Document [original]

APL Photonics 5, 106101 (2020); https://doi.org/10.1063/5.0014921 5, 106101

Quantum dot single-photon emission

coupled into single-mode fibers with 3D

printed micro-objectives

Cite as: APL Photonics 5, 106101 (2020); https://doi.org/10.1063/5.0014921

Submitted: 22 May 2020 . Accepted: 10 September 2020 . Published Online: 01 October 2020

Lucas Bremer, Ksenia Weber, Sarah Fischbach, Simon Thiele, Marco Schmidt, Arsenty Kaganskiy, Sven Rodt,

Alois Herkommer, Marc Sartison, Simone Luca Portalupi, Peter Michler, Harald Giessen, and

Stephan Reitzenstein

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APL Photonics ARTICLE scitation.org/journal/app

Quantum dot single-photon emission coupled

into single-mode fibers with 3D printed

micro-objectives

Cite as: APL Photon. 5, 106101 (2020); doi: 10.1063/5.0014921

Submitted: 22 May 2020 •Accepted: 10 September 2020 •

Published Online: 1 October 2020

Lucas Bremer,1Ksenia Weber,2Sarah Fischbach,1Simon Thiele,3Marco Schmidt,1Arsenty Kaganskiy,1

Sven Rodt,1Alois Herkommer,3Marc Sartison,4Simone Luca Portalupi,4Peter Michler,4

Harald Giessen,2and Stephan Reitzenstein1,a)

AFFILIATIONS

1Institute of Solid State Physics, Technische Universität Berlin, Berlin, Germany

24th Physics Institute and Research Center SCoPE and Integrated Quantum Science and Technology Center IQST,

University of Stuttgart, Stuttgart, Germany

3Institute for Applied Optics (ITO) and Research Center SCoPE, University of Stuttgart, Stuttgart, Germany

4Institut für Halbleiteroptik und Funktionelle Grenzflächen, Center for Integrated Quantum Science and Technology (IQST)

and Research Center SCoPE, University of Stuttgart, Stuttgart, Germany

a)Author to whom correspondence should be addressed: [email protected]

ABSTRACT

User-friendly single-photon sources with high photon-extraction efficiency are crucial building blocks for photonic quantum applications.

For many of these applications, such as long-distance quantum key distribution, the use of single-mode optical fibers is mandatory, which

leads to stringent requirements regarding the device design and fabrication. We report on the on-chip integration of a quantum dot (QD)

microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The practical quantum device is

realized by the deterministic fabrication of the QD-microlens via in situ electron-beam lithography and the 3D two-photon laser writing

of the on-chip micro-objective and fiber chuck. A QD with a microlens is an efficient single-photon source, whose emission is collimated

by the on-chip micro-objective. A second polymer microlens is located at the end facet of the single-mode fiber and ensures that the

collimated light is efficiently coupled into the fiber core. For this purpose, the fiber is placed in an on-chip fiber chuck, which is pre-

cisely aligned to the QD-microlens thanks to the sub-micrometer processing accuracy of high-resolution two-photon direct laser writing.

The resulting quantum device has a broadband photon extraction efficiency, a single-mode fiber-coupling efficiency of 22%, a measured

single-photon flux of 42 kHz (8.9 kHz) under cw (pulsed) optical excitation, which corresponds to 1.5 MHz (0.3 MHz) at the single-mode

fiber output, and a multi-photon probability in terms of g(2)(0) = 0.00±0.04

0.00 (0.13 ±0.05) under cw (pulsed) optical excitation. The stable

design of the developed fiber-coupled quantum device makes it highly attractive for integration into user-friendly plug-and-play quantum

applications.

