148 research outputs found
Optical signatures of a fully dark exciton condensate
We propose optical means to reveal the presence of a dark exciton condensate
that does not yield any photoluminescence at all. We show that (i) the dark
exciton density can be obtained from the blueshift of the excitonic absorption
line induced by dark excitons; (ii) the polarization of the dark condensate can
be deduced from the blueshift dependence on probe photon polarization and also
from Faraday effect, linearly polarized dark excitons leaving unaffected the
polarization plane of an unabsorbed photon beam. These effects result from
carrier exchanges between dark and bright excitons.Comment: 5 pages, 4 figure
Effects of fermion exchanges on the polarization of exciton condensates
Exchange processes are responsible for the stability of elementary boson
condensates with respect to their possible fragmentation. This remains true for
composite bosons when single fermion exchanges are included but spin degrees of
freedom are ignored. We here show that their inclusion can produce a
"spin-fragmentation" of a condensate of dark excitons, i.e., an unpolarized
condensate with equal amount of dark excitons with spins (+2) and (-2). Quite
surprisingly, for spatially indirect excitons of semiconductor bilayers, we
predict that the condensate polarization can switch from unpolarized to fully
polarized, depending on the distance between the layers confining electrons and
holes. Remarkably, the threshold distance associated to this switching lies in
the regime where experiments are nowadays carried out.Comment: 5 pages, 1 figur
Controlling the charge environment of single quantum dots in a photonic-crystal cavity
We demonstrate that the presence of charge around a semiconductor quantum dot
(QD) strongly affects its optical properties and produces non-resonant coupling
to the modes of a microcavity. We first show that, besides (multi)exciton
lines, a QD generates a spectrally broad emission which efficiently couples to
cavity modes. Its temporal dynamics shows that it is related to the Coulomb
interaction between the QD (multi)excitons and carriers in the adjacent wetting
layer. This mechanism can be suppressed by the application of an electric
field, making the QD closer to an ideal two-level system.Comment: 12 pages, 4 figure
From Strong to Weak Coupling Regime in a Single GaN Microwire up to Room Temperature
Large bandgap semiconductor microwires constitute a very advantageous
alternative to planar microcavities in the context of room temperature strong
coupling regime between exciton and light. In this work we demonstrate that in
a GaN microwire, the strong coupling regime is achieved up to room temperature
with a large Rabi splitting of 125 meV never achieved before in a Nitride-based
photonic nanostructure. The demonstration relies on a method which doesn't
require any knowledge \'a priori on the photonic eigenmodes energy in the
microwire, i.e. the details of the microwire cross-section shape. Moreover,
using a heavily doped segment within the same microwire, we confirm
experimentally that free excitons provide the oscillator strength for this
strong coupling regime. The measured Rabi splitting to linewidth ratio of 15
matches state of the art planar Nitride-based microcavities, in spite of a much
simpler design and a less demanding fabrication process. These results show
that GaN microwires constitute a simpler and promising system to achieve
electrically pumped lasing in the strong coupling regime.Comment: 14 pages, 4 figure
Quantum magnetism and counterflow supersolidity of up-down bosonic dipoles
We study a gas of dipolar Bosons confined in a two-dimensional optical
lattice. Dipoles are considered to point freely in both up and down directions
perpendicular to the lattice plane. This results in a nearest neighbor
repulsive (attractive) interaction for aligned (anti-aligned) dipoles. We find
regions of parameters where the ground state of the system exhibits insulating
phases with ferromagnetic or anti-ferromagnetic ordering, as well as with
rational values of the average magnetization. Evidence for the existence of a
novel counterflow supersolid quantum phase is also presented.Comment: 8 pages, 6 figure
Enhanced spontaneous emission in a photonic crystal light-emitting diode
We report direct evidence of enhanced spontaneous emission in a photonic
crystal (PhC) light-emitting diode. The device consists of p-i-n heterojunction
embedded in a suspended membrane, comprising a layer of self-assembled quantum
dots. Current is injected laterally from the periphery to the center of the
PhC. A well-isolated emission peak at 1300nm from the PhC cavity mode is
observed, and the enhancement of the spontaneous emission rate is clearly
evidenced by time-resolved electroluminescence measurements, showing that our
diode switches off in a time shorter than the bulk radiative and nonradiative
lifetimesComment: 10 page
Homologous recombination at the border: Insertion-deletions and the trapping of foreign DNA in Streptococcus pneumoniae
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