPM structures in LNOI thin films and
furthermore periodically poled photonic wires, and to investigate nonlinear interactions
in such a photonic wire. Compared to LiNbO3crystal we discussed above, an ion-
implanted LiNbO3thin film has several special features. First, the LNOI thin film has
been treated with a sequence of processes, including high dose ions implantation and
annealing, in order to achieving successful bonding and splitting. Second, the film has a
thickness of sub-micrometer. Third, surface defects are unavoidable from the fabrication
process. All these features make the poling process significantly different from poling of
a bulk LiNbO3crystal. Several methods are developed in this work to fabricate QPM
structures in LNOI thin film.
1.3 Overview of this thesis
The central contribution of this research consists of two parts:
Part I : Techniques of fabricating periodically poled ridge waveguides on X- and Y-cut
LiNbO3substrates are developed. Simulations of the optical modes and the in-
teraction cross section, show a moderate improvement of both values, in turn an
enhanced nonlinear efficiency. Two nonlinear interactions using such a ridge guide
are studied: (i) second harmonic generation (SHG); (ii) cascaded interaction of
SHG and difference frequency generation (Cascaded SHG/DFG). Experimental
results are presented and discussed.
Part II: Fabrication and characterization of QPM structures in LNOI thin films and in
LNOI photonic wires are investigated. Two methods are developed including (i) di-
rectly bonding of a PPLN substrate; (ii) domain inversion directly in LiNbO3thin
18
films. Periodically poled LNOI photonic wires are fabricated, and nonlinear inter-
actions in photonic wires are studied.
Essential information about QPM nonlinear interaction, including a theoretical ana-
lysis of the nonlinear interactions studied in this work is presented in Chapter 2. Part
I of this research is presented in Chapters 3 to 5. Chapter 3 introduces the techniques
of fabricating a ridge waveguide on a X-cut substrate using plasma etching and the
subsequent local periodic poling. Chapter 4 presents the characterizations of the ridge
waveguide and periodic ferroelectric domains. Chapter 5 presents and discusses the
simulation and experimental results of SHG and cascaded SHG/DFG in the waveguide
described in chapter 3. Part II of this research is presented in Chapter 6. It includes the
fabrication of LNOI thin films and photonic wires and different methods of fabricating
periodically poled LNOI films. Last but not least, Chapter 7 concludes this dissertation
and provides some suggestions for future work.
Appendix A presents the fabrication of sub-micron periodic photoresist gratings by
using holographic lithography. This technique has been used in this work to fabricate
periodical domains in bulk LiNbO3substrates as well as in periodically poled LNOI thin
films.
19
Chapter 2
Theoretical background of QPM
nonlinear interaction
Nonlinear optics deals with nonlinear interaction of light with matter. At high light
intensities, a nonlinear response of the medium is induced. The nonlinear polarization
is excited, which leads to the generation of a new electric field or modification of the
phase and amplitude of the input optical wave. Numerous nonlinear optical phenomena
have been discovered since 1961, when Franken et al. reported the first second harmonic
generation (SHG) observed from a quartz crystal [31]. In this chapter, a theoretical ana-
lysis of second-order nonlinear interaction in waveguide structures is briefly discussed
for understanding this physical phenomenon from the viewpoint of device design. Ba-
sic concepts are presented in this chapter; more detailed discussions can be found in
textbooks [32, 33].
In this chapter, the nonlinear polarization induced by the electric field of an optical
wave is introduced. The coupled-mode method is then used to describe the nonlinear
interaction. Coupled-mode equations of two nonlinear interactions are studied in this
research: SHG and cascaded SHG/DFG. The formulars are derived and discussed. The
concept of QPM is introduced afterwards. The advantages of using a ridge guide and a
LNOI photonic wire for nonlinear interactions are briefly discussed in the last section.
2.1 Nonlinear polarization and coupled-mode equa-
tions
In an optical medium, a dielectric polarization ~
Pis induced by the electric field of an
optical wave ~
E.~
Pcan be expressed in general as (2.1).
~
P=ε0nχ(1) ~
E+χ(2) ~
E~
E+χ(3) ~
E~
E~
E+....o,(2.1)
where ε0is the permittivity of vacuum, χ(q)is the dielectric susceptibility tensor of
the q-th order. The first term in (2.1) is the linear polarization. The second-order
20
nonlinear polarization ~
P(2) =ε0χ(2) ~
E~
Eis crucial for three-wave (with frequency ω1,
ω2,ω3respectively) mixing processes i.e. SHG, when ω3=2ω1=2ω2, and DFG, when
ω3=ω1-ω2. Often a tensor dis introduced as dijk =1
2χ(2)
ijk. According to the Kleinman
symmetry [34], dcan be represented as a 3×6 matrix; for LiNbO3(point group: 3m) it
takes the form:
d=
0 0 0 0 d31 −d22
−d22 d22 0d31 0 0
d31 d31 d33 0 0 0
.(2.2)
For simplicity, we assume purely transverse-mode fields in the waveguide (i.e. the
TE and TM approximation). We therefore have only one transverse component of the
electric fields of the wave, either parallel or perpendicular, with respect to the waveguide
surface. Thus the electric field of the wave at frequency ωcan be expressed as
~
Eω(x, y, z, t) = 1
2{A(z)E(x, y)exp[i(ωt −βz)] + c.c.},(2.3)
where A(z) is the slowly varying amplitude, E(x, y) is the transverse-mode field distri-
bution, βis the propagation constant, “c.c.” stands for complex conjugation.
The electric field of the optical waves in the waveguide is
~
E=~
Eω1+~
Eω2,(2.4)
Substituting (2.3) into (2.4), and using deff denotes the effective value of dijk (in
turn, χ(2)
ijk) for the actual process, the nonlinear polarization P(2) induced by the electric
field ~
Ecan be written as
P(2) =ε0deff ~
E2=ε0deff ~
Eω1+~
Eω22.(2.5)
P(2) =ε0deff 1
4A2
1(z)E2
1(x, y)ei(2ω1t−2β1z)+1
4A2
2(z)E2
2(x, y)ei(2ω2t−2β2z)
+1
2A1(z)A2(z)E1(x, y)E2(x, y)ei((ω1+ω2)t−(β1+β2)z)
+1
2A1(z)A∗
2(z)E1(x, y)E∗
2(x, y)ei((ω1−ω2)t−(β1−β2)z)
+1
2A1(z)A∗
1(z)E1(x, y)E∗
1(x, y) + 1
2A2(z)A∗
2(z)E2(x, y)E∗
2(x, y)
+c.c..(2.6)
For a defined three-wave mixing (ω3=ω1±ω2), only one term from the right side
of (2.6) possibly contributes the appreciable intensity in the generated radiation. The
reason for this behavior is that the nonlinear polarization can efficiently produce an
output signal only if a certain phase matching condition (which will be discussed at the
end of this section) is satisfied, and usually this condition cannot be satisfied for more
than one frequency component of the nonlinear polarization.
Pω3=ε0deff A1(z)A2(z)E1(x, y)E2(x, y)ei((ω1+ω2)t−(β1+β2)z),(2.7)
21
Pω3=ε0deff A1(z)A∗
2(z)E1(x, y)E∗
2(x, y)ei((ω1−ω2)t−(β1−β2)z).(2.8)
The equation (2.7) describes sum frequency generation (SFG) and (2.8) is associated to
difference frequency generation (DFG).
The reason why the polarization plays a key role in the description of nonlinear
optical phenomena is that a time-varying polarization can act as the source of new
components at different frequencies of the electromagnetic field. The nonlinear wave
equation can be written as
∇2−n2
c2
∂2
∂t2~
E=µ0
∂2~
P(2)
∂t2,(2.9)
where nis the refractive index at the frequency of the electric field in concern, cis the
speed of light in vacuum and µ0is the magnetic permeability of vacuum. Substituting
(2.7) into the wave equation (2.9), coupled-mode equations are then derived from (2.9)
under slowly-varying amplitude approximation. The coupled-mode equations describe
the evolution of the fields in the propagation direction. Let us first consider a DFG
process, in which a strong pump wave at frequency ωpis mixed with a (usually weak)
signal wave at frequency ωsto generate a frequency-shifted idler wave at frequency
ωi=ωp−ωsvia the second-order nonlinear susceptibility χ(2). The coupled-mode
equations describing this process with the propagation losses taken into account take
the following form
∂Ai
∂z =−αi
2Ai−iκiAs
∗Ape−i∆βz,
∂As
∂z =−αs
2As−iκsAi
∗Ape−i∆βz,
∂Ap
∂z =−αp
2Ap−iκpAsAie+i∆βz,
(2.10)
where αj(j=p, s, i) is the waveguide propagation loss at the respective wavelength.
The difference of the propagation constants of the interacting waves ∆βis defined as:
∆β=βp−βs−βi,(2.11)
κj=deff ωj
cnj·RREs(x, y)Ei(x, y)Ep(x, y) dxdy
RREj2(x, y) dxdy, j =p, s, i. (2.12)
Solution
The solution of the equations (2.10) generally can only be expressed in integral
formats and require numerical integration to obtain the results. For simplicity, we
assume the waveguide is lossless, the input pump power is much larger than the signal
power, i.e. |Ap,0| |As,0|and the DFG conversion efficiency is low, i.e. the pump
power can be taken as constant in the waveguide. The last assumption is also known as
22
undepleted pump approximation. The equations (2.10) for an interaction through the
propagation length Lare then solved using the boundary condition Ai,0= 0 as
Ap(L) = Ap,0,
As(L) = ei∆β
2LAs,0[cosh gL +i∆β
2gsinh gL],(2.13)
Ai(L) = −iκie−i∆β
2L1
gAp,0sinh gL.
g=sκiκs|Ap,0|2−(∆β
2)
2
.(2.14)
Where the power of the optical wave is calculated as
Pj=ε0cnj|Aj|2
2ZZ Ek2(x, y) dxdy, j =p, s, i. (2.15)
The power levels of the interacting waves at the output of the waveguide (z=L) are
Pp(L) = Pp,0,
Ps(L) = Ps,0cosh2gL, (2.16)
Pi(L) = ωi
ωs
1
g2ηnormPp,0Ps,0sinh2gL,
g=sηnormPp,0−(∆β
2)
2
,(2.17)
ηnorm =2deff 2ωiωs
ε0c3npnsniSeff
,(2.18)
Seff =RREp2(x, y) dxdyRREs2(x, y) dxdyRREi2(x, y) dxdy
RREp(x, y)Es(x, y)Ei(x, y) dxdy2.(2.19)
gand ηnorm are often called the gain coefficient and normalized conversion efficiency
respectively. Seff , denoted as an effective cross section, is the normalized overlap integral
of the transverse mode distributions of the interacting waves. Efficiency optimization in
waveguide devices requires designing the waveguide geometry to maximize the overlap
integral, i.e. minimize the effective area.
The conversion efficiency of DFG is expressed as
ηDF G =Pi(L)
Ps,0
,(2.20)
=ωi
ωs
ηnorm
g2Pp,0sinh2gL, (2.21)
≈ωi
ωs
ηnormL2Pp,0sinc2∆β
2L. (2.22)
23
Equation (2.22) is obtained under the low gain limit, emphasizing the relationship
of conversion efficiency to material properties (deff and n) and device geometry (Seff
and L), coupled pump power and the phase mismatching term (sinc2(∆βL/2)). For
the perfect phase matched case where ∆β= 0, the factor sinc2(∆βL/2) approaches
1, and the intensity of the generated wave P3increases quadratically with z. This is
because the generated wave maintains a fixed phase relation with respect to the nonlinear
polarization and is able to extract energy efficiently from the pump wave. When ∆β6= 0,
the conversion efficiency decreases as ∆βL increases, with some oscillations occurring.
The reason for this behavior is that if Lis greater than approximately 1/∆β, the output
wave becomes out of phase with its driving polarization, and the power flows from the
signal and idler waves back into the pump wave. The length, through which a πphase
shift is built between the output wave and its driving polarization, is defined as the
coherence length Lcof the interaction.
Lc=π
∆β.(2.23)
2.2 Quasi phase matching
The previous discussion shows that the phase matching condition ∆β= 0 must be ful-
filled in order to achieve efficient nonlinear interactions. This is often not automatically
achieved due to the normal dispersion in the crystal. One of the established methods is
using birefringent phase matching (BPM), where the anisotropy of a nonlinear crystal
is used to find a unique propagation direction at which the interacting waves have the
same phase velocity. However, BPM is not feasible in the integrated optics because an
optical waveguide often orients along one of the crystallographic axes, at which BPM are
not fulfilled. Moreover, the specific nonlinear coefficient (i.e. d33, the largest nonlinear
coefficient in LiNbO3) can not be utilized in BPM because it requires that all the inter-
acting waves have the same polarization, which is along Z-axis. Due to the limitations
of BPM, a quasi-phase matching (QPM) technique is introduced.
The theory of QPM, first described by Armstrong, et al. [35] in 1962 and by Franken
and Ward [31] in 1963, has been discussed and extended by numerous authors. In QPM,
the interacting waves (for example, fundamental and SH waves in a SHG process) are
allowed to have different phase velocities, and the phase difference of the wavefront of one
wave relative to another reaches πover a distance called the coherence length Lc. A QPM
LiNbO3structure, as shown in Figure 1.3, has a periodically inverted orientation of the
Z-axis. An inversion of the direction of the Z-axis has the consequence of inverting the
sign of the nonlinear coefficient d33. The period Λ of the alternation of the crystalline
axis is chosen equal to twice of the coherence length Lcof the nonlinear interaction.
The nature of QPM is that, when each time the field amplitude of the generated wave
is about to decrease as a consequence of the phase mismatch, a reversal of the sign of
d33 occurs which allows the field amplitude to continue growing monotonically. Figure
2.1 shows, as an example, the growth of the converted SH power versus the interaction
24
length in BPM, QPM and non-phase matched conditions. In a non-phase matched
interation, no continuous buildup of SH power is obtained. In a BPM interaction,
the SH power increases parabolically. In a QPM interaction using d33, the SH power
grows continuously along the interaction length in a quasi-parabolic manner. With the
advantage of waveguides for light confinement in mind, this growth can be maintained
with high optical intensity within a long distance.
Figure 2.1: The growth of the SH power with propagation distance (divided by Lc) for
(i) quasi phase matched, (ii) birefringent phase matched and (iii) non-phase matched
interactions.
In a QPM structure, the spatially varying d(z) can be expressed in a form of Fourier
series
d(z) = X
m
dmexp(−imKz), K =2π
Λ, m = 1,2,3, ... (2.24)
where mK is the grating vector associated with the mth Fourier component of d(z).
Assuming a rectangular structure of d(z), the coefficient dmgiven by
dm=2
mπ sin(mπD)deff ,(2.25)
is the effective nonlinearity of the mth harmonic of the QPM grating, Dis the duty
cycle of the grating. Substituting d(z) as nonlinear coefficient into the coupled-mode
equations (2.10), the right hand sides of the equations are then expressed by a summation
of the spatial harmonic terms. Only one term of them can contribute substantially to
the nonlinear optical interaction, the other terms do not contribute substantially since
they oscillate rapidly with z. Therefore, the coupled-mode equations have the similar
form as in (2.10) with dmas the nonlinear coefficient and ∆βreplaced by ∆βQP M
∆βQP M = ∆β−mK. (2.26)
The intensity of the generated wave calculated from the equation (2.16) is modified
accordingly.
Most of the studies on QPM nonlinear interactions reported so far are interactions
among copropagating waves as shown in figure 2.2(a). If a QPM structure with a
25
sufficiently short period is available, the nonlinear interaction with counter-propagating
waves, e.g. backward propagation SHG, will become possible, as shown in figure 2.2(b).
Calculation shows that a QPM structure for a backward propagating SHG with fun-
damental wavelength at 1550 nm would be ∼400 nm. There are certain advantages
for counter-propagating nonlinear interaction. If both the input and output facets are
antireflection (AR) coated at the fundamental wavelength, it is much easier to separate
the second harmonic (SH) beam from the fundamental one. This is useful for so called
mirrorless parametric oscillation [36] and parametric down-conversion (PDC). Counter-
propagation interactions have unique spectral properties, for example, a much narrower
spectrum width compared to that of the copropagating PDC, due to that the phase
mismatch is limited by a much stricter condition. The simulation of single photon ge-
neration using counterpropagating PDC was reported by Christ et. al. [37]. It shows
that the FWHM of the wavelength distribution is 0.09 nm for the backward propagating
photon and 0.73 nm for the forward propagating photon. The narrow spectrum band-
width of the backward propagating photon with no further filter required makes it well
suited for a long distance transmission in optical fibers.
(a) (b)
Figure 2.2: Scheme of (a) a copropagating SHG and (b) a counter-propagating SHG.
