44,155 research outputs found

### Bose-Einstein statistics in thermalization and photoluminescence of quantum well excitons

Quasi-equilibrium relaxational thermodynamics is developed to understand
LA-phonon-assisted thermalization of Bose-Einstein distributed excitons in
quantum wells. We study the quantum-statistical effects in the relaxational
dynamics of the effective temperature of excitons $T = T(t)$. When $T$ is less
than the degeneracy temperature $T_0$, well-developed Bose-Einstein statistics
of quantum well excitons leads to nonexponential and density-dependent
thermalization. At low bath temperatures $T_b \to 0$ the thermalization of
quantum-statistically degenerate excitons effectively slows down and $T(t)
\propto 1 / \ln t$. We also analyze the optical decay of Bose-Einstein
distributed excitons in perfect quantum wells and show how nonclassical
statistics influences the effective lifetime $\tau_{opt}$. In particular,
$\tau_{opt}$ of a strongly degenerate gas of excitons is given by $2 \tau_R$,
where $\tau_R$ is the intrinsic radiative lifetime of quasi-two-dimensional
excitons. Kinetics of resonant photoluminescence of quantum well excitons
during their thermalization is studied within the thermodynamic approach and
taking into account Bose-Einstein statistics. We find density-dependent
photoluminescence dynamics of statistically degenerate excitons. Numerical
modeling of the thermalization and photoluminescence kinetics of
quasi-two-dimensional excitons are given for GaAs/AlGaAs quantum wells.Comment: 19 pages, 9 figures. Phys. Rev. B (accepted for publication

### Auger decay of degenerate and Bose-condensed excitons in Cu$_2$O

We study the non-radiative Auger decay of excitons in Cu$_2$O, in which two
excitons scatter to an excited electron and hole. The exciton decay rate for
the direct and the phonon-assisted processes is calculated from first
principles; incorporating the band structure of the material leads to a
relatively shorter lifetime of the triplet state ortho excitons. We compare our
results with the Auger decay rate extracted from data on highly degenerate
triplet excitons and Bose-condensed singlet excitons in Cu$_2$O.Comment: 15 pages, revtex, figures available from G. Kavoulaki

### Electrical control of optical orientation of neutral and negatively charged excitons in n-type semiconductor quantum well

We report a giant electric field induced increase of spin orientation of
excitons in n-type GaAs/AlGaAs quantum well. It correlates strongly with the
formation of negatively charged excitons (trions) in the photoluminescence
spectra. Under resonant excitation of neutral heavy-hole excitons, the
polarization of excitons and trions increases dramatically with electrical
injection of electrons within the narrow exciton-trion bias transition in the
PL spectra, implying a polarization sensitivity of 200 % per Volt. This effect
results from a very efficient trapping of neutral excitons by the quantum well
interfacial fluctuations (so-called "natural" quantum dots) containing resident
electrons.Comment: 18 pages, 4 figure

### Kinetics of indirect excitons in the optically-induced exciton trap

We report on the kinetics of a low-temperature gas of indirect excitons in
the optically-induced exciton trap. The excitons in the region of laser
excitation are found to rapidly -- within 4 ns -- cool to the lattice
temperature T = 1.4 K, while the excitons at the trap center are found to be
cold -- essentially at the lattice temperature -- even during the excitation
pulse. The loading time of excitons to the trap center is found to be about 40
ns, longer than the cooling time yet shorter than the lifetime of the indirect
excitons. The observed time hierarchy is favorable for creating a dense and
cold exciton gas in optically-induced traps and for in situ control of the gas
by varying the excitation profile in space and time before the excitons
recombine.Comment: 4 pages, 3 figure

### Energy relaxation during hot-exciton transport in quantum wells: Direct observation by spatially resolved phonon-sideband spectroscopy

We investigate the energy relaxation of excitons during the real-space
transport in ZnSe quantum wells by using microphotoluminescence with spatial
resolution enhanced by a solid immersion lens. The spatial evolution of the
LO-phonon sideband, originating from the LO-phonon assisted recombination of
hot excitons, is measured directly. By calculating the LO-phonon assisted
recombination probability, we obtain the nonthermal energy distribution of
excitons and observe directly the energy relaxation of hot excitons during
their transport. We find the excitons remain hot during their transport on a
length scale of several micrometers. Thus, the excitonic transport on this
scale cannot be described by classical diffusion.Comment: 4 pages, 4 figure

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