Appl. Phys. Lett. 107, 041105 (2015); https://doi.org/10.1063/1.4927429 107, 041105
© 2015 AIP Publishing LLC.
Single-photon emission at a rate of 143
MHz from a deterministic quantum-dot
microlens triggered by a mode-locked
vertical-external-cavity surface-emitting
laser
Cite as: Appl. Phys. Lett. 107, 041105 (2015); https://doi.org/10.1063/1.4927429
Submitted: 28 May 2015 . Accepted: 15 July 2015 . Published Online: 27 July 2015
A. Schlehahn, M. Gaafar, M. Vaupel, M. Gschrey, P. Schnauber , J.-H. Schulze, S. Rodt, A. Strittmatter,
W. Stolz, A. Rahimi-Iman, T. Heindel , M. Koch, and S. Reitzenstein
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Single-photon emission at a rate of 143 MHz from a deterministic quantum-dot
microlens triggered by a mode-locked vertical-external-cavity surface-emitting
laser
A. Schlehahn,
1
M. Gaafar,
2
M. Vaupel,
2
M. Gschrey,
1
P. Schnauber,
1
J.-H. Schulze,
1
S. Rodt,
1
A. Strittmatter,
1,a)
W. Stolz,
2
A. Rahimi-Iman,
2
T. Heindel,
1,b)
M. Koch,
2
and S. Reitzenstein
1
1
Institut f€ur Festk€orperphysik, Technische Universit€at Berlin, Berlin 10623, Germany
2
Department of Physics and Materials Science Center, Philipps-Universit€
at Marburg, 35032 Marburg,
Germany
(Received 28 May 2015; accepted 15 July 2015; published online 27 July 2015)
We report on the realization of a quantum dot (QD) based single-photon source with a record-
high single-photon emission rate. The quantum light source consists of an InGaAs QD which is
deterministically integrated within a monolithic microlens with a distributed Bragg reflector as
back-side mirror, which is triggered using the frequency-doubled emission of a mode-locked
vertical-external-cavity surface-emitting laser (ML-VECSEL). The utilized compact and stable
laser system allows us to excite the single-QD microlens at a wavelength of 508 nm with a pulse
repetition rate close to 500 MHz at a pulse width of 4.2 ps. Probing the photon statistics of
the emission from a single QD state at saturation, we demonstrate single-photon emission of the
QD-microlens chip with g
(2)
(0) <0.03 at a record-high single-photon flux of (143 616) MHz
collected by the first lens of the detection system. Our approach is fully compatible with resonant
excitation schemes using wavelength tunable ML-VECSELs, which will optimize the quantum
optical properties of the single-photon emission in terms of photon indistinguishability. V
C2015
AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4927429]
A light source emitting single photons at high repetition
rates is highly desirable with respect to future applications in
the field of quantum information technology.
1
Single semi-
conductor quantum dots (QDs) turned out to be promising
candidates for the realization of such non-classical light
sources.
2
In particular, the engineering of electrically oper-
ated devices has been actively pursued within the nanopho-
tonics community
3–7
and recently, even the feasibility of
quantum-key distribution within a 500 m free-space experi-
ment using electrically triggered QD single-photon sources
(SPSs) has been demonstrated.
8
Using suitable pulse-
generators such SPSs can be triggered electrically with fre-
quencies up to the GHz range
9,10
in order to maximize the
single photon flux. On the other hand, quasi-resonant
11,12
or
strict-resonant
13–15
excitation is highly desirable with respect
to the generation of indistinguishable photons due to reduced
temporal jitter and dephasing. These excitation schemes,
however, are not easily compatible with electrical current
injection schemes
16–18
as in most cases the charge carriers
are injected above-band via p-i-n diode structures. Recently,
we tackled this issue by utilizing an integrated electrically
driven microlaser on the same chip as the non-classical light
source,
19
but a very sophisticated device technology is
required in this approach. Therefore, in most quantum optics
experiments QDs are excited optically using commercial
laser systems based on mode-locked Ti:sapphire lasers,
which offer ps- or fs-pulse-widths and can be tuned over
a broad spectral range. These widely used laser systems,
however, are rather bulky, expensive and most importantly
have limited repetition rates of typically 80 MHz, restrict-
ing the achievable single photon flux for a given photon
extraction efficiency.