2.3 Second harmonic generation
SHG is a special three-wave interaction process addressed in the first section, where
two waves have the same frequency (ω1=ω2) and the generated wave (ω3) has the
doubled frequency. The process is symbolically shown in figure 2.3. It can be illustrated
as a process that two photons of the same frequency ωare annihilated and a photon
of frequency 2ωis created. The coupled-mode equations governing SHG using a QPM
waveguide can be expressed in the following form (subscripts ‘SH’ and ‘f’ denote SH
wave and fundamental wave, respectively)
∂ASH
∂z =−αSH
2ASH −iκSH Af2exp[i∆βSHG z],
∂Af
∂z =−αf
2Af−iκfASH Af
∗exp[−i∆βSHG z],
(2.27)
26
Figure 2.3: Schematic sketch of SHG in a waveguide on a X-cut LiNbO3substrate. Inset:
sketch of the energy conservation of SHG.
where αSH ,αfare the waveguide propagation losses at fundamental and SH wavelength,
respectively. Let us consider the actual interactions explored in this work, Type Iprocess
uses the nonlinear coefficient d33, i.e. both waves have the same polarization which
orients along Z-axis. The coupling coefficients are
κSH =d33ωSH
πcnSH ·RR Ef2(x,y)ESH (x,y) dxdy
RR ESH
2(x,y) dxdy,
κf=2d33ωf
πcnf·RR Ef2(x,y)ESH (x,y) dxdy
RR Ef2(x,y) dxdy.
(2.28)
With the QPM structure of the period Λ and a m-th order process implemented, the
phase mismatch ∆βSHG is
∆βSHG =βSH −2βf+m2π
Λ.(2.29)
Solution
In the limit of an undepleted pump, an analytic solution can be derived. The am-
plitudes of the fundamental wave and generated SH wave after a propagation length of
Lare
Af(L) = e−
αf
2zAf,0,
ASH (L) = −iκSH A2
f,0
e(i∆βSHG +αSH
2−αf)L
−1
i∆βSHG +αSH
2−αf
e−
αSH
2L.
(2.30)
The power of the fundamental wave and the SH wave can be calculated from the
amplitudes above using the definition in (2.15); the conversion efficiency in a general
case is then obtained as
ηSHG =PSH,L
Pf,0
=8d2
33
ε0cnSH n2
fλ2
SH Seff
g(L)Pf,0,(2.31)
27
where
Seff =RREf2(x, y) dxdy2RRESH
2(x, y) dxdy
RREf2(x, y)ESH (x, ) dxdy2,(2.32)
g(L) = e−2αfLe∆αL + 12−4cos2∆βSHG L
2e∆αL
∆α2+ (∆βSHG )2,(2.33)
∆α=αf−αSH
2.
Assuming a lossless waveguide, the efficiency in (2.31) can be written as
ηSHG =8d2
33
ε0cnSH n2
fλ2
SH Seff
| {z }
ηSHG,norm
sinc2(∆βL
2)L2Pf,0.(2.34)
For SHG with exact phase matching, the conversion efficiency increases with the
coupled funmdamental power Pf,0and propagation length L. Equation (2.34) provides
guidelines of designing a QPM waveguide such that a high efficiency can be achieved
i.e. by using a long waveguide with small cross section. A normalized SH efficiency
of a conventional channel waveguide with an effective cross section of 57 µm2can be
up to ∼53 %W−1cm−2in a lossless case. It should be noted such an efficiency can be
achieved only with exact phase matching in a lossless waveguide and low fundamental
power. The propagation loss and phase mismatch tend to not only reduce the efficiency
but also broaden the bandwidth. With high fundamental power, the energy transfered
from the fundamental wave into the SH wave can not be neglected. As a consequence,
the generated SH power will increase linearly instead of parabolically with respect to
the fundamental power. What is more, the onset of the photorefractive effect at a high
fundamental power level also deteriorates the conversion efficiency.
2.4 Cascaded nonlinear interaction (SHG/DFG)
Besides the direct χ(2) process discussed above, a cascaded χ(2) :χ(2) process is often
of special interest, when the interacting waves are not in the same frequency band [38].
This is because an optical waveguide, which transmits only the fundamental mode at a
long waveguide (i.e. 1550 nm), can transmit multiple modes at a short waveguide (i.e.
775 nm). The propagation constant of different transverse mode is different. However,
the QPM condition is only fulfilled for a specific transverse mode (typically fundamental
mode) of each wave. Therefore, selectively exciting only the required transverse mode
of each interacting wave is essential for an efficient interaction. In a cascaded χ(2) :
χ(2) process such as a cascaded SHG/DFG, both pump (also fundamental wave in this
28
case) and signal waves are within the same band. The interaction involves the cascading
of SHG and DFG as shown in figure 2.4. The fundamental wave at ωfis converted to
frequency ωSH by SHG. Simultaneously, the generated ωSH serves as pump wave, and is
mixed with the input signal ωsto generate a wavelength-shifted idler ωi(ωi=ωSH −ωs=
2ωf−ωs) by DFG.
Figure 2.4: Schematic sketch of cascaded SHG/DFG in a waveguide on a X-cut
LiNbO3substrate. Inset: sketch of frequency mixing.
Since in such a cascaded scheme all the input waves are within the same band
(ωf≈ωs≈ωiωSH = 2ωf) in this frequency-mixing process, coupling coefficients can
be simplified such as κf≈κs≈κi≡κand κSH = 2κf. The coupled-mode equations
can then be expressed in a simplified manner as
∂Af
∂z =−αf
2Af−iκASH Af
∗e−i∆βSHG z,
∂ASH
∂z =−αSH
2ASH −iκAf2ei∆βSHG z−i2κAsAiei∆βDF G z,
∂As
∂z =−αs
2As−iκAi
∗ASH e−i∆βDF G z,
∂Ai
∂z =−αi
2Ai−iκAs
∗ASH e−i∆βDF G z,
(2.35)
∆βSHG =βSH −2βf,
∆βDF G =βSH −βs−βi.
(2.36)
For such an interaction, significant pump depletion is required to convert the funda-
mental wave to its SH wave, which serves as the pump for DFG. Equations (2.35) also
ignore the possible interaction between the fundamental wave and the signal wave via
SFG, which will happen only when the input signal frequency is tuned too close to the
fundamental frequency. It has been discussed before that such coupled-mode equations
generally can only be solved using numerical methods. In order to get some insight into
the conversion process, we derive a simplified analytic solution by assuming that pump
depletion, propagation loss and group velocity mismatch can be ignored. Therefore, the
29
first two equations in (2.35) would be equivalent to (2.27). We also assume a constant
signal intensity Ps. Under the phase matching condition (∆βSHG = 0 and ∆βDF G = 0),
we substitute the solution for SHG in (2.30) into the last equation of (2.35); under the
low gain limit, we obtain the power of the idler wave Piand the conversion efficiency
from signal to idler in the following form
Pi≈1
4
8d2
33
ε0cnSH n2
fλ2
SH SSHG,eff
| {z }
ηSHG,norm
32d2
33
ε0cninsnfλiλsSDF G,eff
| {z }
ηDF G,norm
L4P2
f,0Ps,0.(2.37)
η=Pi
Ps,0≈1
4ηSHG,norm ηDF G,norm L4P2
f,0.(2.38)
The normalized conversion efficiency increases with the length of the device quadruple
exponentially; thus a long device is advantageous to achieve a high conversion efficiency.
However, in a practical device, due to pump depletion and waveguide propagation losses,
the dependence on the device length is less than the fourth power.
2.5 Nonlinear interactions in ridge-type waveguides
As shown in (2.22), (2.31)and (2.38), the nonlinear conversion efficiency in a second
order nonlinear interaction has a strong dependence on the device geometry, i.e. inter-
action length Land effective cross section Seff . The interaction length one can achieve
is generally limited by the fabrication techniques with the considerations of the propa-
gation loss and quality of the periodic poling. The effective cross section is determined
mainly by the material and the geometric dimension of the waveguide, which can vary
significantly for different types of waveguide. The essential part of designing a highly
efficient LiNbO3waveguide is to minimze Seff . Therefore, a ridge waveguide is of in-
creasing interest since it can improve the light confinement. The calculation in this
section shows that it also improves the overlap between the transverse modes of the
interacting waves, i.e. reduce Seff . In this work, we simulate the optical fields in wave-
guides using a finite difference module provided by the commercial software OlympIOs.
One of the nonlinear interactions of interest in this work is SHG at a fundamental wave-
length of ∼1550 nm utilizing the nonlinear coefficient d33. The discussion in the section
will be focused in this interaction as an example. The analysis and conclusions also
apply to other interactions such as DFG and cascaded SHG/DFG. Two types of ridge
waveguides, Ti in-diffused ridge guide and LNOI photonic wire, are discussed.
The simulation of the optical modes in a Ti in-diffused ridge waveguide using the
finite difference method has been discussed in [39] in detail, and will not be repeated in
this thesis. The distribution of the refractive index through a Ti in-diffusion is calculated
from the distribution of Ti4+ ions taking the material dispersion into account. The
Ti4+ ion concentration is determined by the thickness of the Ti layer and the diffusion
parameters i.e. temperature and duration. In this work, in-diffusion of Ti with the
30
thickness of 70, 80 and 90 nm respectively, is carried out at 1060 ◦C for 8.5 hours in O2
environment. O2is required to reduce the out-diffusion of Li+ions at high temperature.
In comparison, the simulation for a LNOI photonic wire is much more straightforward
as the refractive index is homogeneously distributed in the material as growth.
(a) (b)
Figure 2.5: Simulated optical intensity profiles for TE modes at 1550 nm (red curves)
and 775 nm (blue curves) in (a) a ridge waveguide fabricated on X-cut Y-propagating
LiNbO3with the top width of 7 µm and the height of 3.5 µm and (b) a X-cut photonic
wire with the width of 1 µm and the height of 0.5 µm. The lines represent 10, 30, 50,
70 and 90 % of the peak intensity for both modes.
Figure 2.5 shows the simulated optical modes of both fundamental and SH waves
in a ridge guide and a LNOI photonic wire. This ridge guide in figure 2.5(a) has a top
width of 7 µm and a height of 3.5 µm; the thickness of the Ti stripe was 80 nm. The
LNOI photonic wire in figure 2.5(b) has a width of 1 µm and a height of 0.5 µm. The
improvement of the light confinement becomes evident by comparing the mode sizes
of ridge-type waveguides with that of a conventional Ti in-diffused channel waveguide
of 7 µm width in table 2.1. Let us first compare a Ti in-diffused ridge guide and a
conventional waveguide. The optical mode of a Ti in-diffused ridge guide is reduced
by one third in the horizontal direction compared to a conventional one, mainly due to
the high refractive index contrast between the ridge body and the air. However, the
mode size in the vertical direction does not differ much since the refractive index change
induced by the Ti in-diffusion has a similar profile. In contrast to the Ti in-diffused
ridge guide, which yields an improvement of light confinement mainly in the horizontal
direction, a LNOI photonic wire presents an improvement of the mode size by a factor
of 10 in both horizontal and vertical directions. This is due to the high index contrast
with an almost symmetric geometry in all four interfaces. It is especially important to
note that figure 2.5(b) shows a good coincidence of the centers of two optical modes,
resulting in a nearly perfect mode overlap. Compared to a conventional waveguide, the
effective cross section Seff is reduced by one third using a Ti in-diffused ridge guide and
nearly a factor of 100 using a LNOI photonic wire; accordingly the nonlinear efficiency
could increase by a factor of 1.5 and 100, respectively, without taking the propagation
losses into account.
31
C. Ti in-diffused Ti in-diffused LNOI
waveguide ridge guide Photonic wire
X[µm] 5.28 3.86 0.56
Y[µm] 3.54 3.36 0.4
Seff [µm2] 57 38.5 0.21
Table 2.1: Mode size in horizontal and vertical directions and effective cross section of
a conventional Ti in-diffused channel waveguide, a Ti-diffused ridge guide and a LNOI
photonic wire. The orientation and geometric design of the waveguides are described in
the text above.
An additional improvement of the nonlinear efficiency is possible by using a ridge
guide of different parameters i.e. waveguide dimensions and Ti thickness as shown in
figure 2.6. When the Ti layer is thicker, the Ti4+ concentration inside the ridge is higher,
and the increase of the refractive index is larger; therefore, both fundamental and SH
modes are better confined closer to the upper surface than the ridge guides with lower
Ti layer, resulting in a better overlap. Figure 2.6 also shows that a narrower ridge gives
a better mode overlap. Seff can be enhanced by a factor of 2 in a Ti in-diffused ridge
guide with the optimal design.
Figure 2.6: Comparison of Seff of ridge guides with different parameters.
In practice, due to a relatively higher propagation loss in a ridge guide (i.e. ∼1
dB/cm) compared to a conventional Ti in-diffused channel waveguide (i.e. ∼0.1 dB/cm),
the increase of the SH efficiency would be compromised to a certain degree. Figure 2.7
gives an example of the normalized SHG conversion efficiency deteriorating with the
increase of the propagation loss for different interaction length, assuming fundamental
wave and SH wave have the same propagation loss. However, the propagation losses
could be reduced by improving the fabrication processes (see discussions in chapter 3
and 4). Therefore, a highly efficient nonlinear interaction in a low-loss ridge guide is
achievable.
32
Figure 2.7: Normalized SH efficiency varies as a function of propagation loss and inter-
action length for a ridge guide with Seff = 38.5µm2, given αf=αSH .
Another advantage of using ridge guides is the potential of realizing wavelength con-
version, where the input of one or more high intensity optical fields is required. As briefly
mentioned before that in a congruently melt LiNbO3waveguide the photorefractive effect
is prominent at high intensity level, and therefore deteriorates the conversion efficiency.
Common methods of suppressing photorefractive effect are operating the system at a
high temperature, generally >150 ◦C or using MgO or ZnO doped LiNbO3instead.
It requires either a complicated temperature control system or expensive material, i.e.
increases fabrication complexity and cost. It was first observed by Nishida et.al. that a
ridge waveguide on Z-cut LiNbO3shows high damage resistance at room temperature
[40]. A similar phenomenon is observed in ridge guides on a X-cut LiNbO3substrate in
this work. This interesting phenomenon could be due to that the electrons generated
via photorefraction can move to the surface of the ridge, then be compensated from the
ambient environment due to the unique geometry of the ridge. In this way, no sustained
electric field generated by the photorefraction exists in the waveguide to induce the
change of the refractive index. This property could be of strong interest for industrial
applications.
Last but not least, a ridge-type waveguide, especially a photonic wire, has the poten-
tial in realizing counterpropagating nonlinear interaction. As discussed in section 2.2,
the domain period required for a counter-propagating nonlinear interaction such as back-
ward SHG is often in the submicron range. PPLN of such a period is difficult to be
realized in a LiNbO3bulk crystal due to the difficulty of fabricating submicron elec-
trodes and degradation of electric field contrast. With a ridge and a LNOI thin film,
because the periodically poled volume is 2 - 3 orders of magnitude smaller than that of
a bulk crystal, fabricating submicron periodical electrode is feasible. Several methods
of fabricating submicron domains are studied in this work; results will be discussed in
the corresponding section.
33
2.6 Summary
In this chapter, we have developed coupled-mode equations for guided-wave χ(2) non-
linear interactions such as DFG, SHG and a cascaded SHG/DFG, and demonstrated
theoretically that QPM is an effective phase matching technique in guided-wave non-
linear interactions.
Simplified solutions of all the coupled mode equations are solved to illustrate the
dependence on several important parameters assuming no loss and no pump depletion.
In SHG, we see that the conversion efficiency is typically proportional to the square of
the fundamental power in SHG; in DFG, it is proportional to the product of the pump
power and the signal power. In both cases, the conversion efficiency is proportional to
the square of the interaction length, and inversely proportional to the nonlinear cross
section.
The potential improvement of the nonlinear conversion efficiency using ridge guides
and photonic wires due to a reduced nonlinear cross section has been discussed. The
photonic wire is a favorable choice of waveguide since it can enhance the conversion
efficiency by a factor of 100.
34
35
Chapter 3
Fabrication of PPLN ridge
waveguides
This chapter presents the details of fabricating PPLN ridge waveguides on a X(Y)-cut
LiNbO3substrate. In the first section, the fabrication of ridge waveguides on a X(Y)-
cut LiNbO3substrate using plasma etching is introduced. This fabrication method
was originally developed by Hu [41]. In the second section, local periodic poling of
ridge waveguides is presented. The fabrication processes are illustrated using a X-cut
LiNbO3substrate as an example. The actual processes are identical for X-cut and Y-cut
LiNbO3substrates.
3.1 Ridge fabrication and Ti in-diffusion
Ridge waveguides on a X-cut LiNbO3are fabricated using inductively coupled plasma
(ICP) etching and subsequent Ti in-diffusion. The substrate used in this work is a 1 mm
thick X-cut congruent LiNbO3substrate from CrystalTech Inc. The essential processes
are depicted in figure 3.1. The details of each step are illustrated in the following:
Step 1: Substrate preparation
The fabrication starts with a proper cleaning of the substrate surface. The clean-
liness of the surface is essential for a good adhesion between the substrate surface
and the metal layer which will be coated in the next step. Therefore, we use a
three-steps cleaning process:
•Cleaning in H2SO4:H2O2(9:1), also called ”piranha”, at 50 ◦C for 10 minutes.
