269 research outputs found
Origin of the inner ring in photoluminescence patterns of quantum well excitons
In order to explain and model the inner ring in photoluminescence (PL)
patterns of indirect excitons in GaAs/AlGaAs quantum wells (QWs), we develop a
microscopic approach formulated in terms of coupled nonlinear equations for the
diffusion, thermalization and optical decay of the particles. The origin of the
inner ring is unambiguously identified: it is due to cooling of indirect
excitons in their propagation from the excitation spot. We infer that in our
high-quality structures the in-plane diffusion coefficient is about 10-30cm^2/s
and the amplitude of the disorder potential is about 0.45meV.Comment: 4 pages, 3 figure
Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System
The development of a Turing instability to a spatially modulated state in a
photoexcited electron-hole system is proposed as a novel signature of exciton
Bose statistics. We show that such an instability, which is driven by kinetics
of exciton formation, can result from stimulated processes that build up near
quantum degeneracy. In the spatially uniform 2d electron-hole system, the
instability leads to a triangular lattice pattern while, at an electron-hole
interface, a periodic 1d pattern develops. We analyze the mechanism of
wavelength selection, and show that the transition is abrupt (type I) for the
uniform 2d system, and continuous (type II) for the electron-hole interface.Comment: 5 pages, 3 figure
Theory of condensation of indirect excitons in a trap
We present theoretical studies of condensation of indirect excitons in a
trap. Our model quantifies the effect of screening of the trap potential by
indirect excitons on exciton condensation. The theoretical studies are applied
to a system of indirect excitons in a GaAs/AlGaAs coupled quantum well
structure in a diamond-shaped electrostatic trap where exciton condensation was
studied in earlier experiments. The estimated condensation temperature of the
indirect excitons in the trap reaches hundreds of milliKelvin
Kinetics of Exciton Emission Patterns and Carrier Transport
We report on the measurements of the kinetics of expanding and collapsing
rings in the exciton emission pattern. The rings are found to preserve their
integrity during expansion and collapse, indicating that the observed kinetics
is controlled by charge carrier transport rather than by a much faster process
of exciton production and decay. The relation between ring kinetics and carrier
transport, revealed by our experiment and confirmed by comparison with a
theoretical model, is used to determine electron and hole transport
characteristics in a contactless fashion.Comment: 6 pages, 4 figure
The Interaction in the Macroscopically Ordered Exciton State
The macroscopically ordered exciton state (MOES) - a periodic array of beads
with spatial order on a macroscopic length - appears in the external exciton
rings at low temperatures below a few Kelvin. Here, we report on the
experimental study of the interaction in the MOES. The exciton PL energy varies
in concert with the intensity along the circumference of the ring, with the
largest energy found in the brightest regions. This shows that the MOES is
characterized by the repulsive interaction and is not driven by the attractive
interaction.Comment: 3 pages, 3 figure
Kinetics of the inner ring in the exciton emission pattern in GaAs coupled quantum wells
We report on the kinetics of the inner ring in the exciton emission pattern.
The formation time of the inner ring following the onset of the laser
excitation is found to be about 30 ns. The inner ring was also found to
disappear within 4 ns after the laser termination. The latter process is
accompanied by a jump in the photoluminescence (PL) intensity. The spatial
dependence of the PL-jump indicates that the excitons outside of the region of
laser excitation, including the inner ring region, are efficiently cooled to
the lattice temperature even during the laser excitation. The ring formation
and disappearance are explained in terms of exciton transport and cooling.Comment: 19 pages, 6 figure
Photoluminescence and spectral switching of single CdSe/ZnS colloidal nanocrystals in poly(methyl methacrylate)
Emission from single CdSe nanocrystals in PMMA was investigated. A fraction
of the nanocrystals exhibiting switching between two energy states, which have
similar total intensities, but distinctly different spectra were observed. We
found that the spectral shift characteristic frequency increases with the pump
power. By using the dynamic shift in the spectral position of emission peaks,
we were able to correlate peaks from the same nanocrystal. The measured
correlation is consistent with assignment of low energy lines to phonon
replicas.Comment: 5 pages, 4 figure
- …