In this work, we realize an ultra-bright SPS by opti-
cally exciting a deterministically integrated single-QD
microlens using a frequency-doubled mode-locked verti-
cal-external-cavity surface-emitting laser (ML-VECSEL).
The compact and stable laser system allows us to overcome
the limited repetition rates of commercial mode-locked
(ML) Ti:sapphire lasers and to excite the single-QD micro-
lens with a pulse repetition rate close to 500 MHz and a
pulse width of 4.2 ps at a wavelength of 508 nm. Probing
the photon statistics of the emission of a single QD-state at
saturation, we demonstrate single-photon emission of the
QD microlens with g
(2)
(0) <0.03 at a record-high single-
photon flux of (143 616) MHz collected by the first lens of
the detection system.
The sample used for the fabrication of deterministic QD
microlenses was grown by metal-organic chemical vapor
deposition on GaAs (001) substrate. After a 300 nm thick
GaAs buffer layer, a distributed Bragg reflector (DBR)
consisting of 23 alternating k/4-thick layers of AlGaAs
(77.0 nm) and GaAs (65.7 nm) is grown. Next, 65 nm of
GaAs is deposited followed by a low-density layer of self-
organized InGaAs QDs which are realized in the Stranski-
Krastanow growth mode. Finally, the QDs are capped by
400 nm of GaAs. The thickness of the GaAs capping layer is
optimized for the realization of microlenses with high photon
a)
Present address: Abteilung f€
ur Halbleiterepitaxie, Otto-von-Guericke
Universit€
at, 39106 Magdeburg, Germany.
b)
Author to whom correspondence should be addressed. Electronic mail:
tobias.heindel@tu-berlin.de
0003-6951/2015/107(4)/041105/4/$30.00 V
C2015 AIP Publishing LLC107, 041105-1
APPLIED PHYSICS LETTERS 107, 041105 (2015)
extraction efficiency. The actual QD microlens is then fabri-
cated deterministically onto a single pre-selected InGaAs
QD via a recently established in-situ electron beam lithogra-
phy technique.
20,21
Here, cathodoluminescence spectroscopy
in combination with 3D in-situ electron-beam writing and
subsequent plasma etching is used to precisely define a mon-
olithic microlens on top of the pre-selected QD. The overall
alignment accuracy is 34 nm.
22
The resulting QD micro-
lenses (cf. Fig. 1(c)) feature enhanced photon extraction effi-
ciencies as reported recently in Ref. 21.
To optically excite the QD microlens with high pulse
repetition rates, we implemented a compact laser system
emitting 508-nm ps-pulses based on a ML-VECSEL (see
Fig. 1(a)) into a standard micro-photoluminescence (lPL)
spectroscopy setup (Fig. 1(b)). The laser system itself is
mainly composed of an optically pumped VECSEL chip in
combination with a semiconductor saturable-absorber mirror
(SESAM) and a nonlinear crystal made out of beta-barium
borate (BBO) for frequency conversion via second-harmonic
generation (SHG). The VECSEL chip consists of 10
(InGa)As quantum wells equally spaced by k=2 (GaP)As
barrier layers. The DBR consists of 241
=2pairs of quarter
wavelength GaAs/(AlGa)As layers. Details of the semicon-
ductor layer structure can be found in Ref. 23. Such a chip is
then mounted to a water-cooled heat-sink and optically
pumped by a continuous-wave (CW) diode laser emitting at
808 nm.
24,25
The frequency-doubled emission of the ML-
VECSEL is coupled to the lPL setup via a cold mirror.