(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0014921

., s

INTRODUCTION

The development of real-world quantum communication net-

works1,2 that offer a previously unattained level of data transfer

security3,4 has become very dynamical in recent years. In addi-

tion to the first quantum networks based on optical fibers5,6 and

free-space channels,7,8 expanded solutions in the form of satellite-

based quantum communication networks9,10 have also been devel-

oped. In this context, strongly attenuated pulsed lasers are still

frequently used as the photon sources for decoy-state-based pro-

tocols,11,12 offering the highest transfer rates so far. However,

ultimate performance can only be obtained by using true

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single-photon sources, based, for instance, on semiconductor quan-

tum dots (QDs), which not only promise on-demand operation

but also feature nowadays the purest single-photon emission of all

known quantum light sources.13,14 It is noteworthy that only the

sources with non-classical single-photon statistics can exploit the full

potential of quantum communication.15,16 In addition, the availabil-

ity of the true single-photon sources is crucial for the implementa-

tion of the quantum repeater protocol.1,17 In fact, the on-demand

sources of indistinguishable and entangled photons18–21 are key to

make the dream of long-distance multi-partite networks become a

reality. Here, it is important to ensure that the photons always prop-

agate in the same defined spatio-temporal mode,22 making the use

of single-mode fibers (SMFs) quantum channel indispensable.

The on-chip fiber-coupled sources have the decisive advan-

tage of enabling ultra-stable operation without additional free-beam

optics. This feature reduces the complexity of the source enormously

and, as a result, boosts its flexibility and application relevance. In

the past, multimode fibers23 or SMF bundles24 were used to real-

ize the fiber-coupled single-photon sources. Only recent progress in

nanofabrication techniques has paved the way for the single-mode-

coupling of semiconductor QDs integrated into Fabry–Perot optical

microcavities,25 cylindrical mesas,26 photonic crystal nanobeams,27

or waveguide-based devices.28–30 In the present work, we take a

different approach and want to combine the strength of the QD-

microlenses in terms of extraction efficiency31 with the advantages of

the precise and flexible 3D two-photon direct laser (TPL) patterning

of photonic microstructures.32–36

A significant challenge to be tackled for the realization of effi-

cient coupling of a QD-microlens to a SMF with a core diameter

of only 4.4 μm and a numerical aperture (NA) as low as 0.13 is

that, although the QD-microlenses are mechanically very robust and

provide the broadband enhancement of photon extraction, their

emission is only moderately directed with respect to small NAs.

As a result, a micro-objective design was developed that collects

and collimates the outcoupled radiation of the QD-microlens. For

this purpose, the approach described in Ref. 37 was revised and

extended using ray-tracing calculations with Zemax. In the present

work, we developed a total internal reflection (TIR) microlens, serv-

ing as the light-collection micro-objective, which is printed with

sub-micrometer accuracy35 onto a QD-microlens using 3D fem-

tosecond direct laser writing. The TIR design used here offers two

major advantages over the multi-element objective applied in Ref.

37: First, it collects light from the whole hemisphere above the

quantum dot. This corresponds to a numerical aperture equal to

the refractive index n of the used photopolymer (n ≅1.54). Com-

pared to the best multi element design with an NA of 0.7, this

is an increase of more than a factor of two. Second, the TIR

lens strongly reduces losses at the semiconductor interface as the

photo resist moderates the transition from GaAs to air and the

number of polymer to air interfaces is reduced from four to only

one. A further advantage is that the TIR lens is more compact

and faster to fabricate. The main disadvantages are that the align-

ment needs to be much more precise (<100 nm) in comparison

to that in Ref. 37 (<5μm) and more external strain is induced at

low temperatures. Further details on the lens design can be found

in Ref. 38.

Additionally, similar to Ref. 39, we designed and real-

ized an in situ TPL-printed on-chip fiber chuck aligning the

QD-microlens-system to a coupling lens on the face facet of a

SMF. Lensed fibers have proven to be highly tolerant to displace-

ment along the fiber’s lateral axis in a collimated beam setup with

respect to the coupling efficiency.40 The synergetic combination of

all these components results in a precise, fully integrated, and ultra-

stable micro-optical on-chip photonic system for applications in

fiber-based quantum communication.

