193 research outputs found
Nonlinear dynamics of polariton scattering in semiconductor microcavity: bistability vs stimulated scattering
We demonstrate experimentally an unusual behavior of the parametric polariton
scattering in semiconductor microcavity under a strong cw resonant excitation.
The maximum of the scattered signal above the threshold of stimulated
parametric scattering does not shift along the microcavity lower polariton
branch with the change of pump detuning or angle of incidence but is stuck
around the normal direction. We show theoretically that such a behavior can be
modelled numerically by a system of Maxwell and nonlinear Schroedinger
equations for cavity polaritons and explained via the competition between the
bistability of a driven nonlinear MC polariton and the instabilities of
parametric polariton-polariton scattering.Comment: 5 pages, 4 Postscript figures; corrected typo
Spin multistability of cavity polaritons in a magnetic field
Spin transitions are studied theoretically and experimentally in a resonantly
excited system of cavity polaritons in a magnetic field. Weak pair interactions
in this boson system make possible fast and massive spin flips occurring at
critical amplitudes due to the interplay between amplitude dependent shifts of
eigenstates and the Zeeman splitting. Dominant spin of a condensate can be
toggled forth and back by tuning of the pump intensity only, which opens the
way for ultra-fast spin switchings of polariton condensates on a picosecond
timescale.Comment: 4 pages, 4 figure
Optical Orientation of Excitons in CdSe Self-Assembled Quantum Dots
We study spin dynamics of excitons confined in self-assembled CdSe quantum
dots by means of optical orientation in magnetic field. At zero field the
exciton emission from QDs populated via LO phonon-assisted absorption shows a
circular polarization of 14%. The polarization degree of the excitonic emission
increases dramatically when a magnetic field is applied. Using a simple model,
we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the
absence and presence of magnetic field, respectively. With increasing
temperature the polarization of the QD emission gradually decreases.
Remarkably, the activation energy which describes this decay is independent of
the external magnetic field, and, therefore, of the degeneracy of the exciton
levels in QDs. This observation implies that the temperature-induced
enhancement of the exciton spin relaxation is insensitive to the energy level
degeneracy and can be attributed to the same excited state distribution.Comment: 21 pages, 7 figs - PDF format onl
Nonlinear emission dynamics of a GaAs microcavity with embedded quantum wells
The emission dynamics of a GaAs microcavity at different angles of
observation with respect to the sample normal under conditions of nonresonant
picosecond-pulse excitation is measured. At sufficiently high excitation
densities, the decay time of the lower-polariton emission increases with the
polariton wavevector; at low excitation densities the decay time is independent
of the wavevector. The effect of additional nonresonant continuous illumination
on the emission originating from the bottom of the lower polariton branch is
investigated. The additional illumination leads to a substantial increase in
the emission intensity (considerably larger than the intensity of the
photoluminescence excited by this illumination alone). This fact is explained
in terms of acceleration of the polariton relaxation to the radiative states
due to scattering by charge carriers created by the additional illumination.
The results obtained show, that at large negative detunings between the photon
and exciton modes, polariton-polariton and polariton-free carrier scattering
are the main processes responsible for the filling of states near the bottom of
the lower polariton branch.Comment: 10 pages, 6 figures. This is an author-created, un-copyedited version
of an article accepted for publication in Journal of Physics: Condesed
Matter. IOP Publishing Ltd is not responsible for any errors or omissions in
this version of the manuscript or any version derived from i
Biexciton oscillator strength
Our goal is to provide a physical understanding of the elementary coupling
between photon and biexciton and to derive the physical characteristics of the
biexciton oscillator strength, following the procedure we used for trion.
Instead of the more standard two-photon absorption, this work concentrates on
molecular biexciton created by photon absorption in an exciton gas. We first
determine the appropriate set of coordinates in real and momentum spaces to
describe one biexciton as two interacting excitons. We then turn to second
quantization and introduce the "Fourier transform in the exciton sense" of the
biexciton wave function which is the relevant quantity for oscillator strength.
We find that, like for trion, the oscillator strength for the formation of one
biexciton out of one photon plus a \emph{single} exciton is extremely small: it
is one biexciton volume divided by one sample volume smaller than the exciton
oscillator strength. However, due to their quantum nature, trion and biexciton
have absorption lines which behave quite differently. Electrons and trions are
fermionic particles impossible to pile up all at the same energy. This would
make the weak trion line spread with electron density, the peak structure only
coming from singular many-body effects. By contrast, the bosonic nature of
exciton and biexciton makes the biexciton peak mainly rise with exciton
density, this rise being simply linear if we forget many-body effects between
the photocreated exciton and the excitons present in the sample
The trion: two electrons plus one hole versus one electron plus one exciton
We first show that, for problems dealing with trions, it is totally hopeless
to use the standard many-body description in terms of electrons and holes and
its associated Feynman diagrams. We then show how, by using the description of
a trion as an electron interacting with an exciton, we can obtain the trion
absorption through far simpler diagrams, written with electrons and
\emph{excitons}. These diagrams are quite novel because, for excitons being not
exact bosons, we cannot use standard procedures designed to deal with
interacting true fermions or true bosons. A new many-body formalism is
necessary to establish the validity of these electron-exciton diagrams and to
derive their specific rules. It relies on the ``commutation technique'' we
recently developed to treat interacting close-to-bosons. This technique
generates a scattering associated to direct Coulomb processes between electrons
and excitons and a dimensionless ``scattering'' associated to electron exchange
inside the electron-exciton pairs -- this ``scattering'' being the original
part of our many-body theory. It turns out that, although exchange is crucial
to differentiate singlet from triplet trions, this ``scattering'' enters the
absorption explicitly when the photocreated electron and the initial electron
have the same spin -- \emph{i}. \emph{e}., when triplet trions are the only
ones created -- \emph{but not} when the two spins are different, although
triplet trions are also created in this case. The physical reason for this
rather surprising result will be given
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