This cleaning solution can remove organic pollution on the sample.
•Cleaning in deionized water with soap at 50 ◦C for 10 minutes to eliminate
remaining leftovers from the last step.
•Cleaning in H2O:H2O2:NH4OH (7:2:1), also known as RCA Standard Clean-
ing 1 (RCA SC-1), at 50 ◦C for 30 minutes. RCA SC-1 is useful to remove
particles and organic residues on the surface.
36
Figure 3.1: A flow chart of ridge waveguides fabrication.
Step 2: Fabrication of the Cr mask
The substrate is blow dried with nitrogen after cleaning, and then immediately
put into the sputtering chamber. A 300 - 400 nm thick Cr layer is then sputtered
on the substrate surface. The thickness of the Cr layer is designed depending on
the height of the ridge and the etching rates of Cr and LiNbO3during ICP etching.
With the ICP etching in a mixture of 15 sccm C4F8and 15 sccm Helium, the ratio
of the etching rate between LiNbO3and Cr is greater than 10:1. Once the Cr
layer is deposited, a standard lithography process is made in order to transfer the
waveguide structure from the e-beam written photomask to the sample. This is
accomplished by using the positive photoresist Fujifilm OIR907-17, spun at 6000
rpm during 3 seconds, soft baked at 90 ◦C for one hour and exposed with a UV
lamp in a mask aligner. Then the photoresist is hard baked at 130 ◦C for one hour
and developed in OPD4290 developer. Afterwards, the remaining photoresist is
used as a mask in a wet etching process of the Cr layer in a solution of 100ml H2O,
5ml H2SO4and 40 g Cerium (IV) sulfate. After the etching, a 10% H2SO4solution
is used to remove etching residues that remain on the surface of the substrate. Now
the LiNbO3substrate is covered with Cr stripes of the width 8 - 10 µm, and is
ready for ICP etching.
Step 3: ICP etching
Plasma etching is a very efficient, well controllable process well known from semi-
conductor technology. It is performed in a Plasma100 system (Oxford Instrument).
Plasmas based on fluorine gases are generally used for etching LiNbO3due to the
volatility of fluorinated niobate species at a temperature around 200 ◦C. The etch-
ing gas we use in this work is a mixture of 15 sccm C4F8and 15 sccm He. During
37
etching one of the etching products, LiF, is re-deposited on the surface of the sam-
ple. In the meantime carbon polymers as byproducts accumulate on the substrate
surface. Both effects lower the etching rate. Therefore, we use a repeating process
consisting of multiple runs. After each run of etching (empirically 3 - 4 mins), the
sample needs to be taken out and cleaned in RCA SC-1 solution for 1 minute to
remove the carbon polymers which appear as a brown layer on the substrate. One
run of etching generally induces a depth of 300 - 400 nm. The ridge waveguides
presented in this thesis as examples have a height of 3.5 µm. It requires generally
9 - 10 runs to reach such a depth. After the ICP etching is completed, the remain-
ing Cr stripes have to be removed by a Cerium Sulfate solution and the sample is
carefully cleaned. Figure 3.2 shows an etched ridge on a X-cut LiNbO3substrate.
We use the term “undoped ridge” to refer to this ridge in order to distinguish it
from a Ti in-diffused ridge. The surface of the substrate appears fairly rough due
to the etching process. The width of the ridge top is generally ∼1µm narrower
than on the photomask due to the under-etching effect.
Figure 3.2: A SEM micrograph of an etched ridge.
Step 4: Ti deposition
With the ridge structure, the light confinement in the lateral direction is auto-
matically achieved by the high refractive index contrast between LiNbO3and air.
However, in the vertical direction, one has to increase the refractive index in-
side the ridge artificially. This could be done by either Ti in-diffusion or proton
exchange. In this research, Ti in-diffusion is investigated. It is possible to take ad-
vantage of the unique geometric profile of a ridge structure to accurately create Ti
stripes only on top of the ridges without an additional e-beam written photomask
and lithographic steps. A flood exposure and lift-off technique is implemented as
shown in figure 3.3.
This technique starts with spin-coating of a thin photoresist layer, for example,
the OIR907-17, on the sample. Due to the embossed surface of the ridge top,
the thickness of the photoresist on top of the ridge will be thinner than on the
surrounding. Therefore, it is possible to make a short UV light illumination, i.e.
insufficient exposure dose, which is only able to completely expose the upper layer
38
of the photoresist with the thickness on top of the ridges. Correspondingly, a short
developing is followed to remove the exposed photoresist from the top of the ridges
while leaving the rest of the photoresist on the substrate. A thin layer (70 - 80
nm) of Ti is then deposited homogeneously on the sample. The lift-off of Ti is
done by soaking the sample into acetone till the photoresist and the metal on top
of the photoresist cleared away. Ultrasonic bath is usually applied to speed up
this process and get a better cleaned surface.
Figure 3.3: Scheme of the flood exposure technique used to deposit Ti only on top of
the ridge.
Step 5: Ti in-diffusion
After the Ti stripe is properly fabricated on top of the ridge, the sample is sent to
the diffusion oven where the diffusion procedure is applied:
•Heating up to 1060 ◦C during 1.5 hours in O2atmosphere.
•Diffusion during 7.5 hours in Ar atmosphere.
•Second diffusion during 1 hour in O2atmosphere. The second diffusion in O2
is used to restore the oxygen into the LiNbO3structure that was out-diffused
before.
•Cooling down to room temperature.
A Ti in-diffused LiNbO3ridge waveguide, also called a “doped ridge” in this thesis,
is shown in figure 3.4. Compared to figure 3.2, the surface of the substrate is
significantly smoothed due the diffusion at the high temperature, so is the side
wall of the ridge. In this way, the propagation loss of the ridge waveguide due to
scattering by the rough side walls will be reduced.
39
Figure 3.4: A SEM micrograph of a Ti in-diffused ridge waveguide.
3.2 Local periodic poling
Considering that the spontaneous polarization of a X-cut LiNbO3substrate is parallel
to the surface, within a ridge geometry, this direction happens to be approximately
perpendicular to the side walls of the ridges. The idea of the local periodic poling is
to place the metal electrodes on both sides of the ridge, then apply an electric field
between them, as depicted in figure 3.5. The distance between two side walls (<10 µm)
is two orders of magnitude smaller compared to the thickness of a conventional Z-cut
LiNbO3substrate (typically 0.5 mm), meaning that a roughly two orders of magnitude
lower voltage is needed to overcome the coercive field Eccompared to that of poling a
bulk LiNbO3crystal. The domain inversion presumably happens only in the body of
the ridge as the electric field in the outer region is significantly lower.
Figure 3.5: Scheme of the poling configuration for a ridge on X-cut LN.
The static electric field distribution is calculated in the cross section of the ridge using
a finite difference method. Homogeneous electrodes are assumed for the calculations.
A numeric derivation was made on a non-uniform grid of 512×512 points in an area of
60×50 µm2around the ridge, yielding as result the profile of the relevant component
Ezof the electric field distribution [39]. Figure 3.6 simulated by Miguel Garcia Granda
presents, as an example, the calculated electric field distribution in an undoped ridge
with the height 4 µm and top width 9 µm, assuming 1 V applied to the left electrode and
the right electrode grounded. As the coercive field of LN is ∼21 kV/mm, the applied
40
voltage should be at least 166 V to allow domain inversion to take place. First nucleation
is expected at the upper edge of the ridge where the field is highest. In most of the
cases, due to the imperfection of the electrodes, the actual applied voltage had to be
higher than the simulated value.
Figure 3.6: Calculated Ez(MV/m) in a ridge of 4 µm height and 9 µm width, assuming
1 V applied to the left electrode with the right electrode grounded.
Standard lift-off lithography is used to fabricate comb-like electrodes as depicted
in 3.5. An image reversal photoresist with a relatively high viscosity is required for
successful lift-off on the non-planar substrate. We use a thick photoresist AZ4533 (Mi-
croChemicals) mixed with 1.2% imidazol as the image reversal photoresist. Spincoating
the photoresist at 3000 rpm for 10 seconds results in a ∼4µm thick photoresist layer on
the planar substrate, sufficient to cover the top of the ridge. An E-beam written pho-
tomask is designed containing many groups of comb-like finger structures with period
of 16.2 µm and 16.6 µm and different duty cycle ranging between 25 : 75 and 50 : 50
(finger width : interval width). The consideration of using different duty cycles comes
from our experience of periodic poling of Z-cut LiNbO3, where the domain inversion not
only proceeds along the direction of the electric field but also grows laterally toward
the neighbouring domain. After the photoresist is exposed and developed, a reversed
electrode structure is transferred to the photoresist layer. A 100 nm thick Ti or Cr layer
is then deposited homogenously on the substrate. The metal coated sample is soaked
in acetone for some time in order to remove the photoresist and the metal on top of the
photoresist. Figure 3.7 shows the top view of a fabricated electrode structure under an
optical microscopy.
It is also possible to fabricate comb-like electrodes with fine periods. One approach
is fabricating a fine periodical grating using the holographic lithography discussed in
Appendix A. The fabrication consists of three major steps in the following.
Step 1: A thin layer of photoresist AZ4533 is spincoated over the surface. During the
exposure, the photoresist on top of the ridge is protected by the photomask from
being exposed. After the development, only a thin layer of photoresist on top of
41
Figure 3.7: Top view of the electrodes of 16.6 µm period for a 9 µm wide ridge.
the ridge remains. This part of photoresist is used to avoid the direct connection
between counter electrodes.
Step 2: A thin layer of photoresist SX AR-P 3500/6 is spincoated over the surface. The
coated sample is then exposed using Lloyd’s interferometric optical holography
(see Appendix A) for 15 seconds and developed. Afterwards, a 100 nm thick Ti is
evaporated over the surface. The undeveloped photoresist from the last two steps
and Ti on top of it is removed in aceton bath.
Step 3: Standard lift-off lithography is performed in order to connect all the Ti strips
in the same grating group, therefore, comb-like electrodes can be formed. A pho-
tomask with stripe structures parallel to the waveguide is used during the exposure.
After Ti evaporation and subsequent lift-off, counterparts of comb-like electrodes
are formed.
Fabricated comb-like electrodes of a period of 2 µm are shown in figure 3.8. Due
to the unique topography of a ridge structure, the Ti finger tips are narrower towards
to the ridge side walls. This approach is rather complicated and challenging as the
quality of the resulting electrodes is highly influenced by the precision of lithography
in each step. An advanced alternative could be using E-beam lithography to write
photoresist gratings with fine periods. However, it is often time and cost consuming and
not accessible in this work.
The poling experiments were done in an oil bath to maintain a high resistance be-
tween the two electrodes and to avoid surface currents as much as possible. Voltage
pulses with different pulse durations and different numbers of pulses are applied from
a computer controlled voltage amplifier for poling, and in the meantime the voltage
across the ridge and the current from the ridge are monitored using a circuit shown
in figure 3.9. A DC voltage amplifier controlled by a computer via D/A converter is
used to provide a voltage in the range of 0 - 1000 V. 1% of the output voltage from
the amplifier is output for monitory purpose. Voltages are detected by the computer
via a A/D converter. Resistors R1, R2, R3 and R4 are carefully chosen to have enough
sensitivity to measure the small signal while keeping V1 and V2 lower than the input
42
Figure 3.8: Top view of the electrodes of 2 µm period for a 8 µm wide ridge.
limit of the A/D converter. The actual voltage on the sample VLN is calculated by
VLN =R2+R3
R3V1−V2; the poling current ILN is calculated by ILN =V2
R4.
Figure 3.9: A sketch of the electric circuit for the local poling
Periodic poling in undoped and doped ridges has been investigated. Figure 3.10
presents the voltage/current-time characteristic for both an undoped ridge and a doped
ridge of the same width and height during one single voltage pulse. The voltage is
ramped up with a slope of 80 V/ms, kept constant for a certain time before ramped
down to zero. For both undoped and doped ridge, the poling current abruptly rises
when the voltage reaches a certain value, indicating the start of domain inversion.
In the undoped case (see figure 3.10(a)), the rise of the current starts at voltage
of ∼180 V, which is consistent with the theoretical value calculated from electric field
distribution. With the voltage continuing increasing, a second rising of the current
appears when the voltage ramps up to ∼300 V, indicating domain inversion proceeding
in a larger area. However, the current decrease when the voltage becomes stabilized
is hard to explain. Afterwards, the current nearly stabilizes or decreases slightly with
a constant voltage applied. Then it slowly drops to zero as the voltage ramps down.
Considering the spontaneous polarization of congruent LN of 0.72 µC/mm2at room
43
temperature and the poled area of ∼0.1 mm2(four ridges were poled simultaneously),
the total charge required for poling should be 0.14 µC. This value is significantly smaller
than the measured total charge of ∼1µC. We suspect that there might be some leakage
current along the surface involved. However, the reason behind such a leakage current
appearing only after domain inversion has taken place is not clear.
(a) (b)
Figure 3.10: Poling characteristics of (a) an undoped ridge and (b) a doped ridge of 8
µm width and 3.5 µm height.
In the case of poling a doped ridge waveguide (see figure 3.10(b)), domain inversion
starts at a considerably higher voltage of ∼280 V. On one hand, this could be due to
Li out-diffusion during annealing at the high diffusion temperature since the coercive
field increases when the Li concentration decreases [42]. On the other hand, it has
been observed in Ti in-diffused channel waveguide in Z-cut LiNbO3that the domain
inverted area in Ti doped region is generally smaller than that in undoped region [43].
This phenomenon suggests that the coercive field of Ti doped LiNbO3could be slightly
higher than that of undoped LiNbO3. The higher electric field required for poling a Ti
doped ridge guide could be the consequence of both effects.
When applying a chain of voltage pulses to the ridge, we observe that the largest
charge is accumulated during the first pulse; it decreases fast with increasing pulse
number as shown in figure 3.11. It might indicate that most of the domain inversion
has been achieved by the first pulse alone. However there is still some remaining charge
even in the very last pulse, which might be due to the leakage current. Such a behavior
has already been observed previously [43]. The formation of a leakage current is not
fully understood. One speculation is that the crystal lattices beneath the electrodes
undergo a certain degree of damage due to the strong electric field applied, therefore,
the leakage current can flow through the crystal. In most of our experiments, one or
two pulses with 15 - 30 ms duration are used.
In principle, one can restore the polarization of the inverted domain by applying
the electric field of the opposite polarity. For Z-cut LiNbO3crystal, the electric field
required for restoring the inverted domain was found much lower than that required for
44
Figure 3.11: The accumulated charge per rectangular poling pulse of 270 V as function
of the pulse number.
reversing an as-grow crystal [44]. Poling a ridge on a X-cut LiNbO3substrate using the
electric field of a reversed polarity is tricky due to the existence of the leakage current.
Figure 3.12 shows poling characteristics of periodically poling an undoped ridge on X-
cut LiNbO3using a positive polarity electric field and subsequently a negative polarity
electric field. Poling characteristics using a positive polarity electric field (red curve) is
similar as we discussed in figure 3.10(a). When a negative polarity electric field (green
curve) is applied, the current appears and increases along with the voltage. This near-
ohmic behavior could be due to the leakage. When the voltage remains constant, the
current stays at a lower value compared to the previous poling and decreases fast when
the voltage decreases. One speculation is that, the area involved in the polarization
restoration might be smaller than the poled area in the previous poling. This happens
when the domain growth in the first poling is not complete, i.e. domain inversion starts
from the +Z face has not completely reached the −Z face, resulting in a smaller poling
area in the restoration poling than in the first poling.
The quality of the electrodes is important to the quality of the periodical domains.
In practice, the imperfection from the fabrication, such as unsuitable duty cycle, asym-
metric electrodes with respect to the central line of the ridge, and distorted finger tips,
can lead to the incomplete domain growth. The voltage, duration and pulse number of
the voltage pulses also play a role in domain growth in both longitudinal (direction from
+Z face to −Z face) and lateral (along the ridge) directions. Moreover, a too strong
electric field might cause damage of the crystal. A detailed discussion is given in the
next chapter together with the characterization results of periodic domains.
45
Figure 3.12: Poling characteristics of poling an undoped ridge on X-cut LiNbO3using a
positive polarity electric field (red curve) and subsequently a negative polarity electric
field (green curve). Voltage waveform is given in black curve.
3.3 Summary
In this chapter, the details of fabricating a periodically poled Ti in-diffused ridge guide
on X(Y)-cut LiNbO3have been presented.
A ridge is fabricated using conventional lithography and ICP etching. Afterwards, a
novel lift-off technique is used to define a Ti stripe only on top of the ridge. In-diffusion
is performed in O2at 1060 ◦C for 8.5 hours.
After the ridge guide is fabricated, electric field assisted poling is performed to peri-
odically reverse the spontaneous polarization of the crystal. This technique is called
local periodical poling because domain inversion takes place in the body of the ridge
only. In comparison to pole a bulk LiNbO3substrate, a very low voltage of a few hun-
dred volts is sufficient for local periodical poling. However, a certain amount of leakage
current has been observed after poling took place. The reasons of the leakage current
and irreversible poling characteristics are not fully understood.