DEVICE TECHNOLOGY AND FABRICATION

The sample is based on a wafer heterostructure consisting of

InGaAs QDs grown on a (100) GaAs substrate by metal–organic

chemical vapor deposition. A back-side distributed Bragg reflector

(DBR) consisting of 23 GaAs (67 nm)/Al0.9Ga0.1As (78 nm) λ/4-

thick layer pairs is located underneath the QD layer at a distance

of 67 nm to reflect the light emitted into the lower hemisphere, and

thus, increase the photon-extraction efficiency normal to the sample

surface. The self-assembled QDs are randomly distributed in posi-

tion and wavelength with a center wavelength of around 920 nm.

Above the QD layer, a 420 nm thick capping layer is grown, which

is required for the deterministic structuring of the QD-microlenses

by 3D in situ electron-beam lithography (EBL). With the help of

the numerical optimization of the microlens design, it is possi-

ble to achieve outcoupling efficiencies of almost 30% for an NA

of 0.4.31,41

For the fabrication of the QD-microlenses, a 80 nm thick

electron-sensitive CSAR62 resist film42 is first spin-coated onto the

sample, and then a specially modified electron scanning microscope

is used to record cathodoluminescence maps at T = 10 K. Suitable

QDs are selected based on the emission wavelength and the emis-

sion intensity of the excitonic lines, and a lenticular dose profile is

then introduced into the resist at the positions of selected QDs. In

the later anisotropic, plasma-enhanced reactive ion etching step, the

electron-sensitive resist remaining on the sample surface after expo-

sure and development acts as an etching mask so that the introduced

lens profile is transferred into the semiconductor material, resulting

in the monolithic QD microlenses.31

To realize a high photon coupling efficiency into a SMF, the

radiation pattern of the QD-microlens must be taken into account,

i.e., to maximize the usable photon flux, the emission should be col-

lected from the largest possible solid angle and coupled into the

fiber. We solve this issue by a configuration consisting of two TPL

written polymer micro-optical elements, as illustrated in Fig. 1(a).

A total-internal-reflection micro-objective on the QD-microlens is

used to collimate its divergent emission, while an NA matched (NA

= 0.13) coupling microlens on the single mode fiber is used to focus

the beam down onto the fiber core. One advantage of this opti-

cal system is that it is rather insensitive to the distance between

TIR and coupling lenses, as long as the beam can still be regarded

as collimated. Details on the design of the TPL written lenses, its

optimization, and its relationship to the achievable photolumines-

cence enhancement are given in the supplementary material and

in Ref. 38. For these optimization measurements, high-precision

low temperature deterministic lithography was used43 marking the

emitter for further room temperature fabrication.35,44 Furthermore,

a first characterization of the fiber in-coupling performances was

carried out in a high stability cryostat where the position of the

fiber with respect to the sample can be precisely controlled (see the

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FIG. 1. (a) Optical design for coupling light from a QD-microlens into a single-mode fiber obtained from sequential ray tracing. Light that is emitted from the QD-microlens is

collimated by a TIR lens (output NA = 0.001, see the supplementary material) and focused onto the fiber core by an NA matched (NA = 0.13) spherical focus lens. (b) Scheme

showing the fabrication of the integrated fiber coupled single-photon source. The TIR lens is printed onto the QD-microlens (left), followed by 3D printing the on-chip fiber

chuck and inserting the SMF with the focus lens printed to its end facet (middle). After inserting the fiber, the light from the QD-microlens is efficiently coupled into the fiber

core via the micro-optical system (right). (c) Microscope image of a SMF (THORLABS 780-HP) with an NA matched focus lens printed onto it. (d) Microscope image of an

optical SMF inserted into a fiber chuck via a manual XYZ-flexure stage. (e) Microscope image of a SMF inserted into a fiber chuck and glued to the substrate with UV-cured

glue. Inset: fiber-coupled QD-sample mounted onto a strain-relief copper holder for mounting in a cryostat. (f) Scheme for the second-order autocorrelation measurement.

