146 research outputs found
Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots
A systematic variation of the exciton fine-structure splitting with quantum
dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor
deposition is observed. The splitting increases from -80 to as much as 520
eV with quantum dot size. A change of sign is reported for small quantum
dots. Model calculations within the framework of eight-band k.p theory and the
configuration interaction method were performed. Different sources for the
fine-structure splitting are discussed, and piezoelectricity is pinpointed as
the only effect reproducing the observed trend.Comment: 5 pages, 5 figure
Multi-excitonic complexes in single InGaN quantum dots
Cathodoluminescence spectra employing a shadow mask technique of InGaN layers
grown by metal organic chemical vapor deposition on Si(111) substrates are
reported. Sharp lines originating from InGaN quantum dots are observed.
Temperature dependent measurements reveal thermally induced carrier
redistribution between the quantum dots. Spectral diffusion is observed and was
used as a tool to correlate up to three lines that originate from the same
quantum dot. Variation of excitation density leads to identification of exciton
and biexciton. Binding and anti-binding complexes are discovered.Comment: 3 pages, 4 figure
GaN/AlN Quantum Dots for Single Qubit Emitters
We study theoretically the electronic properties of -plane GaN/AlN quantum
dots (QDs) with focus on their potential as sources of single polarized photons
for future quantum communication systems. Within the framework of eight-band
k.p theory we calculate the optical interband transitions of the QDs and their
polarization properties. We show that an anisotropy of the QD confinement
potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to
a pronounced linear polarization of the ground state and excited state
transitions. An externally applied uniaxial stress can be used to either induce
a linear polarization of the ground-state transition for emission of single
polarized photons or even to compensate the polarization induced by the
structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte
Decay dynamics of neutral and charged excitonic complexes in single InAs/GaAs quantum dots
Systematic time-resolved measurements on neutral and charged excitonic
complexes (X, XX, X+, and XX+) of 26 different single InAs/GaAs quantum dots
are reported. The ratios of the decay times are discussed in terms of the
number of transition channels determined by the excitonic fine structure and a
specific transition time for each channel. The measured ratio for the neutral
complexes is 1.7 deviating from the theoretically predicted value of 2. A ratio
of 1.5 for the positively charged exciton and biexciton decay time is predicted
and exactly matched by the measured ratio indicating identical specific
transition times for the transition channels involved.Comment: 4 pages, 4 figure
Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots
A systematic study of the impact of annealing on the electronic properties of
single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence
spectra are recorded to trace the evolution of one and the same QD over several
steps of annealing. A substantial reduction of the excitonic fine-structure
splitting upon annealing is observed. In addition, the binding energies of
different excitonic complexes change dramatically. The results are compared to
model calculations within eight-band k.p theory and the configuration
interaction method, suggesting a change of electron and hole wave function
shape and relative position.Comment: 4 pages, 4 figure
Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses
Optimized light-matter coupling in semiconductor nanostructures is a key to
understand their optical properties and can be enabled by advanced fabrication
techniques. Using in-situ electron beam lithography combined with a
low-temperature cathodoluminescence imaging, we deterministically fabricate
microlenses above selected InAs quantum dots (QDs) achieving their efficient
coupling to the external light field. This enables to perform four-wave mixing
micro-spectroscopy of single QD excitons, revealing the exciton population and
coherence dynamics. We infer the temperature dependence of the dephasing in
order to address the impact of phonons on the decoherence of confined excitons.
The loss of the coherence over the first picoseconds is associated with the
emission of a phonon wave packet, also governing the phonon background in
photoluminescence (PL) spectra. Using theory based on the independent boson
model, we consistently explain the initial coherence decay, the zero-phonon
line fraction, and the lineshape of the phonon-assisted PL using realistic
quantum dot geometries
Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots
Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1120] and [1100]. Realistic eight-band k·p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are formed either by the A or the B valence band
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