46
47
Chapter 4
Characterization of PPLN ridge
waveguides
Different methods are applied to characterize ridge waveguides and periodically inverted
ferroelectric domains inside the ridge waveguides. Section 1 presents the characterization
of ridge waveguides by measuring the optical mode distribution, propagation loss and
interaction cross section. Periodic ferroelectric domains are characerized using selective
chemical etching and nonlinear confocal laser scanning microscopy(CLSM) respectively.
The corresponding results are presented in section 2.
4.1 Waveguide properties
4.1.1 Propogation losses
The propagation losses are measured by analyzing the low-finesse Fabry-Perot reso-
nances of a waveguide as shown in figure 4.1. The polished end faces of a waveguide
form a low-finesse cavity. The cavity can be tuned by varying, either the wavelength of
the propagating wave or the temperature of the sample. In this work, the temperature
tuning is applied. The length of the sample increases with the increased temperature
of the sample as a function of time. A coherent single-frequency laser beam at ∼1550
nm emitted from an external cavity laser (ECL) is launched into the waveguide and
the output power is detected using an InGaAs photodiode. The time-dependent output
current from the photodiode is recorded by a computer. The propagation losses are
evaluated from the contrast of the resonances [45]:
α=4.34
L(ln R−ln ˜
R),˜
R=1
K(1 −√1−K2)and K =Imax −Imin
Imax +Imin
.(4.1)
Ris the reflectivity of the waveguide end faces; L is the cavity length in cm.Kis the
measured intensity contrast. αdefines the upper limit of the propagation losses inside
the cavity.
48
Figure 4.1: Schematic diagram of the loss measurement setup utilizing the low-finesse
Fabry-Perot contrast method. PD: photodiode, ECL: external cavity laser.
The height and the top width of the ridge waveguides are mainly designed in such a
way that the single mode transmission at a certain wavelength (1550 nm in this work) is
guaranteed. For a Ti in-diffused ridge guide with a height of 3.5 µm and the original Ti
layer of 70 - 80 nm thickness, due to the small refractive index contrast, the cut-off top
width of the ridge guides for guiding the optical wave at 1550 nm is ∼4µm. On the
other hand, the ridge guides with a top width larger than 9 µm generally guide more than
just the fundamental mode. Other factors such as propagation losses, nonlinear effective
cross section and the feasibilty and reproducibility of the fabrication processes are also
taken into account in the design. The propagation losses (mainly scattering losses) of
the optical mode in a ridge guide of a smaller width is generally higher because the
electric field is higher at the interface, and more scattering is induced. The relation
between nonlinear effective cross section Seff and the ridge top width as shown in figure
2.6 in section 2.5 indicates that a top width of 5 - 6 µm is prefered in order to minimize
Seff . With all these factors in mind, the ridge waveguides with a height of 3.5 µm and
a top with of 6 - 8 µm are fabricated and investigated in this work. The lengths of the
ridge guides are 13 - 27 mm.
Propagation loss of the optical mode in a ridge guide is mainly induced from the
scattering of the electric field inside the ridge and at the interface. Plasma etching
and other fabrication processes (lithography, wet etching of Cr, etc.) can bring in
defects such as rough surface and pinholes. These defects will introduce large amount
of scattering centers. Inhomogeneous Ti indiffusion also increases the loss because of
the inhomogeneous refractive index distribution. Moreover, the crystal often undergoes
a certain degree of damage during electric field assisted poling; scattering losses can be
induced by such damage. The table 4.1 gives typical propagation losses at wavelength
1550 nm in TE and TM polarizations in ridge guides of different parameters.
For the ridge guides with top width within the range of 6 - 8 µm, as shown in table 4.1,
there are several points worth noting. Firstly, the propagation loss of the optical mode
49
Ti thickness [nm] 70 80
Top width [µm] 6.5 7.5 6 8
αT E [dB/cm] 1.1 0.7 1.3 1
αT M [dB/cm] 0.4 0.3 0.9 0.5
Table 4.1: Average propagation loss of Ti in-diffused ridge guides on X-cut LiNbO3at
1550 nm in both TE and TM polarization.
in a ridge guide on X-cut LiNbO3is generally higher than that of a ridge guide on Z-cut
LiNbO3(0.2 - 0.5 dB/cm) fabricated by wet etching [13]. This is because the surfaces
of ridges fabricated by plasma etching, especially the side walls, are rougher than those
fabricated by wet etching. Secondly, the propagation loss increases with the decrease
of the ridge top width (see the discussion above). Thirdly, ridge guides in-diffused with
70 nm thick Ti layer generally have lower losses than that with 80 nm thick Ti layer
provided the in-diffusion process is identical and the geometric parameters of the ridge
are comparable. This is because with thicker Ti, Ti4+ concentration is higher in the
ridge and at the interface. This in turn introduces more scattering centers, therefore,
higher loss. Last but not least, propagation loss of the ridge guide on X-cut LiNbO3in
TE polarization is higher than that in TM polarization. This agrees with our experience
in Ti in-diffused channel waveguide on Z-cut LiNbO3. The optical mode with the electric
field oscillating along the extraordinary axis (Z-axis) of LiNbO3usually undergos higher
loss. The possible reason is that the optical mode, in which the electric field oscillates
along Z-axis (TE mode in this work), generally has a smaller size (refer to section 4.1.2.
The optical field confined inside the ridge is stronger, and the scattered optical field is
in turn higher. This results in higher propagation loss.
4.1.2 Optical mode distribution
The transmission mode is measured using the experimental setup sketched in figure 4.2.
The light path before the beam entering the waveguide is identical to that in figure 4.1
in section 4.1.1. An objective lens with high magnification (100X) is placed hehind the
waveguide to magnify the near field distribution at the end face of the waveguide. This
near field distribution is then imaged on an infrared camera. Figure 4.3 shows measured
optical modes in both TE and TM polarizations from a Ti in-diffused ridge guide of a
top width 7.5 µm and a height of 3.5 µm. A 70 nm thick Ti layer was diffused into this
ridge guide. In both polarizations, a clear single mode is presented. The full width half
maximum (FWHM) mode sizes are measured in the table 4.2 for ridge guides of different
parameters. For a ridge waveguide on a X-cut LiNbO3substrate, the transmitted optical
mode is smaller in TE polarization than in TM polarization. The horizontal mode size
is mainly influenced by the top width of the ridge; it decreases with the decreased top
width. The vertical mode size is, on the other hand, almost independent on the top
width. The measured mode sizes agree with the calculated results presented in table
2.1 in section 2.5.
50
Figure 4.2: Schematic diagram of the setup to measure the mode profile. IR: infrared.
(a) (b)
Figure 4.3: Transmitted optical mode from a Ti in-diffused ridge guide of top width
7.5 µm and height 3.5 µm, Ti thickness 70 nm in (a) TE polarization and (b) TM
polarization.
4.2 Periodic ferroelectric domains
4.2.1 Domain visualization using selective chemical etching
It is well known that LiNbO3ferroelectric microdomains can be visualized via selective
chemical etching and imaging. The etching solution HF:HNO3is used for this purpose.
In the inverted LiNbO3domains, both directions of Z- and Y-axis are rotated by 180◦,
while the crystallographic X-axis remains the same. +Z and −Z faces of LiNbO3, as
well as +Y and −Y faces, have significantly different etching rates in HF:HNO3solu-
tion. In HF:HNO3(HF concentration : 24%) solution, the etching rate for −Z face of
51
Ti th. Ridge top TE TM
[nm] width [µm] Hor. [µm] Vert. [µm] Hor. [µm] Vert. [µm]
70 6.5 3.9 2.9 4.8 4.4
7.5 4.3 3.0 5.2 4.2
80 6 3.6 3.2 4.6 4.8
8 4.4 3.0 5.2 4.1
Table 4.2: Measured FWHM horizontal (Hor.) and Vertical (Vert.) mode sizes of Ti
in-diffused ridge guides on X-cut LiNbO3at 1550 nm in both TE and TM polarization
of ridge guides with different top width and Ti thickness (th.).
LiNbO3is 4.7 µm per hour at 60 ◦C; while +Z face remains unaffected [46]. Therefore,
the difference in height between +Z and −Z faces is generated after etching, and can be
observed using an optical microscope.
The samples are first immersed in the etching solution for 10 minutes, and then
observed from the top using differential interference contrast optical microscopy. The
samples are then cut through the ridges to reveal the cross sections of the ridges. The
cross section faces are carefully polished before immersed into the etching solution.
Another 10 minutes of etching is applied to reveal the domain patterns in the cross
sections. Undoped ridges on X- and Y-cut LiNbO3and Ti in-diffused ridge on X-cut
LiNbO3are investigated using this method.
Undoped ridge
Figure 4.4 presents corresponding results of an undoped ridge on X-cut LiNbO3.
From figure 4.4(a) and (b), a periodical etching of the ridge side wall is observed from
the top and the side. The side walls of the ridge consist of a sequence of +Z- and
−Z-faces, whereas the top surface is still a homogeneous X-face. The sample is then
cut through the body of the ridge as shown in figure 4.4(d) and polished before etched
again. The periodic domains in the body of the ridge are observed as shown in figure
4.4(c). A hexagonal shape of the domain wall is observed as marked with white dash
lines in 4.4(c). The depth of the domains (in X-direction) corresponds to the height of
the ridge as expected. The duty cycle of the periodical domain structure is ∼7:3 (width
of inverted to non-inverted domain) in comparison with the duty cycle of the finger
electrodes of 1:1. This indicates a certain degree of overpoling.
Figure 4.5 presents similar results of an undoped ridge on Y-cut LiNbO3. From
figure 4.5(a) and (b), a periodical etching of the ridge side wall is observed from the top
and the side. In addition to the periodic etching on the side wall, a periodic modulation
of the height of the ridge can be seen because the top face contains a sequence of +Y-
and −Y-faces. The periodic domains in the body of the ridge are observed as shown in
figure 4.5(c). An improved duty cycle of the domain structure close to 1:1 is attributed
to the optimized design of the finger electrodes as well as the voltage pulse. The shape of
the domains is significantly different with a depth somewhat smaller than on the X-cut
substrate.
52
Figure 4.4: Selectively etched, periodically poled undoped ridge on X-cut LiNbO3. (a)
Top view. (b) Side view. (c) Side view after cutting the ridge. (d) Cutting scheme.
Figure 4.5: Selectively etched, periodically poled undoped ridge on Y-cut LiNbO3. (a)
Top view. (b) Side view. (c) Side view after cutting the ridge. (d) Cutting scheme.
53
Ti in-diffused ridge
Ti in-diffused ridge waveguides are etched in the etching solution. The top view of
an etched ridge guide (etching time: 3 hours) is shown in figure 4.6(a); the etched cross
section of this ridge guide is shown in figure 4.6(b) (cutting scheme is referred to figure
4.4(d)). In the top view, a periodically modulated pattern can be seen in both original
+ and −Z faces (ridge sidewalls). This indicates that the inverted domains extend
from +Z face to −Z face. A hexagonal domain shape can be observed as indicated as
dashed line in figure 4.6(b). Both figures show similar features as in figure 4.4(a) and
(c). The duty cycle of the domain pattern is close to 1:1, suitable for efficient nonlinear
interaction. Figure 4.7 presents the etched Z-face of another ridge guide on a X-cut
LiNbO3substrate. The inverted domain pattern in the cross section of the ridge and on
the side walls are presented respectively. A hexagonal shape of the inverted domain is
clearly observed in figure 4.7.
(a) (b)
Figure 4.6: (a) Top view and (b) cross section of a selectively etched, periodically
poled Ti in-diffused ridge on X-cut LiNbO3. The white dashed line in (b) indicate the
hexagonal domain shape.
Ridge with periodical domains of 2 µm
As discussed in section 3.2, comb-like electrodes with fine periods can be fabricated.
A ridge with such electrodes was poled and subsequently etched. The top view of the
selectively etched ridge is presented in figure 4.8. The arrows indicate the locations
of electrode fingers. However, the quality of the periodical domains, especially the
duty cycle and the homogeneity is limited by the quality of the electrode fabrication
(see the corresponding discussion in section 3.2). It can be improved by using E-beam
lithography.
54
Figure 4.7: Side views of a selectively etched, periodically poled doped ridge on X-cut
LiNbO3.
Figure 4.8: Top view of a selectively etched, periodically poled ridge of periodicity 2
µm on X-cut LiNbO3.
Ferroelectric domains with different poling parameters
It has been briefly mentioned in section 3.2 that the voltage and duration of the
voltage pulse used in the electric field assisted poling play an important role in domain
growth. Figures 4.9, 4.10 and 4.11 show how the growth of the inverted domain is
influenced by the pulse duration and voltage.
Figure 4.9 shows the results from a doped ridge on X-cut LiNbO3. It has been known
that, domain inversion generally starts at randomly distributed cites, also called “nu-
cleation cites”. Seed-like inverted domains then immediately grow from the nucleation
cites. These seed-like domains are called “filament”. With a very short voltage pulse,
only a few filaments can be seen as in figure 4.9(a). With a long pulse, the inverted
domain continues growing through the ridge, and simultaneously growing laterally. The
consequence of the lateral growth is that the filaments will finally merge and a complete
domain is formed and likely increases its volume with the pulse duration as shown in
figures 4.9(b) and (c), and in figures 4.10 (a)-(c) as well.
Figure 4.11 shows the comparison of the domain inversion in the ridge poled using a
voltage pulse of a fixed duration but of different voltage levels. The images in figure 4.11
are top views of the chemically etched ridges on Y-cut LiNbO3. On the ridge poled with
300 Volts pulse as shown in figure 4.11(b), only a few flaments are observed, indicating
55
Figure 4.9: Selectively etched doped ridge of top width 8 µm on X-cut LiNbO3poled
using a voltage pulse of (b) 500 Volts, 3 ms duration; (c) 400 Volts, 30 ms duration and
(d) 500 Volts, 60 ms duration. (a) Cutting scheme, the circled area is imaged.
Figure 4.10: Selectively etched doped ridge of top width 7 µm on Y-cut LiNbO3poled
using a voltage pulse of 600 Volts and different duration (b) 20 ms, (c) 30 ms and (d)
50 ms respectively. (a) Cutting scheme, the circled area is imaged.
56
Figure 4.11: Top views of selectively etched undoped ridge of top width 8 µm on Y-cut
LiNbO3poled using a 20 ms voltage pulse of (b) 300 Volts, (c) 500 Volts and (d) 700
Volts respectively. (a) Imaging scheme and +zand −zindicate the crystallographic
orientation of the side walls of the ridge.
an insufficient domain inversion. The inverted area is evidently broader when a pulse of
higher voltage was applied.
Material damage during poling
High voltage and long pulse duration not only broaden the inverted domains, but also
induce surface damage in the region under the electrodes with positive polarity. This is
due to the large amount of charges migrating through the surfaces rapidly during poling.
The damaged crystal can be attacked by the etching solution much more easily than
the intact crystal. Figure 4.12 shows examples of some surface damages of the ridge
guides revealed by chemical etching. Dashed lines indicate the location of electrodes
which have been removed by chemical etching. Figure 4.12(a) is taken from the top
surface of a periodically poled ridge guide in which the tips of the comb-like electrodes
reached the top surface of the ridge. Since the domain inversion typically starts in
the vicinity of the electrodes tips, the region around the tips, the top surface near the
edge in this case, suffers a certain degree of damage. Figure 4.12(b) is taken from the
ground surface of a periodically poled ridge guide. An atomic force microscopy (AFM)
image of a scratch-like mark is shown in figure 4.12(c). The damage appears typically as
scratch-like lines along one direction which orients at ∼60◦with respect to the Z-axis.
The surface damage induced by poling could introduce additional scattering centers;
therefore, increase the propagation loss.
57
Figure 4.12: Images of surface damage on periodically poled ridge guides on a X-cut
LiNbO3substrate revealed by chemical etching: (a) a top surface of a ridge guide where
the electrode tips are on the top surface, (b) ground surface of the ridge guide; (c) AFM
image of the scratch-like surface damage on the substrate.
4.2.2 Domain visualization using CLSM
Confocal laser scanning microscopy (CLSM) is regarded as a powerful nondestructive
method of imaging material. Nonlinear CLSM is especially helpful for 3-D imaging of
ferroelectric domains in the material such as LiNbO3. Visualizing ferroelectric domains
using CLSM is done in collaboration with the research group of “Optoelektronik und
Spektroskopie an Nanostrukturen” (University of Paderborn); the measurements are
performed by Berth et.al..
Experimental nonlinear CLSM setup used in this work is sketched in figure 4.13. In
this special configuration the incoming laser beam is focused by an infinity-corrected
microscope objective to a diffraction-limited spot. The signal to be detected is focused
likewise by a detector lens onto the confocal pinhole. A pinhole placed in front of the
detector is responsible for the confocal characteristic of the system. A pinhole of 2 µm
diameter is typically used in this measurement. Information, which does not originate
from the focal plane of the microscope objective, is faded out by this arrangement.