The fiber-coupled sample is inserted into a liquid helium can and excited via an off-resonant excitation laser. The fiber-coupled QD signal is then spectrally analyzed and sent

to a Hanbury Brown and Twiss setup to obtain the time-correlated signal.

supplementary material). A maximum fiber-coupling efficiency of

26% ±5% was observed. This motivated the fabrication of the fiber

chuck to replicate this performance in a mechanically stable man-

ner. Controlling the reciprocal position further showed that in the

transverse direction, precise alignment with sub-micrometer accu-

racy of all components is required.33,34 The micro-optics are fab-

ricated from IP-Dip photoresist45 with a commercial femtosecond

two-photon 3D laser printer (Photonics Professional GT, Nano-

scribe GmbH). The laser beam is scanned in the lateral direction by

two galvanometric mirrors through a 63×microscope objective in

dip-in configuration (NA = 1.4).33

In the case of the TIR lens, the photoresist is applied onto the

semiconductor sample and the objective is immersed into it, while

for the fiber lens, the photoresist is applied directly onto the objective

and the fiber is inserted into it subsequently. In the axial direction,

the semiconductor sample is moved by a piezoelectric crystal, while

the fiber is left stationary and the objective is moved instead.

The TIR lens is aligned to the QD-microlens using prefabri-

cated alignment markers. As the TIR lens will cover the etched mark-

ers after its fabrication, new 3D printed markers of the same shape

are printed along with the TIR lens. The SMF core is located by shin-

ing light from a red LED onto the other end facet of the fiber, which

leads to the fiber core lighting up in the microscope image for pre-

cisely placing the coupling lens (more details can be found in Ref.

46). The total printing time for the TIR lens was about 5 min. Due

to its substantially larger volume, the coupling lens took about 1 h

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to be produced. After the printing, the structures were developed in

mr-Dev 600 (micro resist technology GmbH) for several minutes.

During this process, the unexposed photoresist is removed. After

pre-characterizing the QD-microlenses with the TIR lenses printed

onto them by micro-PL spectroscopy [see Fig. 2(a)], fiber chucks

are fabricated around the lenses using the 3D printed markers for

alignment. Due to the large size of the fiber chuck, the polymer IP-S,

which is a lower resolution photoresist and an objective with a larger

field-of-view (25×, NA = 0.8) are used in this processing step. The

printing time for the fiber chuck is roughly 45 min. After develop-

ment, the fiber integration is performed. For this, the SMF with the

coupling lens printed on top [shown in Fig. 1(c)] is mounted onto a

FIG. 2. (a) micro-PL spectrum of the QD-microlens before (black) and after (red)

SMF-coupling. The induced compressive strain of the micro-objective causes a

blue-shift of the QD emission lines of about 4 nm. The spectrum of the fiber-

coupled sample is shown with the doubled intensity for better comparability. In

gray, an additional spectrum of the QD-microlens is shown, which was generated

during the optimization of the micro-objective geometry. It reveals that the blue-

shift is reversible by removing the micro-objective. (b) Single-photon rate under

the cw optical excitation of the QD emission line marked in (a). For the right y-

axis, the setup efficiency was taken into account and the photon flux detected by

a SPCM was corrected accordingly. (c) Measured lifetime for the QD line from (a)

(red). The lifetime τ1= (0.92 ±0.03) ns corresponds to the inverse of the slope

of a linear fit to the falling edge of the pulse. A lifetime measurement taken during

the pre-characterization of the QD microlens yields τ1= (0.92 ±0.02) ns and is

shown in blue (horizontally shifted for clarity). The lifetime is not influenced by the

fiber-coupling.

manual XYZ-flexure stage and inserted into the fiber chuck under a

microscope. A mirror angled at 45○is used to monitor the position

of the fiber relative to the fiber chuck opening in all directions.

