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Permanent tuning of quantum dot transitions to degenerate microcavity resonances

TL;DR: In this paper, the authors demonstrate a technique for achieving spectral resonance between a polarization-degenerate micropillar cavity mode and an embedded quantum dot transition based on a combination of isotropic and anisotropic tensile strain effected by laser-induced surface defects, thereby providing permanent tuning.
Abstract: We demonstrate a technique for achieving spectral resonance between a polarization-degenerate micropillar cavity mode and an embedded quantum dot transition. Our approach is based on a combination of isotropic and anisotropic tensile strain effected by laser-induced surface defects, thereby providing permanent tuning. Such a technique is a prerequisite for the implementation of scalable quantum information schemes based on solid-state cavity quantum electrodynamics.

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Summary

  • The authors demonstrate a technique for achieving spectral resonance between a polarization-degenerate micropillar cavity mode and an embedded quantum dot transition.
  • The authors approach is based on a combination of isotropic and anisotropic tensile strain effected by laser-induced surface defects, thereby providing permanent tuning.
  • Such a technique is a prerequisite for the implementation of scalable quantum information schemes based on solid-state cavity quantum electrodynamics.

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Permanent tuning of quantum dot transitions to degenerate
microcavity resonances
Gudat, J.; Bonato, C.; Nieuwenburg, E. van; Thon, S.M.; Kim, H.; Petroff, P.M.; ... ;
Bouwmeester, D.
Citation
Gudat, J., Bonato, C., Nieuwenburg, E. van, Thon, S. M., Kim, H., Petroff, P. M., …
Bouwmeester, D. (2011). Permanent tuning of quantum dot transitions to degenerate
microcavity resonances. Applied Physics Letters, 98(12), 121111. doi:10.1063/1.3569587
Version: Not Applicable (or Unknown)
License: Leiden University Non-exclusive license
Downloaded from: https://hdl.handle.net/1887/65891
Note: To cite this publication please use the final published version (if applicable).