The advantage of out-fading information from above or below the focal plane enables
the confocal microscope to perform depth-resolved measurements. Image acquisition
is accomplished by scanning the sample with a nano-positioner under the condition of
a spatially fixed laser focus. A 3-D image can be obtatined by scanning of sequential
levels. The adjustment of the optical system (pinhole modules, objectives, intermedi-
ate imaging, fiber-coupling, etc.) is driven by piezo-actuators, which are realized as
piezoelectric inertial-drives. Two display-units (LED, CCD, beam splitter) are used for
conventional microscopy. A detailed introduction of nonlinear CLSM and its application
in imaging Z-cut PPLN can be found in [47].
In the linear operation mode, a Ti:sapphire laser (780 - 820 nm) operating in the
continuous wave mode as well as a 100 mW diode pumped solid state laser (DPSSL)
with an emission wavelength of 473 nm are available. By using the DPSSL the lateral
58
Figure 4.13: Sketch of the experimental nonlinear CLSM.
resolution was determined to be ∼300 nm and the axial resolution to ∼500 nm. A
tomography image of the sample can be obtained using the linear mode. In the nonlinear
operation mode, a 20 fs mode-locked Ti:sapphire laser (Femtosource C-20) is pumped by
a frequency doubled Nd:YVO4laser. The Ti:sapphire laser with an average output power
of 500 mW at a centre wavelength of 800 nm emits ∼100 fs pulses with a repetition rate
of 80 MHz. For the spectral separation of the fundamental and the SH signal, dichroic
beam splitters and additional colour bandpass filters are placed in the detection path.
The SH signal is detected by single photon counters based on avalanche photodiode
modules. A Glan-Thomson-prism is used for polarization analysis. Undoped ridge on
X- and Y-cut LiNbO3has been investigated using nonlinear CLSM in this work. Figure
4.14 presents images of the top face of of an undoped ridge of 9 µm width and 2 µm height
on X-cut LiNbO3using linear 4.14(a) and nonlinear 4.14(b) modes. Since the sample
has been slightly etched in HF:HNO3beforehand, a faint periodic edge modulation can
be seen in 4.14(a).
Figures 4.15 and 4.16 represent nonlinear CLSM images of undoped ridge on X-cut
and Y-cut LiNbO3respectively. In both figures, strong SH signals are seen periodically
along the ridge, and the period coincides with the duty cycle of the domain pattern.
Signals are also detected from the surface on both sides of the ridge with somewhat
different intensity; it is typically strong on the side where the positive electrode was
originally placed. It is understood that the domain inversion in X(Y)-cut LiNbO3using
a countering comb-like electrode takes place in a way, that starts from the positive side
of the electrode, grows towards the countering electrode, in the meantime expands in
59
Figure 4.14: CLSM images of the top surface of an undoped ridge of 9 µm width and 2
µm height in (a) linear mode and (b) nonlinear mode.
the area beneath the positive electrode [43]. It is also noted that the samples have been
previously etched in HF:HNO3solution. The crystal under the positive electrode often
suffers damage from large amount of charge migration, the damaged crystal is easily
attacked by the etching solution, resulting in rough surface. The strong signal seen from
the surface originally under the positive electrode could be the scattered signal from the
damaged surface.
Figure 4.15: Nonlinear CLSM image of the top surface of a periodically poled undoped
ridge on X-cut LiNbO3.
However, the reason for strong signal detected inside the area where domain inversion
took place is not entirely clear. According to the previous research in Z-cut PPLN,
domain wall on the surface tends to induce less SH signal than the area far from the
wall due to the scattering, whereas the situation becomes opposite when imaging the
domain beneath the surface. What has been seen in X(Y)-cut LiNbO3is completely
different from that in Z-cut PPLN. One possible cause could be the existence of the
domain filaments or fragments embeded under the surface. In that case, the clear
domain wall as we see in 4.4 does not exist, instead a group of small domain walls
surrounding filaments or fragments may induce complex scattering and interference of
the light. This can be seen more clearly in figure 4.17. Figure 4.17(b) shows a nonlinear
CLSM image of scanning the top face of a periodically poled ridge, while the chemical
etched cross section of this ridge is shown in figure 4.17(a). The inverted domains inside
the ridge appear smaller and having more seed-like structures surrouding.
60
Figure 4.16: Nonlinear CLSM image of the top surface of a periodically poled undoped
ridge on Y-cut LiNbO3.
Figure 4.17: Charaterization of a periodically poled undoped ridge on X-cut LiNbO3us-
ing (a) nonlinear CLSM and (b) selective chemical etching. Graph (b) represents the
etched cross section whose position is indicated as a dashed line in graph (a).
One advantage of using a nonlinear CLSM is that the inner structure of the material
can be studied undestructively via 3-D scanning. In figure 4.18, images are taken in
the Y-Z plane at different depth from the top of the ridge down to the substrate. The
evolution of the SH signal inside the ridge overall decreases; the region of high intensity
SH signal slightly shrinks along the depth and the intensity decreases faster in this
region. Figure 4.19 scanning in the X-Y plane along the central line of the ridge shows a
similar behavior as in figure 4.18. Both figures indicate that the domain inversion region
is as deep as ∼4µm which is consistent with the results discussed in the last section.
61
Figure 4.18: Nonlinear CLSM image of a periodically poled undoped ridge on X-cut
LiNbO3at different depth.
Figure 4.19: (a) Depth resolved nonlinear CLSM image of a periodically poled undoped
ridge on X-cut LiNbO3scanned along the plane sketched in (b).
4.3 Summary
In this chapter, ridge guides and ferroelectric domains have been characterized.
Ridge guides are characterized in terms of waveguide propagation loss and mode
size in both TE and TM polarizations. The propagation loss (mainly scattering loss)
varies with the geometric design of the ridge and the thickness of Ti layer. In general,
propagation loss in TE polarization is higher than in TM polarization; within the top
width of 5 - 9 µm, the propagation loss decreases with the increase of the top width.
Ridge guides, which have been used in this work to demonstrate nonlinear ineteraction,
have the propagation loss of 0.7 - 1.2 dB/cm. The mode sizes of the ridge guide are
evidently smaller than that of the conventional channel waveguide, typicallly ∼4µm
horizontally and ∼3µm vertically in TE polarization.
Ferroelectric domains are characterized by two different methods: selective chemical
etching and imaging; nonlinear CLSM imaging. Selective chemical etching is performed
62
on the surface of the sample as well as the cross section of the ridge after cutting through
the ridge and polishing. The result reveals that a successful poling in the ridge on X-cut
LiNbO3induces a hexagonal domain shape which agrees with the common knowledge of
LiNbO3ferroelectric domains. The inverted domain is as deep as up to ∼5µm, sufficient
to overlap the transmitted mode inside the ridge guide. The duty cycle of the periodic
domains can be controlled by optimizing the duty cycle of the comb-like electrodes
together with the duration and voltae of the voltage pulse(s) applied during poling.
Nonlinear CLSM imaging, as a powerful nondestructive method, has been applied to
investigate some of our ridge guides. The domain profile on the surface as well as in
the depth are imaged using this method. The results agree with that obtained from the
selective chemical etching. Although a solid explanation of why the inverted domain
induces higher signal has not been reached, the existence of filaments in the inversion
area could be one reasonable speculation.
63
Chapter 5
Nonlinear optical interactions
Nonlinear optical interactions, specifically SHG and cascaded SHG/DFG, are ivestigated
using a periodically poled Ti in-diffused ridge guide on X-cut LiNbO3. The experimental
setups are presented. The tuning characteristics and power characteristics of SHG are
discussed in section 1. The experimental results of cascaded SHG/DFG are discussed
in section 2.
5.1 Second harmonic generation (SHG)
SHG is investigated using periodically poled Ti in-diffused ridge waveguides on X-cut
LN of different geometric dimensions (top width, ridge height and length). The period
of the periodically poled section is 16.2 µm and 16.6 µm respectively. The two different
periodicities combined with temperature tuning allow to vary the phase matching wave-
length up to 1560 nm. The end faces of the waveguide have been carefully polished in
order to couple the light efficiently. The measurement setup is sketched in figure 5.1. An
external cavity laser (ECL) is used to tune the fundamental wavelength λfin steps of 1
pm around λf=1550 nm. A polarization controller is used to adjust the polarization of
the beam. The interaction is designed for converting a fundamental wave of TE polar-
ization into a SH wave of TE polarization. An erbium doped fiber amplifier (EDFA) is
then used to amplify the optical power of the fundamental wave. In this way, the tuning
characteristics can also be measured at high fundamental power. The fundamental wave
is coupled to the waveguide by fiber butt coupling with some index matching oil between
fiber and waveguide end face. In this way the reflectivity of the front face was reduced
to ∼3.6 % from ∼14 %, determined by the index step between LN and air. The light is
coupled out from the waveguide by either free space coupling using a focusing lens or by
fiber butt coupling. A Silicon photodiode is used to measure the generated SH power.
The experiments are operated at room temperature; a temperature controller is used to
keep the sample at 25 ◦C.
Figure 5.2 presents a typical SHG tuning characteristic as generated second harmonic
power PSH versus λf=∼1548 nm together with the transmitted fundamental power Pf.
The ridge waveguide is 13 mm long and has a domain period of 16.6 µm, and it has a
64
Figure 5.1: Sketch of a setup for SHG characterization. ECL: external cavity laser. PC:
polarization controller. EDFA: erbium doped fiber amplifier. PD: photodiode.
cross section of 6.5 µm width and 3.5 µm height. Fabry-Perot resonances of both waves
are observed. The resonances are induced by the cavity with approximately 3.6 % and 14
% mirror reflectivities. Similar to the method used in section 4.1.1, the resonances allow
evaluating precisely the propagation losses of 1.0 dB/cm for this waveguide at 1550 nm.
The intra-cavity modulated fundamental power in turn modulates the generation of the
second harmonic power. Due to the nonlinear interaction the corresponding resonances
become narrower; moreover, they are also modified by Fabry-Perot resonances at the
second harmonic wavelength λSH (see inset of the figure 5.2). Fitting a theoretical SHG
tuning characteristic (red curve in figure 5.2) to the average of the experimental result
yields a good agreement of theoretical and measured bandwidth of 0.8 nm demonstrating
the excellent homogeneity of the periodically poled waveguide along the interaction
length.
Figure 5.2: Generated SH and transmitted fundamental powers as a function of the
fundamental wavelength for a 14 mm long, periodically poled ridge waveguide on X-cut
LN. Inset: Results around the maximum efficiency plotted with higher resolution.
SHG was also investigated as function of the fundamental power up to ∼700 mW;
the tuning characteristics at different coupled fundamental power levels are shown in
figure 5.3. With increasing fundamental power, the phase matching wavelength (the
peak of the tuning curve) shifts slightly to shorter wavelength, and the tuning charac-
65
teristic becomes asymmetric. This could be due to the influence of the photorefractive
effect at high fundamental power. As the scanning is from shorter wavelength to longer
wavelength, the increase of the effective index induced by photorefraction leads to the
blue-shift of the phase matching wavelength.
Figure 5.3: Tuning characteristics at different coupled fundamental power levels: SH
power vs. fundamental wavelength.
The power characteristics of this ridge waveguide are measured at different funda-
mental power levels as shown in figure 5.4. At low power levels (see figure 5.4(a)) a
parabolic dependence is observed, well described by an efficiency of 28 % W−1nor-
malized to the coupled fundamental power. It is calculated as the ratio of out-coupled
SH power and the square of in-coupled fundamental power. The efficiency normalized
to coupled fundamental power and the interaction length is 16.5 % W−1cm−2. This
value is ∼50 % increased compared to the efficiency of a conventional Ti in-diffused
channel waveguide. However, the measured efficiency is still lower than the theoretical
conversion efficiency of 36 % W−1cm−2. This could be due to a non-ideal duty cycle of
the domain structure, and larger ridge dimensions (therefore, a larger nonlinear cross
section) than necessary for a maximum SHG. At higher fundamental power levels (see
figure 5.4(b)), SH power first increases parabolically with respect to the fundamental
power up to ∼260 mW. At even higher fundamental power levels, the growth of SH
power appears to be approximately linear with respect to the increasing fundamental
power. This is due to pump depletion and the onset of photorefractive effects. The SH
power up to ∼50 mW is generated by this ridge guide at a fundamental power of ∼700
mW.
In spite of the influence of photorefraction, the high SH power shows a good stability
with time at room temperature, as shown in figure 5.5. In other words, this ridge
waveguide on X-cut LiNbO3has a lower susceptibility to the photorefractive effects than
a conventional channel waveguide. This phenomenon has been first observed in a Z-cut
66
(a) (b)
Figure 5.4: Power characteristics at (a) low fundamental power levels and (b) high
fundamental power levels: SH power vs. fundamental power.
LiNbO3ridge guide by Nishida et. al. [40]. The mechanism of the low susceptibility
of photorefractive effects could be that the electrons generated via photorefraction can
move to the surface of the ridge, then compensated from the ambient environment due
to the unique geometry of the ridge. Therefore, no sustained electric field generated
by the photorefraction exists in the waveguide to induce the change of the refractive
index. Such a high power stability is of special interest for the practical use of PPLN
waveguides. In this way, no complicated heating system is required in the nonlinear
optical devices such as wavelength converter.
Figure 5.5: Generated SH power as a function of time at room tempeature.
The tuning of the SH wavelength with respect to the fundamental wavelength could
be achieved by temperature tuning. Figure 5.6 shows the tuning characteristic at the
67
same coupled fundamental power at different temperature. The phase matching wave-
length shifts towards longer wavelength with increasing temperature. This is due to the
dependence of the refractive index of LiNbO3on the temperature. The actual SH power
at the phase matching wavelength slightly decreases at the higher temperature due to
the degradation of the coupling efficiency.
Figure 5.6: Tuning characteristics at different temperature.
Tuning characteristic of the waveguide in a broader fundamental wavelength range
(1500∼1570 nm) is measured in a ridge guide of 14 mm long, 16.6 µm QPM structure
and a cross section of 8.5 µm width and 3.5 µm height. The tuning characteristic is
shown in figure 5.7. Three phase matching peaks are observed at λf=∼1503, ∼1548
and ∼1560 nm respectively. The SH modes at the peak fundamental wavelengths are
taken by a CCD camera as shown in the insets of figure 5.7. The highest SH power at the
fundamental wavelength λf=∼1548 nm is due to TE00 mode of SH wave generated from
TE00 mode of fundamental wave. The SH mode observed at the fundamental wavelength
λf=∼1560 nm is due to TE10 mode of SH wave generated from TE00 mode of the
fundamental wave. Similarly, the SH mode observed at the fundamental wavelength
λf=∼1503 nm is due to TE20 mode of SH wave generated from TE00 mode of the
fundamental wave. However, the generated SH power of these two modes are much
weaker than the SH power of TE00 mode generated at the phase matching at λf=∼1548
nm.
68
Figure 5.7: SH generation in a broad fundamental wavelength range: SH power vs.
fundamental wavelength.
5.2 Cascaded second harmonic generation and dif-
ference frequency generation (cSHG/DFG)
Wavelength conversion based on a cascaded scheme as discussed in section 2.4 is demon-
strated using a periodically poled Ti in-diffused ridge waveguides on X-cut LN of 13
mm length and 16.6 µm period. The schematic diagram of the experimental setup is
sketched in figure 5.8. The fundamental wave is generated by ECL, then amplified using
an EDFA. The signal wave is supplied by a distributed feedback laser (DFB) at ∼1555.5
nm. Both the fundamental wave and signal wave are coupled into the waveguide via
a circulator and fiber butt coupling. A fiber bragg grating (FBG) is inserted between
DFB and circulator in order to reflect the fundamental wave back to the circulator.
Polarization controllers are used to adjust both fundamental and signal waves to TE
polarization. The spectrum from the waveguide is measured using an optical spectrum
analyser (OSA).
The calculated evolutions of the power levels of the interacting waves are shown in
figure 5.9 as a function of the interaction length (the coupled fundamental power of
69
Figure 5.8: Sketch of a setup for cascaded SHG/DFG characterization. ECL: external
cavity laser. PC: polarization controller. EDFA: erbium doped fiber amplifier. DFB:
distributed feedback laser. OSA: optical spectrum analyser. FBG: fiber bragg grating.
∼200 mW). According to the discussion in section 5.1, the normalized efficiency of SHG
is 16.5 %W−1cm−2. However due to the short length of the sample (13 mm), the overall
conversion efficiency for SHG is 28 %W−1. Because of the low power of a SH wave,
the following DFG can be considered as a non-pump depletion process. In figure 5.9,
the fundamental power decreases with the interaction length due to both propagation
loss and pump depletion in SHG process. The SH power increases parabolically along
the interaction length. The signal power decreases due to the propagation loss. The
generated idler wave first increases quadruply as a function of the interaction length,
then shows a weaker growth due to both the propagation loss and the decrease of signal
power. The conversion efficiency of this cascaded process is −26.5 dB, evaluated as the
generated idler power devided by signal power at the end of the interaction length.
