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
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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.
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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
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