Permanent tuning of quantum dot transitions to degenerate microcavity resonances
Jan Gudat, Cristian Bonato, Evert van Nieuwenburg, Susanna Thon, Hyochul Kim, Pierre M. Petroff, Martin
P. van Exter, and Dirk Bouwmeester
Citation: Appl. Phys. Lett. 98, 121111 (2011); doi: 10.1063/1.3569587
View online: https://doi.org/10.1063/1.3569587
View Table of Contents: http://aip.scitation.org/toc/apl/98/12
Published by the American Institute of Physics
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Permanent tuning of quantum dot transitions to degenerate microcavity
resonances
Jan Gudat,
1
Cristian Bonato,
1,a
Evert van Nieuwenburg,
1
Susanna Thon,
2
Hyochul Kim,
2
Pierre M. Petroff,
2
Martin P. van Exter,
1
and Dirk Bouwmeester
1,2
1
Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands
2
University of California Santa Barbara, Santa Barbara, California 93106, USA
Received 11 February 2011; accepted 1 March 2011; published online 23 March 2011
We demonstrate a technique for achieving spectral resonance between a polarization-degenerate
micropillar cavity mode and an embedded quantum dot transition. Our approach is based on a
combination of isotropic and anisotropic tensile strain effected by laser-induced surface defects,
thereby providing permanent tuning. Such a technique is a prerequisite for the implementation of
scalable quantum information schemes based on solid-state cavity quantum electrodynamics.
© 2011 American Institute of Physics. doi:10.1063/1.3569587
Single self-assembled quantum dots QDs embedded in
microcavities are interesting systems for quantum informa-
tion applications. Cavity-induced Purcell enhancement of the
emitter spontaneous emission rate has been exploited to
demonstrate efficient and reliable single photon sources.
13
Moreover, quantum information schemes employing cavity
quantum electrodynamics with QDs coupled to semiconduc-
tor microcavities have been proposed and implemented.
48
Such system would provide a scalable platform for hybrid
quantum information protocols, in which photonic qubits are
used for long-distance transmission and matter qubits for lo-
cal storage and processing.
9,10
Several quantum information applications require a
polarization-degenerate cavity mode that is spectrally reso-
nant with a specific QD optical transition.
7,8
Polarization-
degeneracy is needed in order to transfer an arbitrary polar-
ization state of a photon to the spin of a single electron
confined in the dot, or vice versa. In the case of micropillar
cavities, due to residual strain in the structure or small shape
asymmetries, the fundamental cavity mode often consists of
two linearly-polarized submodes, energy split by an amount
E. An important issue to note is the fact that the optical
properties of a self-assembled QD strongly depend on its
specific size and local strain, neither of which is determinis-
tically controllable in the growth process. Therefore post-
fabrication tuning techniques are crucial to achieving exact
spectral resonance.
The most flexible tuning technique is Stark-shifting: em-
bedding the dots in a diode structure and applying a voltage
leads to a shift in the optical transition frequency by the
quantum confined Stark effect.
11
Such shifts can be finely
tuned to a limited range of a few hundred microelectron volt,
making the technique most effective in combination with
some other coarse tuning procedures, such as temperature or
strain. Temperature tuning, of either the whole sample
12
or a
local spot
13
is an effective approach, with energy shifts on
the order of 1–2 meV reported in the literature. The tempera-
ture can, however, only be adjusted in the range of about
4–50 K: at higher temperatures the dot luminescence
quenches. Moreover, if one is interested in the spin of a
single electron in the QD, it is crucial to keep the tempera-
ture below 30 K, in order to avoid reducing the spin relax-
ation time.
14
Strain-tuning, via piezoelectric actuators or mechanical
tips, has also been extensively investigated.
1517
Recently, it
was shown that strain control by means of laser-induced sur-
face defects can be used to fine-tune the optical properties of
semiconductor microcavities.
18,19
By focusing a strong laser
beam on a small spot, far away from the cavity center to
preserve the optical quality of the device, the local birefrin-
gence can be modified. Here we show that, by a controlled
combination of anisotropic and isotropic strain, one can si-
multaneously get a polarization-degenerate cavity and tune a
dot optical transition into resonance with the cavity mode.
Since the defects are permanent, no external tuning equip-
ment is needed during an experiment, and this makes our
technique ideal for scalability purposes.
We investigated a sample with QDs embedded in micro-
pillar cavities, grown by molecular-beam epitaxy on a GaAs
100 substrate. The microcavity consists of two distributed
Bragg reflector DBR mirrors, made by alternating / 4 lay-
ers of GaAs and Al
0.9
Ga
0.1
As. Between the mirrors, the ac-
tive -GaAs layer contains embedded InGaAs/GaAs self-
assembled QDs and sits underneath an AlAs oxidation layer.
Trenches are etched down to the bottom DBR and the sample
is placed in a steam oven to define an AlO
x
oxidation front in
the AlAs layer, providing transverse optical-mode confine-
ment which results in high quality factors.
20
Using micropil-
lars defined by trench shapes, intracavity electrical gating of
multiple devices is possible by the fabrication of a PIN-diode
structure see Fig. 1 for a sample diagram.
a
Electronic mail: bonato@molphys.leidenuniv.nl.
FIG. 1. Color online Sketch of the micropillar structure used in the
experiments.
APPLIED PHYSICS LETTERS 98, 121111 2011
0003-6951/2011/9812/121111/3/$30.00 © 2011 American Institute of Physics98, 121111-1