The output of the waveguide is measured in the wavelength range of 1540 - 1557 nm
at the fundamental power of ∼200 mW, as shown in figure 5.10. The fundamental wave
at 1548.5 nm generates SH wave at 774.25 nm. This SH wave and the signal wave at
1555.5 nm then generate the idler wave at 1541.5 nm. A conversion efficiency of −29
dB is obtained from the spectrum. This efficiency is close to the calculated conversion
efficiency in figure 5.9. The generated idler wave also shows a good stability as shown
in figure 5.11, in accordance with the stable SHG discussed in section 5.1.
70
Figure 5.9: Calculated power evolution of fundamental, SH, signal and idler waves in
cSHG/DFG interaction assuming a coupled pump power of ∼200 mW: fundamental
power and SH power are in linear scale on the left vs. interaction length; signal power
and idler power are in logarithm scale on the right vs. interaction length.
Figure 5.10: Measured spectral power of fundamental, signal and idler waves in
cSHGDFG interaction at a coupled pump power of ∼200 mW (Resolution: 0.1 nm):
spectral power density vs. wavelength.
71
Figure 5.11: Generated idler power vs. time at room temperature.
5.3 Summary
In this chapter, nonlinear interactions are investigated using periodically poled Ti in-
diffused ridge waveguides on X-cut LiNbO3at room temperature.
The tuning characteristics and power characteristics of SHG in Ti in-diffused ridge
guides are investigated. A normalized conversion efficiency of 16.5 % W−1cm−2is ob-
tained. This normalized efficiency is 50 % higher than that in a Ti in-diffused conven-
tional channel waveguide. A stable SH power up to ∼50 mW is obtained.
Wavelength conversion based on a cascaded SHG/DFG scheme is also demonstrated
using such a ridge waveguide. A conversion efficiency of −29 dB from a signal wave to
an idler wave is measured when ∼200 mW fundamental power is coupled, close to the
calculated efficiency of −26.5 dB.
Good stability of both the generated SH wave in SHG and the idler wave in cascaded
SHG/DFG at room temperature shows that the Ti in-diffused ridge waveguides on X-cut
LiNbO3have lower susceptibility to the photorefractive effects compared to Ti in-diffused
LiNbO3channel waveguides. This property enables room temperature operation of high
power nonlinear interaction, leading to broad interest in the actual applications.
The conversion efficiency in both processes could be further improved. This will be
achieved by reducing the propagation loss of the waveguide and improving the quality
of the periodical domains especially the duty cycle and using a longer waveguide.
72
73
Chapter 6
Periodically poled LNOI material
platform and photonic wires
As discussed in sections 1.2 and 2.5, LiNbO3waveguides of high refractive index con-
trast are of increasing interest. The high refractive index contrast enables ultra-small
waveguide cross sections below 1 µm2(see figure 6.1 (a)) and bending radii smaller than
10 µm (see figure 6.1 (b)). Therefore, ultra-compact photonic devices and circuits can
be developed. Researchers from CUDOS [2] have blueprinted the next generation of
optical systems built in miniaturised photonic chips of the order of only millimetres in
size (see figure 6.2).
In contrast to photonic wires in Silicon-On-Insulator (SOI) [48], LN offers excellent
electro-optic, acousto-optic, and nonlinear optical properties; moreover, it can be easily
doped with rare-earth ions to become a laser active material. Therefore, LN photonic
wires will enable the development of a wide range of active integrated devices. They
comprise electro-optical modulators, tunable filters, nonlinear wavelength converters,
and amplifiers and (tunable) lasers of different types. Due to high mode intensities even
at moderate optical power levels, devices of a high efficiency can be expected. It is
promising that ultra-compact and highly efficient LiNbO3based photonic chips will be
realized in the near future.
The most promising method of fabricating high index contrast waveguides is to
use ion-slicing and crystal-bonding techniques, also called “smart-cut” technique. The
concept of Lithiun Niobate-On-Insulator (LNOI) (in this work, the insulator is SiO2) and
its fabrication technique are inspired from Silicon-on-Insulator (SOI) technology [49]. In
this work, 3” LNOI wafers fabricated by Hu [50], are used to produce ridge waveguides
of submicrometer dimensions, also called LNOI “photonic wires”. In order to explore
nonlinear interactions in a photonic wire, a periodical domain structure is required to
enable QPM interactions. Two approaches of fabricating periodically poled LNOI thin
films are investigated in this work in collaboration with Hu: (i) direct bonding of PPLN
and (ii) poling of a LNOI thin film. The progress of the fabrication of periodically poled
LNOI (PPLNOI) material platform is presented in the first section of this chapter. In
the second section, the fabrication of PPLNOI photonic wires and the results of SHG
in such photonic wires are presented and discussed.
74
(a) (b)
Figure 6.1: (a) Relation between maximum single mode core size and relative index dif-
ference ∆. (b) Relation between minimum bending radius and relative index difference
∆. ∆ = n12−n22
2n12. The red dots refer to LiNbO3as the core material. The figures are
taken from [1].
Figure 6.2: The next generation of optical systems built in a photonic chip, taken from
the website of CUDOS [2].
75
6.1 PPLNOI material platform
6.1.1 Fabrication of LNOI
The key to fabricate LNOI wafers is the ion-slicing and crystal-bonding process devel-
oped by Hu [3, 50]. The basic fabrication procedure is sketched in Figure 6.3, and it
consists of the following steps:
Figure 6.3: Fabrication of a LNOI wafer: (a) Ion implantation of the LiNbO3wafer
A; (b) SiO2deposition on the LiNbO3wafer B; (c) Crystal bonding and splitting; (d)
Annealing and CMP polishing.
Step 1: Wafer preparation (see figure 6.3 (a) and (b))
Two Z-cut congruent LiNbO3wafers of 0.5 mm thickness are prepared in parallel.
−Z face of wafer A (in figure 6.3 (a)) and +Z face of wafer B (in figure 6.3 (b))
are well polished. Wafer A is then implanted by 250 - 350 keV He+ions with a
dose of 4 ×1016 ions/cm2forming an amorphous layer underneath the surface.
Wafer B, used as a handle wafer, is coated with a SiO2layer by plasma enhanced
chemical vapour deposition (PECVD).
Step 2: Crystal bonding (see figure 6.3 (c))
76
Before crystal bonding, the surfaces of both wafers are carefully cleaned. Two
wafers are then bonded together. The bonded pair of wafers is then annealed at ele-
vated temperatures (165 ◦C and then 190 ◦C) to improve the bonding strength. By
a further annealing procedure at 228 ◦C, a thin LN layer splits along the He+im-
planted layer and remains on the SiO2-LiNbO3substrate.
Step 3: Annealing and Polishing (see figure 6.3 (d))
After crystal bonding, the sample is further annealed at 450 ◦C for 8 hours to
increase the bonding strength. A chemical mechanical polishing (CMP) process is
applied to reduce the surface roughness to below 1 nm and to remove the amor-
phous layer induced by ion implantation. A LNOI wafer of 3 ” diameter fabricated
by Hu is shown in figure 6.4.
Figure 6.4: An optical micrograph of a fabricated LNOI wafer of 3” diameter taken from
[3]
According to the application, various types of LNOI wafers have been fabricated
using the same process but with slight modifications. For example, wafer A could be a
doped LiNbO3or a periodically poled LiNbO3substrate; additional layers such as metal
layers can be added into the stacked structure. These structures are discussed in more
detail in the following section.
6.1.2 Direct bonding of PPLN
As mentioned above, wafer A can be a periodically poled LiNbO3substrate. In this
approach, He+ions are implanted into a PPLN substrate using the same ion energy
and dose as presented in section 6.1.1. Because ion-implantation is not sensitive to
the orientation of the spontaneous polarizations of LiNbO3, a uniform amorphous layer
is formed beneath the surface of the PPLN substrate. The rest of the fabrication is
77
Figure 6.5: Sketch of a PPLN-SiO2-LN thin film.
identical to the process sketched in figure 6.3. Figure 6.5 is a sketch of a PPLN-SiO2-LN
thin film.
In order to fabricate such a periodically poled LNOI thin film, PPLN substrates
of a good quality are required. The basic concept of fabricating of a PPLN substrate
is illustrated in figure 6.6. A thin layer (2 - 3 µm thick) of photoresist is spin-coated
on a bulk LiNbO3substrate (6.6 (a)). The sample is then baked for 30 minutes at
∼90 ◦C. Afterwards, the sample is exposed and developed using either conventional
photolithography or holographic lithography depending on the periodicity (6.6 (b)). A
hard-baking process follows to improve the adhesion between the photoresist and the
substrate surface. The baked sample is then placed inside a customized sample holder in
which a high electric field is applied to the sample to invert the spontaneous polarization
(6.6(c)). The detailed processes such as exposure, hard-baking and poling are different
for fabricating a QPM structure of a large periodicity (generally ≥4µm) and small
periodicity. The poling concepts applied in these two different periodicity regimes are
illustrated schematically in figures 6.7 (a) and (b). The details are discussed in the
following.
Figure 6.6: Fabrication of a PPLN substrate: (a) spin-coating of photoresist; (b) expo-
sure and development of photoresist; (c) periodical poling.
78
Figure 6.7: Two poling schemes: (a) conventional poling (Q= 2×Ps×A); (b) overpoling
(Q > 2×Ps×A). The arrows represent the directions of the spontaneous polarization.
A QPM structure of large periodicity (Λ≥4µm)
To create a large periodicity QPM structure, a conventional photolithography and pol-
ing method is applied. The AZ4533 photoresist is spin-coated on the +Z surface of a
LiNbO3substrate as shown in figure 6.7 (a). After the sample is exposed by a UV radi-
ation through a photomask for ∼60 seconds and subsequently developed in a developing
solvent, a photoresist grating of the desired periodicity is generated. The sample then
undergoes a three-step hard-baking process (100 ◦C, 120 ◦C and 140 ◦C in sequence, 1
hour at each temperature) to progressively improve the adhesion between the photoresist
and the substrate surface. During periodical poling, a positive high voltage is applied
on the +Z face of the sample to overcome the coercive field strength of LiNbO3of ∼21
kV/mm while the −Z face is grounded. The domain nucleations start from the edges of
the electrode on the +Z face, then grow towards the −Z face and simultaneously spread
laterally but at a much lower speed. The poling process is controlled by monitoring the
current flow through the crystal. The applied voltage is reduced gradually to 0 V after
the accumulated charge reaches an empirically determined value to get a 50 % duty
cycle of the domain structure. This calculated charge Qis determined by
Q= 2 ×Ps×A, (6.1)
where Psis the spontaneous polarization of LiNbO3(∼0.72 µC/mm2), Ais the area of
the region where domain inversion is supposed to happen. This poling technique is well
established to fabricate a large periodicity QPM structure. When the required periodi-
city comes down to a few micrometers, the photoresist grating of a sufficient thickness
and duty cycle is difficult to be realized using conventional photolithography due to its
resolution limit. Therefore, the contrast of the electric field strengths between a clear
region and a photoresist covered region is not large enough to invert the spontaneous
polarization only in the clear region. This results in uncontrolled domain inversion in
the whole region.
After poling and carefully cleaning the sample, the PPLN substrate is ready for
ion-implantation and the subsequent processes as discussed in section 6.1.1.
79
A QPM structure of small periodicity (Λ<4µm)
As the conventional poling method (see above) has a limitation in the small perio-
dicity range, the “electric field over-poling” (also called “surface poling”) technique [51]
using a photoresist grating defined by holographic lithography is applied in this work.
The mechanism of electric field over-poling can be understood in the following (see figure
6.7 (b)). A photoresist grating of the desired periodicity is fabricated on the −Z face
of the LiNbO3substrate using holographic lithography (see Appendix A). Afterwards,
the sample undergoes a hard-baking (100 ◦C and 130 ◦C in sequence, 30 minutes at each
temperature) to progressively improve the adhesion between the photoresist and the
substrate surface without seriously deteriorating the photoresist pattern. The sample
is then ready for periodical poling. In electric field over-poling, a positive high voltage
is applied on the unpatterned +Z face to generate an electric field strength somewhat
higher than the coercive field strength of LiNbO3of ∼21 kV/mm, generally 21.5 - 23
kV/mm. A typical poling characteristic in figure 6.8 shows the applied voltage, the
current through the sample and the accumulated charge with respect to the time. The
current starts flowing when the applied voltage ramps up to ∼10.5 kV, indicating the
onset of domain nucleations. Domain nucleations start on the unpatterned +Z face and
grow towards the patterned electrode on the −Z face; simultaneously the domains grow
laterally and start merging with the neighboring domains. After the inverted domains
reach the −Z face, they continue merging underneath the photoresist. The key of over-
poling is to stop the merging of the neighbouring domains in time when small regions
underneath the photoresist remain unpoled. This is achieved by controlling the accumu-
lated charges. The applied voltage is reduced gradually after the accumulated charges
reach a value which is higher than the calculated value Qdetermined from equation
(6.1). Empirically we use 2 ×Q. Therefore, the duty cycle of the periodical domain
grating would be slightly larger than 50%. On the contrary to the literature [51], which
claims that the over-poling can be done with photoresist gratings on either the +Z or
−Z face, we observe the successful over-poling only when the photoresist gratings are
on the −Z face.
QPM structures of periodicities of 1 - 3 µm have been successfully fabricated in
LiNbO3by using this technique. Figure 6.9 presents a PPLN substrate of a periodicity
of 1.7 µm fabricated using this method. This sample is etched in HF:HNO3solution
for a few minutes to reveal the domain patterns. The original −Z face (figure 6.9 (a))
shows clearly a periodical pattern without serious degradation of the duty cycle. The
micrograph of the original −Y face (figure 6.9 (b), cross section) shows clearly how the
domains merge laterally. However, the depth of the unpoled area is generally larger
than ∼3µm, significantly deeper than the thickness of the thin film (<1µm), which
is to be fabricated later by the “smart-cut” process. For a sub-micrometer periodicity,
the quality of the QPM structure (i.e. homogeneity and duty cycle) is degraded due to
the inhomogeneous photoresist pattern and the insufficient thickness of the photoresist
grating (see discussion in Appendix A). Figure 6.10 presents images of two areas of an
etched original −Z face of a PPLN substrate of a periodicity of 800 nm. In figure 6.10
(b), the large area of the surface is homogeneously poled, only some small regions (see
the “islands”) are periodically poled.
80
Figure 6.8: A typical characteristic of an electric field over-poling experiment: voltage
(black curve), current (blue curve) and charge (red curve) vs. time.
(a) (b)
Figure 6.9: Optical micrographs of a PPLN substrate of a periodicity of 1.7 µm after
selective chemical etching: (a) the original −Z face and (b) the original −Y face.
The fabricated PPLN substrate is carefully cleaned. A small part of the substrate is
cut out for the purpose of evaluating the domain pattern. The rest of the substrate is
used for fabricating a LNOI thin film as discussed in section 6.1.1. A periodically poled
LNOI thin film of a periodicity of 3.2 µm is then fabricated, as shown in figure 6.11.
Figure 6.11 (a) shows the original −Z face of the small part of the PPLN substrate after
selective chemical etching; 6.11 (b) shows the surface of the periodically poled LNOI
thin film of the same periodicity.
81
(a) (b)
Figure 6.10: Optical micrographs of two different regions on the original −Z face of a
PPLN substrate of a periodicity of 0.8 µm after selective chemical etching.
(a) (b)
Figure 6.11: Optical micrographs of (a) a PPLN substrate of a periodicity 3.2 µm and
(b) a LNOI thin film of the same periodicity.
6.1.3 Poling of LNOI
The second approach of fabricating a periodically poled LNOI thin film is to directly
invert the spontaneous polarization of the LN thin film. To realize it, both surfaces
(upper and lower) of the LN thin film must have a direct contact with the elctrodes.
This is shown in figure 6.12: a LN thin film is sandwiched between a top electrode and
a thin Ti layer as bottom electrode. The top surface of the thin film is the +Z face. The
top electrode is fabricated using either photolithography or holographic lithography. A
voltage is applied on the top electrode and the bottom Ti layer is connected to the
82
ground. An electric field of the strength higher than 21 kV/mm is generated between
the top and bottom electrodes in order to invert the spontaneous polarization in the
LiNbO3thin film. Because the thickness of the thin film is less than 1 µm, it can be
directly poled by applying a very low voltage (a few tens of volts). The great potential
of this approach is that a high quality top electrode of a sub-micrometer periodicity can
be fabricated using holographic lithography since only a very thin photoresist grating
is required. Due to the small thickness of the film, a high contrast electric field can be
generated to induce the poling.
Figure 6.12: A sketch of a LN-Ti-SiO2-LN structure with a top electrode and poling
scheme.
During poling, the applied voltage and the generated current are monitored. Figure
6.13 presents the measured current as a function of the applied voltage. First, a negative
voltage is applied to the +Z face of the LiNbO3thin film. As the electric field has the
same direction as the spontaneous polarization, we expect no current flowing. However,
with the increase of the voltage, the current starts to rise when the voltage is greater
than 50 V and continues increasing exponentially with the increased voltage. When
the voltage decreases from the 200 V to 0 V, the current decreases correspondingly.