Here, a microscope objective (MO) with a numerical aper-
ture of 0.4 focuses the laser light onto the QD microlens,
which is mounted onto the cold-finger of a liquid-helium-
flow cryostat for cooling the sample to a temperature of
20 K. The same MO serves as the first lens of the detection
system and collects the emission of the QD-microlens chip.
The collimated emission is then spectrally analyzed using a
double-grating spectrometer with attached charge-coupled de-
vice (CCD) camera enabling a spectral resolution down to
25 leV. To perform time-resolved (TR) measurements, a
multi-mode (MM) fiber attached to a second output port of the
spectrometer is coupled to a Si-based avalanche photodiode
(APD) with a timing resolution of 40 ps. The photon statistics
of the QD emission is analyzed by means of photon-
autocorrelation measurements using a 50:50 MM fiber-
coupled beam-splitter (BS) and two Si-APDs in a Hanbury-
Brown and Twiss (HBT) type setup with an overall timing re-
solution of 380 ps. Both the HBT and TR setup utilize time-
correlated single-photon counting (TCSPC) electronics for
coincidence measurements.
Prior to single QD experiments, the utilized laser system
was analyzed in terms of its spectral and temporal emission
features. Fig. 2(a) displays the spectrum of the ML-VECSEL
emission after SHG, which shows a full width at half maxi-
mum of 0.2 nm. The maximum output power of the laser
after the BBO crystal was measured to be 5 lW. A TR mea-
surement of the ML laser emission recorded via a Si-APD
with a timing resolution of 40 ps is depicted in Fig. 2(b).
Here, a pulse train with equidistant pulse separations of
2.025 ns can be observed corresponding to a pulse repetition
rate of 494 MHz. The shape of a single laser pulse of the
1016 nm VECSEL emission recorded with a commercial in-
tensity autocorrelator is shown in Fig. 2(c), which exhibits a
pulse width of 4.2 ps. Next, the ML-VECSEL was used to
optically excite the QD sample. Fig. 3(a) shows a lPL spec-
trum of a single-QD microlens under ML-VECSEL excita-
tion at a laser power of 1.3 lW. The spectrum is dominated
by the emission of the positively charged trion state X
þ
at an
emission energy of 1.335 eV. The assignment of the emission
to a specific QD state was carried out via polarization and
power dependent measurements as described in Ref. 26.At
lower energies around 1.331 eV, the emission of other exci-
tonic states of the same QD is visible. The exceedingly
bright emission of the X
þ
state (maximum count rate of
65 kHz per CCD-pixel) in comparison to other QD states is
FIG. 1. Schematic of the experimental setup used to operate a quantum dot
(QD) single-photon source at high repetition rates of 494 MHz using a
mode-locked vertical-external-cavity surface-emitting laser (ML-VECSEL).
(a) The laser system comprises an optically pumped VECSEL chip operating
at 1016 nm in combination with a semiconductor saturable-absorber mirror
(SESAM) and a nonlinear crystal (BBO) for second-harmonic generation of
508 nm light pulses (OC: output coupler). (b) Micro-photoluminescence
setup: Emission of the QD sample is collected via a microscope objective
(MO) serving as first lens of the detection system and spectrally analyzed
using a double-grating spectrometer. Time-resolved and Hanbury-Brown
and Twiss (HBT) type measurements can be performed at a second output
port of the spectrometer. (c) Atomic force microscopy image of a QD micro-
lens fabricated via in-situ 3D electron beam lithography. In the experiments,
a deterministic single QD microlens with a base diameter of 2 lm acted as
single-photon emitter.
FIG. 2. (a) SHG spectrum of the frequency-doubled VECSEL emission
under mode-locked operation at 494 MHz. (b) Corresponding time-resolved
measurement on a laser pulse train. (c) Autocorrelation measurement of the
1016 nm VECSEL emission revealing a pulse width of 4.2 ps.
041105-2 Schlehahn et al. Appl. Phys. Lett. 107, 041105 (2015)
attributed to an intrinsic p-type background doping during
growth, which is typically observed in this type of QD sam-
ple.