A sketch of the fiber integration is shown in Figs. 1(b) and a

microscope image of the inserted fiber in Fig. 1(d). The fiber in its

chuck is then fixed to the substrate by depositing a small droplet of

UV-cured glue with another piece of the optical fiber mounted onto

a second XYZ-flexure stage. To avoid the fiber chuck to detach from

the substrate when cooled down to cryogenic temperatures, the UV

glue is spread to cover the entire fiber chuck [see Fig. 1(e)]. The sam-

ple is then mounted onto a copper holder with a built-in strain relief,

as shown in the inset of Fig. 1(e). The substrate is glued to the bot-

tom of the holder with a conductive silver paste and the SMF is fixed

about 1 cm away from it between two pieces of polytetrafluoroethy-

lene (Teflon) with two small screws. This makes the device highly

robust and easy to ship and allows it to be mounted in a specially

designed sample holder located at the lower end of a dip stick. The

dip stick is inserted into a user-friendly standard helium (He) trans-

port vessel so that the fiber-coupled device can be studied with the

setup shown in Fig. 1(f).

RESULTS AND DISCUSSION

In the following, we discuss the optical properties of the QD-

microlens and that of the complete fiber-coupled single-photon

source. To quantify the fraction of QD photons emitted by the

QD-microlens and coupled into the SMF, the micro-PL spec-

tra were recorded before and after processing of the TIR objec-

tive and subsequent fiber coupling. Figure 2(a) shows a micro-

PL spectrum of a QD-microlens before the processing of the TIR

objective (black trace). The setup shown in Fig. 1(f) was used,

except that the sample was placed in a He-flow cryostat instead

of immersing it directly in liquid helium. Furthermore, a micro-

scope objective with an NA of 0.65 was used for the measurement.

In the micro-PL spectrum, the bright trion line at 916 nm is par-

ticularly noticeable besides the neutral excitonic complex around

918 nm. The excitonic states were identified by polarization and

excitation power dependent micro-PL measurements. In addition,

the time-resolved measurements were used to confirm the assign-

ment based on the decay constants of the excitonic states. Due to

the highest micro-PL intensity, the trion line was chosen for the

more detailed investigations in the following, including photon-

autocorrelation measurements to confirm the quantum nature of

emission.

Figure 2(a) shows in red the spectrum of the same QD-

microlens under comparable excitation conditions, i.e., close to sat-

uration, after SMF coupling. Each micro-PL spectrum is normalized

by the setup efficiency (He-flow cryostat configuration: 5.1% ±0.5%,

fiber-coupled configuration: 2.8% ±0.3%) to ensure the comparabil-

ity of the intensities. In addition, the intensity of the fiber-coupled

spectrum has been doubled to increase visibility. It is apparent that

the fiber-coupling of the QD-microlens shifts the QD spectrum to

lower wavelengths by a blue-shift of about 4 nm, comparable to the

previously observed values for the 3D printed hemispheric lenses.35

This blue shift was observed reproducibly for two cooling cycles.

Likewise, a second QD-microlens sample, which was fiber-coupled,

showed a blue shift of 4 nm (see the supplementary material). A

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much smaller shift of 0.2 nm due to the influence of a TPL pat-

terned on-chip micro-objective was already discussed in Ref. 37.

However, in the present work, we use a different design for the

micro-objectives, which, in addition to the on-chip fiber chuck, may

explain the larger blue-shift. In general, we have found that the dif-

ferent geometries of micro-objectives consistently lead to blue shifts

of 0.2 nm–4.0 nm, with a tenuous correlation between the lens diam-

eter and the resulting wavelength shift—the larger the diameter, the

smaller the blue shift. The blue-shift is attributed to the compres-

sive stress of the semiconductor material including the QD caused

by the printed micro-objective.35 The relationship of the blue shift

to the polymer structure on top of the sample is also justified by

the fact that the wavelength shift is reversible, i.e., the QD spec-

trum red-shifts to the original position after the removal of the

micro-objective and the fiber chuck [gray trace in Fig. 2(a)]. This

was observed when optimizing the process flow iteratively, whereby

different objective geometries were printed on a variety of QD-

microlenses whose emission properties were monitored after each

processing step. However, the basic optical properties of the QD

do not seem to be influenced by this strain effect. The linewidth of

<25 μeV is still limited by the resolution of the spectrometer (25 μeV)

and the lifetime [Fig. 2(c)], and the multi-photon probability (Fig. 3)

is not affected.