This current we observed in the negative voltage range could be the leakage current.
In the positive voltage range, the current appears when the voltage surpasses ∼25 V.
This voltage is ∼60 percent higher than ∼16 V calculated from the coercive field of
bulk LiNbO3of ∼21 kV/mm. This might be due to the partial oxidation of the Ti
layer beneath LiNbO3thin film during annealing. The oxidation results in an TiO2
layer beneath the LiNbO3thin film. Since TiO2is a dielectric material, the extra
voltage required for poling compensates the voltage drop in this layer. Another possible
explanation is that the coercive field of LiNbO3thin film becomes higher than that of
the bulk LiNbO3during the fabrication processes, for example, due to Li+out-diffusion
during annealing. After this current appears, the material shows an ohmic behavior (see
the following discussion). We suspect that the observed current is not entirely induced
by the domain inversion, because it is higher than the expected current by an order of
magnitude.
Figure 6.14 shows, as an example, the poling characteristic of inverting the sponta-
83
Figure 6.13: The induced current as a function of the applied voltage.
neous polarization of such a thin film. According to the progression of the current, the
poling characteristic can be divided into four different regions. In region I, a current of
∼0.8 µA appears during the ramping of the applied voltage due to the dielectric property
of LiNbO3. The structure of a thin layer of LiNbO3sandwiched between top and bottom
electrodes essentially functions as a capacitor. Given the area of the electrodes and the
thickness of the film, the calculated capacitance of the structure used in this experiment
is ∼100 pF. Theoretically this results in a charging current of 0.5 µA when the voltage
ramps up at a rate of 5 kV/s. In region II, the voltage on the sample is kept constant
and there is no detectable current. In region III, a current up to 3.5 µA appears. The
voltage in this region decreases because of the voltage drop on the series resistor (see
figure 3.9). It is worth to point out that, at which moment this current appears is rather
unpredictable. It varies randomly from sample to sample. The voltage is ramped down
gradually to 0 V in region IV. The ohmic behavior in this region indicates that the
crystal might be damaged due to dielectric breakdown. Therefore, the current observed
in region III might be the leakage current induced from dielectric breakdown and not
the charge migration induced from poling. This leads to the difficulty of controlling the
progress of the domain inversion.
The poling is first done with a homogeneous top electrode as shown in figure 6.12.
The top electrode is fabricated using the lift-off technique (see section 3.2). Domain
inversion is observed after the sample has been selectively etched. Figure 6.15 presents
the domain inversion in two different samples. The domain inverted area appears as a
cluster of small triangle regions. This is surprising since it is well known that LiNbO3do-
84
Figure 6.14: The poling characteristic during the voltage pulse of 26 V: the applied
voltage and measured current vs. time.
mains on the +Z face have a hexagon shape. Because the domain inverted area is much
smaller than the electrode covered area, we speculate that the domain inversion pro-
ceeded for a short time but was automatically terminated after the dielectric breakdown
set in. A similar experiment is done with a grating top electrode as shown in figure 6.16
(a). Domain inversion is observed in a small region which contains a few grating fingers
and part of the connecting strip as shown in figure 6.16 (b).
(a) (b)
Figure 6.15: Optical micrographs of the surfaces of poled LN thin films after selective
chemical etching.
After poling is completed, the metallic Ti layer beneath the LN film has to be oxidized
to TiO2in order to avoid absorption losses. This can be done by a high temperature
annealing for many hours until the film becomes transparent. A preliminary experiment
of annealing the sample for 6 hours at 450 ◦C and 600 ◦C subsequently increases the
transmission of the sample from 10% to 40%, indicating partial oxidation of the metallic
Ti layer. The optimum annealing parameters are under investigation.
85
(a) (b)
Figure 6.16: (a) Scheme of periodical poling of a LNOI thin film. (b) Optical micro-
graphs of the domain inverted area after selective chemical etching. The circle in (a)
indicates how the domain inverted region is corresponding to the electrode pattern.
6.2 PPLNOI photonic wires
Fabricating photonic wires in a PPLNOI platform and the first experimental results
of investigating nonlinear interactions in PPLNOI photonic wires are presented and
discussed in this section. This part of work is done in collaboration with Hu.
6.2.1 Fabrication and optical properties
PPLNOI photonic wires are fabricated by plasma etching a PPLNOI thin film. Accord-
ing to our experience in Ar milling, the rates of milling a +Z face and a −Z face of
LiNbO3are the same. Photoresist (OIR 907-17) stripes of 1.7 µm thickness and 1 - 7
µm width are used as etch mask, defined by photolithography. The sample was etched
by Ar milling for 60 min in a Plasma100 system (Oxford Instruments). The etching
rate is ∼10 nm per minute. Figure 6.17 (a) shows a SEM micrograph of a photonic wire
of 1 µm top width and ∼730 nm height; figure 6.17 (b) is a sketch of the cross section
of this wire. The dark stripe underneath is the SiO2cladding. The side walls of the
wire have a slope of ∼27◦. Figure 6.18 presents the top views of the fabricated PPLNOI
photonic wires of 9 µm and 3.2 µm periodicity, respectively, observed using an optical
microscope.
The optical mode distribution and the propagation loss are measured using the
methods discussed in section 4.1. The experimental setups are similar to the ones shown
in figures 4.1 and 4.2 with a certain modification in order to couple the light in and out
of the photonic wire efficiently. The objective lens used for coupling the light into the
86
(a) (b)
Figure 6.17: (a) SEM micrograph of a LNOI photonic wire of 1 µm top width taken
from [3]. (b) A sketch of the cross section of the photonic wire.
Figure 6.18: Optical micrographs of PPLNOI photonic wires of 1 µm width and a
periodicity of (a) 9 µm and (b) 3.2 µm.
wire has a 60X magnification (NA: 0.8). An objective lens of a 100X magnification (NA:
0.9) is used to magnify the near field distribution of the guided mode. Figure 6.19 shows
the measured (6.19 (a)) and calculated (6.19 (b)) distribution of the fundamental TM
mode in a photonic wire of 1 µm top width. The measured propagation losses of the
TM mode and TE mode are 9.9 dB/cm and 12.9 dB/cm, respectively.
The effective refractive indices neff of the photonic wire at different wavelengths are
required to determine the periodicity of the domain structures for nonlinear interactions.
Due to the high index contrast of the photonic wires and their small cross section
dimensions, neff of the wire varies considerably as function of the wavelength between
the bulk index of LN and that of SiO2.neff for a fundamental mode of TM polarization
in the photonic wire discussed above was calculated as function of wavelength using a
FDTD solver (Lumerical Inc.), as shown in figure 6.20. The group index ngis measured
using the method described in [52, 53, 54]. The measured and calcuated group indices
as shown in figure 6.20 are in good agreement.
87
(a) (b)
Figure 6.19: Measured (a) and calculated (b) intensity distribution of the fundamental
TE mode in a photonic wire of 1 µm top width and 730 nm height, taken from [3].
Figure 6.20: Calculated effective indices neff , calculated and measured group indices
ngfor the fundamental mode of TM polarization in a photonic wire of 1 µm top width
versus the wavelength. The refractive indices of bulk LiNbO3and SiO2are displayed
for comparison. This graph is taken from [3].
6.2.2 Nonlinear interactions in PPLNOI photonic wires
As shown in figure 6.20, the effective index of the photonic wire is significantly different
than the index of bulk LiNbO3. Therefore, the periodicity of a QPM structure required
for a nonlinear interaction is also significantly different. Figure 6.21 presents the calcu-
88
lated periodicities of a QPM structure in a photonic wire required for 1st order type-0
SHG in which fundamental and SH waves are of TM polarization. It is called 1st order
because the 1st harmonic term of the periodically modulated d(z) contributes to the
nonlinear interaction, i.e. m=1 in equation 2.26. The periodicities are calculated for
photonic wires of a height of 730 nm and a width of 1 µm and 7 µm respectively. Only
the nonlinear interaction between TM00 mode of the fundamental wave and TM00 mode
of the SH wave is considered. Due to the small size of the photonic wire, the effective
index of the optical mode in the photonic wire varies significantly in wires of different
geometric dimensions. Therefore, a slight deviation of the periodicity of the periodical
domains will induce a large shift of the phase matching wavelength. For a fundamental
wavelength of 1064 nm, a QPM structure of 3 µm periodicity is required in a photonic
wire of 1 µm width, while a 3.3 µm periodicity is required in a photonic wire of 7µm
width.
Figure 6.21: Periodicity of the QPM structure for 1st order SHG in photonic wires of a
height of 730 nm and a width of 1 µm (red curve) and 7 µm (black curve) as function
of fundamental wavelength. Fundamental and SH waves are in TM polarization.
Wavelength-tuning curves of SHG in periodically poled photonic wires of different
lengths and periodicities are calculated and shown in figure 6.22 as examples. The
photonic wires have a height of 730 nm and a width of 7 µm. The calculations are based
on the assumption of lossless waveguides and no pump depletion. For an interaction
length of 1.5 mm (i.e. the length of the periodically poled section), the phase matching
wavelength in a photonic wire of 3.2 µm shifts by 24 nm towards the shorter wavelength
compared to that in a photonic wire of 3.3 µm periodicity. The bandwidth of the
wavelength tuning is 1.6 nm. SHG in a photonic wire of 0.1 mm interaction length and
3.2 µm periodicity has a lower efficiency, however, a broader bandwidth of 24 nm due
to the short interaction length (see discussions in section 2.3).
89
Figure 6.22: Calculated wavelength-tuning curves of SHG in a photonic wire of a height
of 730 nm and a width of 7µm: Generated SH power as function of fundamental wave-
length. Input pump power is 1 mW. The periodicity and the interaction length of the
photonic wire are 3.2 µm and 1.5 mm (red curve), 3.3 µm and 1.5 mm (black curve),
and 3.2 µm and 0.1 mm (blue curve, 100 times of the actual values) respectively.
SHG in a photonic wire of 9 µm periodicity
In the first attempt to demonstrate nonlinear interactions in photonic wires, a peri-
odically poled LNOI photonic wire of 730 nm height, 1 µm width and 9 µm periodicity
is used. The periodically poled sections is ∼100 µm long.
As discussed above, the 1st order SHG in such a photonic wire requires a periodicity
of 3 µm; 9 µm periodical domains enable 3rd order SHG (i.e. m=3 in equation 2.26).
The fundamental wave is provided by a diode laser at a wavelength of 1064 nm. A
photomultiplier tube which is sensitive in the wavelength range of 300 - 650 nm is used
to measure the generated SH power. The dependence of the generated SH power on the
input fundamental power is shown in figure 6.23 (a); a parabolic dependence is observed.
The mode distributions of the fundamental and SH modes are measured as shown in
figure 6.23 (b) and (c) respectively.
SHG in a photonic wire of 3.2 µm periodicity
1st order SHG is demonstrated in a periodically poled photonic wire of 730 nm height,
7µm width, 3.2 µm periodicity and 1.5 mm interaction length. As it is discussed in
figure 6.22, the fundamental wavelength of 1064 nm is far outside the tuning range where
efficient SHG can take place. Nevertheless, SHG is observed and the optical modes and
90
Figure 6.23: (a) Measured (dots) and fitted (line) SH power versus input fundamental
power (measured in front of the coupling lens) in a PPLN photonic wire of 1 µm top
width and 9 µm periodicity. Measured mode distributions at (b) fundamental and (c)
SH wavelengths. This graph is taken from [4].
the powers of fundamental wave and generated SH wave are measured as shown in figure
6.24. A parabolic dependence of the generated SH power versus the fundamental power
is observed. The mode distributions of the fundamental and SH modes, however, appear
as overlappings of multiple optical modes.
Figure 6.24: (Measured (dots) and fitted (line) SH power versus input fundamental
power (measured in front of the coupling lens) in a PPLN photonic wire of 7 µm top
width and 3.2 µm periodicity. Measured mode distributions at (b) fundamental and (c)
SH wavelengths.
91
6.3 Summary
In this chapter, the recent progress of fabricating a periodically poled LNOI thin film
and photonic wire and nonlinear interactions in such photonic wires are presented.
A LNOI thin film is fabricated using ion-slicing and crystal bonding (i.e. “smart-
cut”) technique. A high energy ion-implantation leaves a damaged layer ∼1µm beneath
the crystal surface. After carefully designed bonding and annealing steps, a thin layer
of LiNbO3is left on top of a SiO2-on-LiNbO3substrate, resulting in a LiNbO3-on-
Insulator thin film structure. A photonic wire is then fabricated by using ICP etching.
The photonic wire has a high refractive index contrast with respect to the surrounding
media and typical sub-micrometer dimensions (1 µm×730 nm in this work). This gives
rise to an enhanced light confinement and enables a reduced bending radius; therefore,
it is a very promising material platform enabling an ultra-dense optical integration and
highly efficient nonlinear interactions.
To enable efficient nonlinear interactions in a LNOI photonic wire, QPM structures
of a suitable periodicity are required. Two methods are investigated to fabricate a peri-
odically poled LNOI thin film. The first method is to directly bond an ion-implanted
PPLN substrate onto a SiO2-on-LiNbO3substrate. The PPLN substrates are fabri-
cated using either conventional lithography and electric field poling (for a periodicity
typically ≥4µm) or holographic lithography (for a smaller periodicity) and electric field
over-poling. Periodically poled LNOI thin films of a periodicity of 9 µm and 3.2 µm
are successfully fabricated. The second method is to add a metal layer between the
LiNbO3thin film and the SiO2layer serving as a bottom electrode, then to directly
invert the spontaneous polarization of the LiNbO3thin film by applying an electric field
between the lithographically defined top electrode and the bottom electrode. Domain
inversion has been observed on the LiNbO3thin films; however, due to a dielectric
breakdown, the electrode covered regions were not completely poled. The challenge of
this method is to avoid dielectric breakdown during poling. This includes developing
methods to recover the crystal defects induced by ion implantation, and improving the
quality of the top and bottom electrodes as well as the contact between LiNbO3and
the electrode material.
1st and 3rd order nonlinear interactions including SHG and SFG are demonstrated
in periodically poled photonic wires of different parameters. Fof SHG in photonic wires
of 3.2 µm and 9 µm periodicity, the parabolic dependences of the generated SH power
vs. the fundamental power are observed and the mode distributions of both waves
are measured. The low conversion efficiency is due to several factors including a high
waveguide loss, a short interaction length and an unsuitable domain period. Further
work is to improve the quality of the photonic wire and QPM structures as well as to
precisely define the domain period.
92
93
Chapter 7
Conclusions and Outlook
7.1 Conclusions
The motivation of this research is to develop LiNbO3integrated nonlinear optical devices
which have an enhanced nonlinear efficiency and enable nonlinear interactions such as
counter-propagating nonlinear interactions. Novel LiNbO3waveguides of an enhanced
light confinement and QPM structures are essential for developing such a device. We
focused our efforts on two promising types of such waveguides: PPLN ridge waveguides
and LNOI photonic wires. The technologies of fabricating periodically poled ridge wave-
guides and photonic wires are developed and nonlinear interactions in the waveguides
are investigated. The main accomplishments are summarized in the following.
PPLN ridge waveguides on X(Y)-cut LiNbO3substrates
Ridge waveguides on a X-cut LiNbO3substrate are fabricated using conventional
lithography and plasma etching followed by Ti in-diffusion. Electric field assisted local
periodical poling is then applied. An electric field is applied in the body of the ridge
to periodically reverse the spontaneous polarization of the crystal. Compared to the
domain inversion of a bulk LiNbO3substrate, a very low voltage of a few hundred volts
is sufficient for local periodical poling.
Single mode transmission at 1550 nm wavelength is observed in the ridge guides of a
top width of 5 - 8 µm. The propagation losses in the ridge guides decrease with the in-
crease of the top width. Ridge guides, which have been used in this work to demonstrate
nonlinear interactions, have propagation losses of 0.7 - 1.2 dB/cm in TE polarization.
The mode sizes of the ridge guide are evidently smaller than that of the conventional
channel waveguide, typically ∼4µm in the horizontal direction and ∼3µm in the verti-
cal direction in TE polarization. Ferroelectric domains are characterized by two different
methods: selective chemical etching and nonlinear CLSM imaging. Selective chemical
etching reveals that a successful poling in the ridge on X-cut LiNbO3induces a hexa-
gonal domain shape which agrees with the common knowledge of LiNbO3ferroelectric
domains. The inverted domain is as deep as up to ∼5µm, sufficient to overlap the
transmitted mode inside the ridge guide. The duty cycle of the periodic domains can
94
be controlled by optimizing the duty cycle of the comb-like electrodes together with the
duration and voltage of the voltage pulse(s) applied during poling.
Nonlinear interactions are investigated using periodically poled Ti in-diffused ridge
waveguides on X-cut LiNbO3at room temperature. A normalized SHG conversion
efficiency of 16.5 % W−1cm−2is obtained at a fundamental wavelength of ∼1550 nm.
It is 50 % higher than that in a Ti in-diffused conventional channel waveguide. A stable
SH power up to ∼50 mW is generated at room temperature. Wavelength conversion
using a cascaded SHG/DFG scheme is also demonstrated using such a ridge waveguide.