20,26
To probe the achievable occupation of the X
þ
state,
excitation-power dependent lPL spectra have been eval-
uated. In Fig. 3(b), the integrated intensity of the X
þ
emis-
sion is depicted as a function of the ML-VECSEL excitation
power at 508 nm in logarithmic scaling. An almost linear ex-
citation power dependence with a slope of 1.10 60.03 is
observed up to an excitation power of 0.3 lW. Further
increasing the excitation power results in saturation of the
X
þ
state, until the maximum emission intensity is reached at
P
exc
¼1.3 lW (indicated by red arrow in Fig. 3(b)). In order
to gain insight into the photon statistics of the QD emission
under these excitation conditions, we performed measure-
ments of the second-order photon-autocorrelation g
(2)
(s). For
this purpose, the spectrally filtered emission of the X
þ
state
(indicated by red arrows in Fig. 3(a)) was coupled to the
fiber-based HBT setup. The excitation power of the ML-
VECSEL was set to 1.3 lW for saturation of the QD state,
resulting in a combined detection rate of 71.5 kHz at the
APDs. The resulting coincidence histogram of g
(2)
(s)ispre-
sented in the upper panel of Fig. 3(c). A strongly reduced
number of coincidences with a measured value of 0.22 can
be observed at zero time delay (s¼0), which indicates
single-photon emission of the QD microlens. Furthermore,
coincidence maxima with a periodicity of 2.025 ns, corre-
sponding to the pulse repetition rate of the ML-VECSEL,
can be clearly identified at finite time delays. This gets even
more evident by comparing the data with the g
(2)
(s) histo-
gram measured directly on the frequency-doubled emission
of the ML-VECSEL, which is shown as a reference in the
lower panel of Fig. 3(c). The strongly overlapping coinci-
dence peaks observed in this measurement are due to the
high repetition rate of the excitation laser generating pulses
with a separation close to the lifetime (1.5 ns) of the QD
state. To quantitatively extract the suppression of two-
photon emission events, we modeled the data with equidis-
tant photon pulses represented by Lorentzian profiles with
2.3 ns full-width at half maximum and a pulse separation
according to the 494 MHz repetition rate of the exciting
laser. The area of the pulses was assumed to be constant,
except for the zero-delay peak. The fitted model function (cf.
Fig. 3(c)) nicely retraces the experimental data and allows to
deduce an upper bound for the antibunching of g
(2)
(0) <0.03
by dividing the area of the almost vanishing zero-delay peak
by the area of the exemplarily displayed peak at finite time
delay. Such strong suppression of two-photon emission
events unambiguously proves the triggered emission of
single-photons from the deterministic QD microlens even at
saturation of the QD state.
A major advantage of using a ML-VECSEL for the exci-
tation of a single quantum emitter, as demonstrated in
this work, is given by the prospect and achievement of sig-
nificantly higher excitation repetition rates compared to con-
ventional laser systems such as Ti:sapphire lasers. To
quantitatively analyze the emission of the QD microlens in
terms of the single-photon flux, we experimentally deter-
mined the detection efficiency g
setup
of our experimental
setup.
21
This was achieved by focusing a CW diode laser
tuned to the emission wavelength of the X
þ
state onto a gold
mirror mounted in the cryostat. The laser was attenuated
using neutral-density filters in front of the monochromator to
achieve APD count-rates at the HBT setup similar to those
observed for the QD emission. Taking into account the laser
power, the reflection of the gold mirror, the transmission of
the cryostat window, the attenuation of the density filters,
and the maximal count-rates on the APDs, we determined
our setup efficiency to g
setup
¼(0.50 60.06) 10
3
. The X
þ
state excited at saturation (P
exc
¼1.3 lW) showed a com-
bined detection rate on the APDs of R
det
¼(71.5 62.0) kHz,
which corresponds to a single-photon flux F
SPS
emitted into
the microscope objective of F
SPS
¼R
det
/g
setup
¼(143 616)
MHz. Taking into account the repetition rate of the
ML-VECSEL of 494 MHz, this corresponds to a photon
extraction efficiency of the deterministic single QD micro-
lens of (29 63)%. The demonstrated single-photon flux of
143 MHz represents a significant improvement compared to
the previous reports on SPSs based on tapered nanowires
27
as well as optically
28
and electrically
29
triggered QD micro-
pillar cavities.