Defects are created on the sample surface by a laser
beam about 100 mW/
m
2
, =532 nm tightly focused on
the structure for about 30 s by a high-numerical aperture
NA0.6 aspheric lens L
1
, with focal length f
0
=4.2 mm.
The material is locally melted and evaporated, leaving a hole
which is approximately 2
m wide and at least 2
m deep.
The whole process is performed in a helium-flow cryostat, at
a temperature of 4 K.
The first step consists of reducing the fundamental cavity
mode to polarization-degeneracy, following the procedure
described in Bonato et al.
19
The built-in strain can be com-
pensated by applying anisotropic strain, through holes burnt
at proper positions. The direction of the original built-in
strain is however unknown, so one must use a trial-and-error
procedure, illustrated in Fig. 2. We first start burning a hole
at a random orientation, for example along the direction la-
beled in the figure as x
1
. If the splitting gets larger, we move
to the orthogonal direction. If the splitting decreases, we
keep burning holes until the splitting stops decreasing. In the
example shown in Fig. 2, the first hole reduces E from
140 4
eV to 541
eV, but a second one slightly in-
creases it. This is an indication that all the strain along that
particular direction was compensated. We repeat the same
procedure on a reference system rotated by 45° with respect
to x
1
, y
1
. In the example, we start burning the third hole
along y
2
, which increases the splitting to E
=82.60.4
eV. Therefore we switch to the orthogonal di-
rection x
2
. Burning holes along this direction reduces E to
around 15
eV. The procedure can be further iterated along
directions in between x
1
and x
2
and generally leads to split-
tings smaller than the mode linewidth in our system about
50
eV, which is the requirement for quantum information
experiments. No appreciable change in the cavity quality fac-
tor was observed.
Strain affects the optical transitions of the QDs as well.
In Fig. 3, we show plots of voltage-resolved photolumines-
cence from the same microcavity analyzed in Fig. 2.We
pump the sample nonresonantly with about 1
W/
m
2
la-
ser beam at 785 nm, above the GaAs bandgap, and we spec-
trally resolve the photoluminescence with a spectrometer
resolution 25
eV/ pixel. Scanning the voltage of the mi-
crostructure PIN-diode, different charged states of the dot
can be selected
21
and the frequency of the optical transitions
can be tuned by the Stark effect.
11
The flat lines in the plots
correspond to the fixed frequency emission of the fundamen-
tal cavity mode, split into two orthogonally-polarized sub-
modes. The Stark-shifting lines correspond to QD optical
transitions.
The effect of laser-induced defects is always a redshift in
the optical transition, independent of the actual position of
the hole. The shift in the dot transition is generally much
larger than the corresponding shift in the cavity mode, and
from a sample of more than one hundred holes burnt, the
ratio of the shifts was found to be on average 5:1. These
findings can be explained with a simple model.
22
The fact
that the optical transition always redshifts suggests that by
burning holes we effectively apply tensile strain to the struc-
ture. This could be explained by assuming that, by removing
material, we release some compressive strain that pre-exists
FIG. 2. Color online Frequency splitting of the two orthogonally-polarized
submodes of the fundamental cavity mode as a function of the burnt holes.
FIG. 3. Color online Voltage-resolved photoluminescence plots for the
holes described in Fig. 2. Originally, the cavity mode is nondegenerate
splitting around 140
eV and QD-3 is around 0.5 meV detuned to the
blue-side of the cavity mode. Burning 6 holes reduces the splitting to about
15
eV and QD-3 is about 0.1 meV detuned. Applying isotropic strain, by
burning pair of holes along orthogonal direction, the dot can be brought into
resonance with the cavity mode, without destroying the mode degeneracy
see plot for 11 holes, bottom right. See Fig. 2 for the position of the holes.
121111-2 Gudat et al. Appl. Phys. Lett. 98, 121111 2011

due to lattice-mismatch in the dot. Such tensile strain affects
the band-structure both of the InAs dot material and of the
bulk surrounding GaAs, reducing the InAs energy gap and
the width of the confining potential well. The change in the
band-structure profiles can be shown to be independent of
the direction of the strain in the plane of the dot.
22
The difference in the way the cavity mode and the dot
transition are affected by hole-burning can be exploited to
tune a QD transition into resonance with a polarization-
degenerate cavity. In Fig. 3 one can see that, while burning
the first six holes, needed to reduce the splitting E, the
optical transitions of the dots redshift, so that the transitions
labeled as QD-1 and QD-2, originally resonant with the non-
degenerate fundamental cavity mode, tune out of resonance.
After burning six holes we have a polarization-degenerate
cavity mode, with a QD transition labeled QD-3 about
100
eV detuned on the blue-side. Now the challenge is to
shift this transition into resonance, without perturbing the
cavity mode degeneracy. This can be done by applying iso-
tropic strain: we can burn sets of two holes at orthogonal
directions, for example, one along x
2
and the other along y
2
,
at the same distance from the center. This leaves the splitting
E unaltered while redshifting the dot transition. The results
are shown in the bottom two pictures in Fig. 3, correspond-
ing to the tenth and eleventh hole burnt. The dot is finally on
resonance and the fundamental cavity mode splitting is
13 1
eV right side of Fig. 2, for holes 7–11.
In conclusion, we demonstrated a tuning technique for
micropillar cavities with embedded QDs, which allows us to
obtain polarization-degenerate micropillars with a QD tran-
sition on resonance. Our technique is a crucial prerequisite
for the implementation of scalable quantum information sys-
tems involving photon polarization and the spin of a single
carrier trapped in the dot.
This work was supported by the NSF Grant No.
0901886, the Marie-Curie Award No. EXT-CT-2006-042580,
and FOM\NWO Grant No. 09PR2721-2. We thank Andor for
the charge-coupled device camera.
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