A conversion efficiency of −29 dB from a signal wave to an idler wave is measured when
∼200 mW fundamental power is coupled. Good stability of both the generated SH
wave in SHG and the idler wave in cascaded SHG/DFG at room temperature shows
that the Ti in-diffused ridge waveguides on X-cut LiNbO3are much less sensitive to the
photorefractive effects compared to Ti in-diffused LiNbO3channel waveguides.
Periodically poled LNOI photonic wires
A LNOI thin film is fabricated using ion-slicing and crystal-bonding (“smart-cut”)
technique. Photonic wires are fabricated in a LNOI thin film by using plasma etching.
LNOI photonic wires have a high refractive index contrast with respect to the surround-
ing media and typical sub-micrometer dimensions (1 µm×730 nm in this work). The
propagation losses in a photonic wire of 1 µm width and 0.7 µm height are 9.9 dB/cm
and 12.9 dB/cm in TM and TE polarization, respectively.
In order to explore nonlinear interactions in LNOI photonic wires, periodical do-
main structures in the wires are required. Two different approaches are developed
in this work. In the first approach, an ion-implanted PPLN wafer is bonded onto a
SiO2-on-LiNbO3substrate. The PPLN wafers are fabricated using either conventional
lithography and electric field poling (for a periodicity typically ≥4µm) or holographic
lithography and electric field over-poling (for a smaller periodicity), respectively. Peri-
odically poled LNOI thin films of a periodicity of 9 µm and 3.2 µm are successfully fab-
ricated. In the second approach, a thin layer of Ti is deposited between the LiNbO3thin
film and the SiO2layer serving as a bottom electrode. Electric field assisted poling is
then applied directly to the LiNbO3thin film by applying an electric field between the
lithographically defined top electrode and the bottom electrode. Triangular inverted
domains are observed on the LiNbO3thin films after selective chemical etching.
1st and 3rd order SHG are investigated in PPLNOI photonic wires of 3.2 µm and 9
µm periodicity, respectively. The photonic wires are fabricated using the first approach
of direct bonding of PPLN. A parabolic dependence of the generated SH power vs. the
fundamental power is observed. However, the conversion efficiency is low due to high
propagation losses, short interaction length and an unsuitable domain periodicity.
95
7.2 Outlook
Despite the accomplishments of this research, the great potentials of LiNbO3ridge type
waveguides especially of LNOI photonic wires are not yet fully demonstrated. Efforts
must be continued in optimizing the fabrication techniques to enhance the nonlinear
efficiency to a higher level, and investigating new techniques of fabricating high quality
QPM structures of sub-micrometer periodicity. These not only increase the nonlinear
efficiency tremendously, but also enable a broad range of interactions such as counter-
propagating interactions. Therefore, highly efficient, ultra-compact and versatile inte-
grated nonlinear optical devices can be realized.
PPLN ridge waveguides on X(Y)-cut LiNbO3substrates
The normalized conversion efficiency of SHG in a Ti in-diffused ridge waveguide
(16.5 % W−1cm−2) is significantly lower than the theoretically predicted value (36
%W−1cm−2). A further improvement of the conversion efficiency can be achieved
by optimizing the dimensions of the ridge and the duty cycle of the periodical domains.
For example, the nonlinear cross section Seff of the ridge guides in this work is ∼40
µm2. This value can be reduced to 30 µm2by using a Ti in-diffused ridge guide of 5
µm width (Ti thickness: 100 nm). The duty cycle of the periodical domains can be
improved by improving the electrode quality, mainly the contact between the electrode
material and the crystal and the symmetry of the counter electrodes with respect to
the centerline of the ridge. We are confident that by improving these two aspects, a
conversion efficiceny close to the theoretical prediction can be achieved.
Another attractive property of such a ridge guide is its lower susceptibility to the
photorefractive effects. It enables room temperature operation of high power non-
linear interaction which generally is achieved by using expensive doped LiNbO3crystals
(MgO:LiNbO3or ZnO:LiNbO3). The characterization of SHG and cSHG/DFG at a
high pump power level in a long ridge waveguide is to be studied in more detail. Besides
SHG and cSHG/DFG, nonlinear interactions such as sum-frequency-generation (SFG),
optical-parametric-scillation (OPO), can also be realized in a Ti in-diffused ridge guide.
Packaged devieces can be designed without a complicated temperature control system
as well as other complications introduced by high temperature operation.
Periodically poled LNOI photonic wires
LNOI photonic wires have a high refractive index contrast as discussed. This en-
ables also a small bending radius (∼10 µm), which facilitates a high density integration
of multiple functions in such a platform. Concerning nonlinear interactions, the high
index contrast and the small cross section of the waveguide result in an extremely small
mode size and excellent overlap between different modes, leading to an extremely high
nonlinear efficiency in theory.
However, the fabricated photonic wires typically have high propagation losses (≥∼10
dB/cm) and short interactions lengths (a few milimeters). This limits the nonlinear
96
efficiency significantly. Possible solutions are to improve the quality of the crystal-
bonding (i.e. cleanliness of the bonding surfaces, low pin-hole deposition of SiO2, etc.)
and to reduce the surface roughness by the means of polishing and well controlled plasma
etching.
In terms of fabricating periodically poled photonic wires, challenges exist in both
approaches. In the 1st approach of direct bonding a PPLN substrate, the quality of
the resulting photonic wire is mainly limited by the quality of crystal-bonding. For
an efficient nonlinear interaction to take place, one has to precisely define the domain
periodicity. This is especially critical in photonic wires because the required periodicity
varies significantly with respect to the geometric dimensions of the waveguides. For this
purpose, a computer controlled goniometer with a precise angle control is required in
Lloyd holography setup. Another challenge is to fabricate high quality sub-micrometer
periodical domain structures in a bulk LiNbO3substrate because a high contrast of the
electric field strength between the electrode covered region and uncovered region is diffi-
cult to achieve using photoresist gratings. One proposal is to use SiO2gratings instead
of photoresist gratings as the poling mask in over-poling [55]. It is highly promising
that a high nonlinear efficiency close to the theoretical prediction can be achieved by
improving the quality of both photonic wire and domain structures.
In the 2nd approach of directly inverting the spontaneous polarization of a LiNbO3
thin film, the main challenge is to prevent the material from dielectric breakdown which
inhibits the lateral progression of the domain inversion. We suspect that such a di-
electric breakdown took place in the defect sites in the crystal. It is known that high
energy ion-implantation tends to induce such defects into the material. The possible
solution is to recover the crystal defects induced by ion-implantation as much as pos-
sible. Meanwhile, improving the quality of the top and bottom electrodes as well as
the contact between LiNbO3and the electrode material enables domain nucleation to
take place homogeneously under the electrode, therefore, domain inversion in a larger
area can be realized. Moreover, using this approach, a high quality top electrode of a
sub-micrometer periodicity can be fabricated using holographic lithography since only a
very thin layer of photoresist grating is required. Due to the sub-micrometer thickness of
the film, a high contrast electric field can be generated to induce the poling. Therefore,
a LiNbO3waveguide of a submicron periodicity can be finally realized. This will enable
a new class of nonlinear processes: counter-propagating nonlinear interactions as, for
example, in a mirrorless optical parametric oscillator (OPO).
Although the concept of a mirrorless OPO using the QPM technique was proposed
first in 1996 [56], it was not experimentally demonstrated until 2007 by Canalias et.
al. [57] in a bulk PPKTP crystal. Fabricating a mirrorless OPO in a LiNbO3wave-
guide poses much higher challenges compared to in a bulk PPKTP crystal, mainly due
to the difficulties of fabricating high quality submicron periodical domain structures in
LiNbO3as previously discussed. Nevertheless, we would like to point out the advan-
tages of such a device to encourage upcoming research in this area. Taking an OPO
for the mid infrared spectral range (2.5 µm - 3.5 µm) as an example, the momentum
conservation conditions and tuning characteristics of the counter-propagating (mirror-
less) and conventional co-propagating OPOs are compared theoretically in figure 7.1.
97
The OPO is pumped at a wavelength around 1.55 µm. This allows the use of available
high performance pump sources (DFB-lasers, tunable extended cavity semiconductor
lasers, Erbium-doped fiber amplifiers). The required domain periodicity for the 1st or-
der counter-propagating interaction is ∼720 nm. In a counter-propagating scheme, the
signal and idler waves propagate in opposite directions. This puts much stronger con-
straints on the momentum mismatch and gives rise to a special tuning behavior of the
counter-propagating idler wave [57]. The bandwidth of the idler wave (red curve) is two
orders of magnitude narrower than that of the co-propagating OPO, leading to possibil-
ities of tuning a narrowband mid-infrared idler wave with high precision. This feature is
of special interest also in quantum information processing using, for example, parametric
down-conversion [37]. The other unique feature of the counter-propagating interaction is
the inherent feedback of the idler wave, which means, no alignment, external mirrors or
surface coatings are required to reflect the beam, hence, “mirrorless”. This will simplify
an OPO system design and enhance the stability of the device significantly.
Figure 7.1: (a) Schemes of an OPO : co-propagating and counter-propagating. β: pro-
pagation constant; K: wave vector associated to periodical domain structures; Λ: pe-
riodicity of the domain tructures. (b) Calculated tuning characteristics for counter-
propagating and conventional co-propagating OPOs. The red and blue curves represent
the backward coupled idler wave and the forward coupled signal wave in the counter-
propagating OPO; The black dashed curve represents the signal/idler wave in the co-
propagating OPO.
In this thesis, we developed different techniques such as local poling, electric field
over-poling assisted with holographic lithography, which are promising to achieve a small
periodicity. Moreover, our ongoing progress in LNOI thin films certainly opens a new
chapter of submicron domain engineering with great promises. We believe that, with
further optimized techniques, high efficiency nonlinear interactions as well as counter-
propagating interactions in a LiNbO3waveguide (photonic wire) can become a reality.
98
99
Appendix A
Holographic lithography
A simple Lloyd interferometric optical holography setup is used to fabricate submicron
photoresist gratings on glass and LiNbO3substrates. The experimental setup and its
schematic structure are shown in figure A.1 (a) and (b), respectively. An Ar-Ion laser
(Innova 90C-6, Coherent Inc.) is used to provide a laser beam of an output power up to
2 W at λ= 488nm. Before the beam reaches the sample, it is expanded and collimated
using Keplerian telescope scheme, which consists of a focal lens and a collimating lens.
The rear focal point of the focal lens coincides with the front focal point of the collimating
lens. A pinhole (φ= 8µm) as a spatial filter is placed at the mutual focal point of these
two lenses to remove aberrations in the beam. A rectangular aperture (20 mm X 5 mm)
is placed behind the collimating lens to form a rectangular beam. The average intensity
of the rectangular beam is in the range of 100 - 300 mW/cm2.
The interference fringe pattern required for grating fabrication is generated using
a Lloyd-type interferometer as shown in figure A.2 ((a) schematic diagram and (b)
photograph of the experimental setup). The mirror and the sample holder are mounted
on a rotational sample stage, and the mirror is oriented perpendicular to the surface of
the sample. The sample stage is adjusted with respect to the laser beam such that the
lower half of the beam reaches the sample directly and the upper half of the beam is
reflected by a mirror and then reaches the sample. Due to the reflection, the phases of
the wavefronts of the upper half of the beam changes slightly with respect to the lower
half. As a result, when the directly incident beam and the reflected beam meet on the
surface of the sample, the constructive and distructive interferences create a sinusoidal
intensity pattern of a peiodicity Λ. By rotating the sample stage, we can easily change
the incident angle of the laser beam onto the sample and thus the periodicity Λ of the
interference patterns, which is determined by:
Λ = λ
2 sin θ,(A.1)
where λis the wavelength of the laser beam, θis the incident angle to the sample.
Figure A.3 explains how the sinusoidal intensity pattern of the illumination (figure
A.3 (a) and (b)) is transferred to a photoresist grating of a rectangular profile (figure A.3
(c)). This works due to a nonlinear behavior of the developing rate of the photoresist in
a diluted solvent with respect to the illumination intensity. There exists such a threshold
100
Figure A.1: Lloyd interferometric optical holography setup: (a) photograph of the setup
and (b) schetch.
Figure A.2: Lloyd interferometer: (a) schematic diagram and (b)photograph of the
sample stage.
intensity, below which the illuminated photoresist can not be developed in the solvent.
As a consequence, the photoresist, which is illuminated with the intensity below the
threshold will remain after developing; hence, a rectangular profile of the photoresist
grating is obtained as shown in figure A.3 (c). The threshold intensity depends on the
chemical composition of the photoresist and the concentration of the developing solvent.
101
Figure A.3: Scheme of fabricating photoresist grating with rectangular profile: (a) in-
terference of the beams; (b) the profile of the modulated illumination intensity; (c) the
photoresist grating after exposure and development. P: Intensity of the illumination,
Pth: the intensity threshold of the illumination, below which the photoresist can not be
resolved in the developing solvent, T: thickness of the photoresist after development.
To fabricate photoresist gratings of a sub-micrometer periodicity, a thin layer of
photoresist (SX AR-P 3500/6) is spin-coated on the sample and baked for 30 minutes
at 90 ◦C. The sample is then mounted in the sample holder as shown in figure A.2
for exposure. The exposure time is determined by the actual irradiation density (∼700
mW/cm2) and the thickness of the photoresist. Generally the sample is exposed for 10 -
20 seconds. Afterwards, the sample is immersed in a developing solvent. The developing
solvent is often diluted in order to enhance the developing contrast. Photoresist gratings
of a periodicity down to 350nm have been fabricated on glass substrates. Figures A.4
and A.5 display photoresist gratings of 720 nm and 420 nm periodicity, respectively, on
glass substrates.
Fabricating submicron periodical photoresist gratings on a LiNbO3substrate is more
challenging due to the much higher refractive index contrast between LiNbO3(∼2.2) and
photoresist (∼1.6). With such a high index contrast, the light is reflected by the interface
between photoresist and LiNbO3, as well as by the rear surface of the LiNbO3substrate.
The back-reflected light disturbs the interference pattern of the incident light, resulting
in an inhomogeneous interference pattern. Another challenge is to create a photoresist
grating of a thickness larger than 1.5 µm, because the irradiation density decreases with
the depth due to the absorption in the photoresist. This results in either an uncleared
surface (when it is not developed sufficiently) or a decreased height of the remaining
photoresist (when it is developed for a longer time). Both situations are detrimental for
subsequent applications such as periodical poling (see section 6.1.2). Figure A.6 displays
a photoresist grating of 720 nm periodicity on a LiNbO3substrate.
102
Figure A.4: Photoresist graing of 720nm periodicity on a glass substrate.
Figure A.5: Photoresist graing of 420nm periodicity on a glass substrate.
Figure A.6: Photoresist graing of 720nm periodicity on a LiNbO3substrate.
103
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109
Acknowledgment
I would like to thank all the people who have helped me to make this work possible.
First of all, I would like to express my gratitude to Prof. Sohler, who had kindly
accepted me as one of his Ph.D. students in his group nearly four years ago. I was
fortunate to study in an internationally recognized “integrated optics group”led by him.
This thesis work has been benefitted enormously from his guidance helpful discussions
and advices. I have learned from him not only the science but also his passion and
scientific attitude in the research.
This work was funded by the Deutsche Forschungsgemeinschaft (DFG) within the
framework of the project “Materials World Network: Nanoscale Structure and Shaping
of Ferroelectric Domains” in collaboration with four other research groups in Germany
and US. I am thankful to DFG and the people who were involved in our collaboration
and contributed many discussions and suggestions.
This work has been carried out in close collaboration with Dr. Hu. He developed
the techniques of fabricating ridge waveguides and LNOI structure, provided excellent
material platforms which enable novel poling techniques. He also kindly shared with
me a lot of his technological experiences. I thank also Dr. Berth and Mr. Wiedemeier
from Prof. Zrenner’s group for characterizing ferroelectric domains in our samples using
CLSM.
I would like to thank Dr. Suche and Dr. Herrmann for their help in my work. I have
learned a lot from them in many aspects such as domain engineering, designing electric
circuit for poling, characterization and etc.
During my experiments of fabricating waveguides and performing periodical poling,
Raimund Ricken and Viktor Quiring had helped me a lot with the technologies. I thank
them for their continuous support in the cleanroom.
I give special thanks to Mrs. Zimmermann for her invaluable help not only in all the
official works but also in my everyday life in Paderborn. She helped me to deal with all
sorts of problems which a foreigner could encounter here.
I would like to thank also my other friends who I met during these years, Sergey
Orlov, Selim Reza, Ansgar Hellwig, Stephan Krapick, etc. Special thanks go to those
who study with me in the same period of time, Daniel Buechter, Mathew George, Abu
Thomas and Rahman Nouroozi, who have helped me in many academic aspects as well
as in my everyday life.
Special thanks to Miguel who is extremely patient and caring, and companied me
through the difficult moments during my Ph.D. time.
Finally, I want to thank my family, who are far far away and know “nothing” about
physics, but they are always my spiritual support wherever I am.
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