89 research outputs found
Large and uniform optical emission shifts in quantum dots externally strained along their growth axis
We introduce a method which enables to directly compare the impact of elastic
strain on the optical properties of distinct quantum dots (QDs). Specifically,
the QDs are integrated in a cross-section of a semiconductor core wire which is
surrounded by an amorphous straining shell. Detailed numerical simulations show
that, thanks to the mechanical isotropy of the shell, the strain field in a
core section is homogeneous. Furthermore, we use the core material as an in
situ strain gauge, yielding reliable values for the emitter energy tuning
slope. This calibration technique is applied to self-assembled InAs QDs
submitted to incremental tensile strain along their growth axis. In contrast to
recent studies conducted on similar QDs stressed perpendicularly to their
growth axis, optical spectroscopy reveals 5-10 times larger tuning slopes, with
a moderate dispersion. These results highlight the importance of the stress
direction to optimise QD response to applied strain, with implications both in
static and dynamic regimes. As such, they are in particular relevant for the
development of wavelength-tunable single photon sources or hybrid QD
opto-mechanical systems
Polarization Control of the Non-linear Emission on Semiconductor Microcavities
The degree of circular polarization () of the non-linear emission in
semiconductor microcavities is controlled by changing the exciton-cavity
detuning. The polariton relaxation towards \textbf{K} cavity-like
states is governed by final-state stimulated scattering. The helicity of the
emission is selected due to the lifting of the degeneracy of the spin
levels at \textbf{K} . At short times after a pulsed excitation
reaches very large values, either positive or negative, as a result of
stimulated scattering to the spin level of lowest energy ( spin for
positive/negative detuning).Comment: 8 pages, 3 eps figures, RevTeX, Physical Review Letters (accepted
Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: A combined experimental and theoretical approach
We present a combined experimental and theoretical approach for the
determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe
heterojunctions with underlying zincblende crystal structure. On the
experimental side, the optical transition of the type II interface allows for a
precise measurement of the type II band gap. We show how the excitation-power
dependent shift of this photoluminescence (PL) signal can be used for any type
II system for a precise determination of the VBO. On the theoretical side, we
use a refined empirical tight-binding parametrization in order to accurately
reproduce the band structure and density of states around the band gap region
of cubic CdSe and ZnTe and then calculate the branch point energy (also known
as charge neutrality level) for both materials. Because of the cubic crystal
structure and the small lattice mismatch across the interface, the VBO for the
material system under consideration can then be obtained from a charge
neutrality condition, in good agreement with the PL measurements.Comment: 11 pages, 5 figure
Probing exciton localization in non-polar GaN/AlN Quantum Dots by single dot optical spectroscopy
We present an optical spectroscopy study of non-polar GaN/AlN quantum dots by
time-resolved photoluminescence and by microphotoluminescence. Isolated quantum
dots exhibit sharp emission lines, with linewidths in the 0.5-2 meV range due
to spectral diffusion. Such linewidths are narrow enough to probe the inelastic
coupling of acoustic phonons to confined carriers as a function of temperature.
This study indicates that the carriers are laterally localized on a scale that
is much smaller than the quantum dot size. This conclusion is further confirmed
by the analysis of the decay time of the luminescence
Dielectric GaAs Antenna Ensuring an Efficient Broadband Coupling between an InAs Quantum Dot and a Gaussian Optical Beam
We introduce the photonic trumpet, a dielectric structure which ensures a
nearly perfect coupling between an embedded quantum light source and a Gaussian
free-space beam. A photonic trumpet exploits both the broadband spontaneous
emission control provided by a single-mode photonic wire and the adiabatic
expansion of this mode within a conical taper. Numerical simulations highlight
the outstanding performance and robustness of this concept. As a first
application in the field of quantum optics, we report the realisation of an
ultra-bright single-photon source. The device, a GaAs photonic trumpet
containing few InAs quantum dots, demonstrates a first-lens external efficiency
of
Bright single-photon sources in bottom-up tailored nanowires
The ability to achieve near-unity light extraction efficiency is necessary
for a truly deterministic single photon source. The most promising method to
reach such high efficiencies is based on embedding single photon emitters in
tapered photonic waveguides defined by top-down etching techniques. However,
light extraction efficiencies in current top-down approaches are limited by
fabrication imperfections and etching induced defects. The efficiency is
further tempered by randomly positioned off-axis quantum emitters. Here, we
present perfectly positioned single quantum dots on the axis of a tailored
nanowire waveguide using bottom-up growth. In comparison to quantum dots in
nanowires without waveguide, we demonstrate a 24-fold enhancement in the single
photon flux, corresponding to a light extraction efficiency of 42 %. Such high
efficiencies in one-dimensional nanowires are promising to transfer quantum
information over large distances between remote stationary qubits using flying
qubits within the same nanowire p-n junction.Comment: 19 pages, 6 figure
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