Besides the blue-shift, it is also noticeable that the continuous

background of the spectrum is significantly reduced by more than

2/3 due to the fiber-coupling, and thus, a clear and background-free

spectrum can be observed from the fiber-coupled QD-microlens.

This is very advantageous and is probably because the single-mode

fiber facet acts effectively as a local pinhole, thereby selecting the

emission of the target QD and suppressing contributions from

FIG. 3. Measured second-order autocorrelation function. The histograms show

single-photon emission with low multi-photon probability of the fiber-coupled

device. (a) Normalized histogram for non-resonant cw excitation. The red solid

line shows an exponential fit of the measured data without the consideration of

the time resolution of the setup (without deconvolution). This results in a value

of g(2)(0)=0.00±0.04

0.00. (b) Photon autocorrelation histogram for non-resonant

pulsed excitation at 80 MHz. The red line represents a fit to the data, assuming a

mono-exponential radiative decay of τ1= 0.92 ns and considering the overall time

resolution of the HBT setup of 495 ps. In this approach, one obtains a value of

g(2)(0) = 0.13 ±0.05.

the wetting layer and possibly other non-intentionally integrated

QDs centered at other positions and wavelengths. Very clean QD

emission spectra are of particular interest for the use of the fiber-

coupled device in stand-alone applications, where for practical rea-

sons, narrow fiber-coupled optical filters are used instead of bulky

and expensive monochromators.26 These filters have usually a higher

bandwidth than the state of the art monochromators, which could

increase multi-photon events.

To determine the coupling efficiency into the single-mode fiber,

which is coupled and glued to the chip, we compare the integrated

spectral area of the trion line before and after the processing. It

should be noted that the fiber-coupling efficiency is based on the

comparison of the trion intensity detected with a microscope objec-

tive with an NA of 0.65 of the pure QD microlens without TIR lens

compared to the full fiber-coupled lens system. To account for the

different NAs under which the light was collected in both mea-

surements, the value for the reference measurement before fiber-

coupling is increased by a factor of 1.17 ±0.01. This correction is

based on the numerical simulations of the QD microlenses,31 which

show that the photon extraction efficiency increases slightly from

22.6% at NA = 0.65% to 26.4% at NA = 1.0. This comparison yields

an in-coupling efficiency from the QD-microlens into the fiber of

22% ±2%, which reflects the excellent performance of the matched

lens geometries despite the non-ideal far-field pattern of the QD-

microlens. If no lens system is used to focus the collimated beam

into the fiber core, a coupling efficiency of <1% would have to be

expected. However, according to our ray tracing model and the

fiber coupling efficiency tool from Zemax/OpticStudio, a coupling

efficiency of 81.8% should theoretically be possible for the present

design. However, it should be noted that the ray tracing model sim-

plifies the situation and, among other things, does not take into

account the non-angular-isotropic radiation characteristic of the

QD-microlens, instead it assumes it as a point source due to its small

diameter compared to the TIR lens. This has a strong influence on

the expected fiber-coupling efficiency, which is overestimated in the

ray tracing simulation since this model does not take into account

the resulting non-ideal mode overlap of the field distribution with

the Gaussian mode of the SM fiber. Furthermore, the dielectric

losses due to reflections at material interfaces were not considered,

which we estimate to amount to at least 24%. It is noticeable that

in addition to the sample shown, another QD-microlens was fiber-

coupled, resulting in a comparable value for the in-coupling effi-

ciency from the QD-microlens into the fiber of 24% ±2% (see the

supplementary material). In addition, in the validation of the cou-

pling system,38 the coupling efficiency of 26% ±5% was measured,

which is a good value for low-dimensional single-mode fiber cou-

pling, even if there is a large discrepancy to the simplified ray tracing

model mentioned above. We expect to reach significantly higher

fiber-coupling efficiencies in the future, e.g., by using, for instance,

near-field coupling in the circular Bragg grating single-photon

sources.47

If the trion line of the spectrum shown in Fig. 2(a) is spectrally

selected with a monochromator and detected with an avalanche-

photodiode-based single-photon counting module (SPCM), we

achieve a count rate of 42 kHz at saturation under cw excitation at

532 nm for this fiber coupled configuration. Considering the over-

all transmission of our setup of 2.8% ±0.3%, this corresponds to a

single-photon flux of 1.5 MHz at the end of the single-mode fiber.