In summary, we realized an ultra-bright QD SPS by
using a frequency-doubled ML-VECSEL operating at
494 MHz pulse repetition rate to excite a single QD deter-
ministically integrated within a monolithically fabricated
FIG. 3. Analysis of the emission of a deterministic QD microlens excited by
a frequency-doubled ML-VECSEL at a repetition rate of 494 MHz. (a) lPL
spectrum of the single QD microlens, revealing bright emission of a posi-
tively charged exciton state (X
þ
) at an excitation power of P
exc
¼1.3 lW.
(b) Spectrally integrated lPL intensity of the X
þ
emission in dependence on
the ML-VECSEL excitation power. The dashed blue line corresponds to a
linear fit to the experimental data. The maximum X
þ
emission intensity is
reached at 1.3 lW corresponding to saturation of the QD state (dashed hori-
zontal line). At this working point, the single-photon flux emitted by the
QD into the first lens of the setup amounts to 143 MHz. (c) Photon-
autocorrelation histogram measured on the X
þ
emission at saturation
(spectral filtering is indicated by arrows in (a) as well as the working point
in (b)). The coincidence data reach a minimal value of 0.22. The model
curve (solid black line) reveals an upper bound to the antibunching value of
g
(2)
(0) <0.03, clearly proving single-photon emission. The lower panel
shows for comparison the photon-autocorrelation histogram measured on
the ML-VECSEL emission.
041105-3 Schlehahn et al. Appl. Phys. Lett. 107, 041105 (2015)
microlens. This compact and long-term-stable laser system
allows us to overcome the repetition rate limit of standard
commercial ML Ti:sapphire lasers and to achieve record-
high single-photon fluxes of (143 616) MHz collected by
the first lens of the setup, which corresponds to a photon
extraction efficiency of (29 63)%. Probing the photon statis-
tics of the emission of a single QD-state at saturation, we
demonstrate triggered single-photon emission of the micro-
lens with g
(2)
(0) <0.03. This first proof of principle experi-
ment carried out under above-band excitation points out the
high potential of our approach, which can be extended in the
future towards resonant excitation of QD-microlenses or
other QD based nanophotonic devices. This could be accom-
plished by deterministically matching the wavelength of re-
spective QD transitions to the emission of a ML-VECSEL,
or by exploiting wavelength-tunable ML-VECSELs in the
future. In this light, even the generation of indistinguishable
photons at unprecedented emission rates seems to be feasi-
ble. Further improvements towards practical SPSs might
include spectral tuning via piezoelectric actuators
30
or the
integration of microlenses onto arrays of site-controlled
QDs.
31,32
Moreover, it would be interesting to combine our
approach with materials providing stable room-temperature
operation,
33,34
or alternatively by utilizing compact table-top
cryocoolers.
26
This work was financially supported by the German
Federal Ministry of Education and Research (BMBF)
through the VIP-project QSOURCE (Grant No. 03V0630)
and the German Science Foundation (Deutsche
Forschungsgemeinschaft, abbreviated as DFG) within the
Collaborative Research Center SFB 787 “Semiconductor
Nanophotonics: Materials, Models, Devices.” M. Gaafar
acknowledges financial support from the Yousef Jameel
scholarship funds, and W. Stolz, A. Rahimi-Iman, and M.
Koch acknowledge support from the German Science
Foundation (DFG: GRK 1782, SFB 1083). We gratefully
acknowledge expert QD sample preparation by R. Schmidt
as well as VECSEL chip processing by C. M€
oller, B.
Heinen, and the NAsP III/V GmbH.
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