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Figure 2(b) indicates the count rate also for lower excitation pow-

ers. In order to estimate the overall efficiency of the fiber-coupled

device starting with the QD’s emission, pulsed excitation with a rep-

etition frequency of 80 MHz at 800 nm was used, and the photon

fluxes of the relevant excitonic-lines (exciton and two trion states)

were summed up to (12.5 ±0.5) kHz, corrected by the setup effi-

ciency resulting in a photon flux of (0.45 ±0.05) MHz and divided

by the excitation rate (80 MHz). This results in an overall efficiency

of 0.56% ±0.07%. This value could presumably be significantly

increased by optimizing the photon-extraction efficiency of the QD-

microlens used. This becomes clear when the outcoupling efficiency

of the pure QD-microlens is calculated from the fiber-collection effi-

ciency (22%) and the overall efficiency (0.56%). This results in a

value of 3%, which is significantly lower than the previously achieved

values.31 It indicates that the QD is either not integrated in the cen-

ter of the microlens and/or that it suffers from low internal quantum

efficiency.

Figure 2(c) depicts in red a time-resolved measurement of the

lifetime of the trion-transition of the fiber-coupled sample, which

can be fitted using an exponential decay function to quantify the

decay time τ1= (0.92 ±0.03) ns. The result is in quantitative agree-

ment with the value (τ1= (0.92 ±0.02) ns) recorded on the free-space

configuration before fiber-coupling and shows that fiber-coupling

does not influence the lifetime of the QD-microlens.

The emission of the trion line is spectrally filtered by a

monochromator and coupled at the monochromator exit slit into a

fiber-based HBT setup for coincidence measurements [see Fig. 1(f)].

The off-resonant excitation power under cw excitation was chosen

to saturate the trion line [see Fig. 2(b)]. In Fig. 3(a) the measured

second-order photon autocorrelation function is plotted showing

clear antibunching at τ= 0 ns. A physical description of the mea-

surement is possible by a two-sided exponential function,

g(2)(τ)=1−((1−g(2)(0))e−∣τ∣

τsp ). (1)

The fitting of the equation to the measurement data yields g(2)(0)

=0.00±0.04

0.00, which verifies a very low probability of multi-photon

emission events. Noteworthy, the spontaneous decay time of

τsp = (0.93 ±0.05) ns resulting from this fit is in very good agreement

with the lifetime measurement shown in Fig. 2(c).

Furthermore, the quantum nature of emission was also char-

acterized under the more application relevant pulsed excitation. For

this purpose, the wavelength of a tunable titanium–sapphire laser

was set to 865 nm, i.e., resonant with the wetting layer, and the

excitation power was reduced until the detected count rate of the

trion line had halved to reduce the contribution of uncorrelated

background emission, which increases in relative strength with the

excitation power. Figure 3(b) displays the corresponding correlation

histogram.

To determine the g(2)(0)-value, we fitted the experimental data

with a sequence of equidistant photon pulses, each represented by

the convolution of a two-sided exponential decay (τ1= 0.92 ns)

with a Gaussian function of 495 ps width (full width at half maxi-

mum), considering the time resolution of the HBT setup. Assuming

a constant area A of the finite-time-delay pulses, the g(2)(0)-value is

expressed by the ratio A0/A, where A0corresponds to the area of the

zero-time-delay peak. This evaluation results in g(2)(0) = 0.13 ±0.05.

The differences in the g(2)(0)-values under cw and pulsed

excitation can presumably be attributed to charge-carrier recap-

ture processes in the case of pulsed excitation,48 which can no

longer be neglected and, in addition, the autocorrelation of the

laser using the HBT-setup indicated that the laser pulses of lower

intensity were also observed outside the expected time windows

given by the repetition rate. Nevertheless, the device fulfills the

requirements of a fiber-coupled pure single photon source on

demand, whereby the slight increase of multi-photon events in the

case of pulsed excitation compared to cw excitation could cer-

tainly be improved by resonant excitation schemes. The results

of another fiber-coupled sample are given in the supplementary

material. The device shows very comparable performance in terms

of count rates, coupling efficiency, and multi-photon suppression,

which underlines the high-reproducibility of our approach. In fact,

as detailed in the supplementary material, the biexciton emission

shows a measured count rate of 111 kHz (3.9 MHz at the end

of the fiber, fiber-coupling efficiency: 24% ±2%) in saturation,

while the exciton line has a count rate of 48 kHz (corrected: 1.7

MHz) under cw excitation, and a g(2)(0) of 7% ±2% under pulsed

excitation.

CONCLUSION

In summary, we realized a user-friendly single-mode fiber-

coupled single-photon source with excellent optical and quantum

optical properties. The device fabrication benefits from the syn-

ergetic combination of 3D electron-beam lithography and fem-

tosecond 3D direct laser writing, which enables us to join the

advantages of both methods and to realize a robust, single-mode

fiber coupled micro-optical on-chip system based on a single pre-

selected semiconductor QD. Explicitly, a QD-microlens produced

using a 3D in situ technique was combined with a total-internal-

reflection micro-objective using 3D direct laser writing. In addi-

tion, a fiber chuck was written with sub-micrometer alignment

accuracy onto the deterministically fabricated QD-microlens, which

allows a SMF with a 3D printed incoupling lens on its facet to

be inserted and permanently attached to the QD-microlens–micro-

objective assembly. A coupling efficiency of 22% was determined

for the fiber-coupled source, leading to a measured count rate of

42 kHz (8.9 kHz) under cw (pulsed) optical excitation from which

we deduce a maximum single-photon rate of 1.5 MHz (0.3 MHz)

at the output of the fiber by considering the setup efficiency (2.8%).

This technology concept has a high potential to pave the way for

the single-mode coupled stand-alone single-photon sources of a

high emission rate and quantum optical quality in the future, as

demanded with regard to the implementation of scalable quantum

networks.

SUPPLEMENTARY MATERIAL

See the supplementary material for a discussion on the choice

of the solid immersion lens shape on top of single-QD devices and

its implications for improving collection efficiency. In addition, the

optical data are presented on a second fiber-coupled QD microlens

to demonstrate the reproducibility of the developed fiber-coupling

concept.

APL Photon. 5, 106101 (2020); doi: 10.1063/5.0014921 5, 106101-6

APL Photonics ARTICLE scitation.org/journal/app

ACKNOWLEDGMENTS

S.R. acknowledges funding from the German Federal Min-

istry of Education and Research (BMBF) through the Project No.

Q.Link.X, the German Research Foundation via the Project No.

Re2974/10-1, and from the Project Nos. EMPIR, 14IND05, MIQC2,

and SIQUST (the EMPIR initiative is co-funded by the Euro-

pean Union’s Horizon 2020 research and innovation program

and the EMPIR Participating States). H.G., K.W., S.T., and A.H.

acknowledge funding by ERC (AdG ComplexPlas, PoC 3DPrinte-

doptics), BMBF (Q.Link.X, Printoptics, Printfunction), MWK BW

(ZAQuant), BW Stiftung (Opterial) and DFG (Grant Nos. SPP1839

and 1929). We thank the Physikalisch-Technische Bundesanstalt

Berlin for technical support. S.L.P., M.S., and P.M. greatly acknowl-

edge funding from the BMBF (German Federal Ministry of Edu-

cation and Research) via the Project No. Q.Link.X (Grant No.

16KIS0862) and support via the Project No. EMPIR 17FUN06

SIQUST. This project received funding from the EMPIR program

cofinanced by the Participating States and from the European

Union’s Horizon 2020 research and innovation program.

DATA AVAILABILITY

The data that support the findings of this study are available

from the corresponding author upon reasonable request.

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