188 research outputs found
Coherent Oscillations in an Exciton-Polariton Josephson Junction
We report on the observation of spontaneous coherent oscillations in a
microcavity polariton bosonic Josephson junction. The condensation of exciton
polaritons takes place under incoherent excitation in a disordered environment,
where double potential wells tend to appear in the disordered landscape.
Coherent oscillations set on at an excitation power well above the condensation
threshold. The time resolved population and phase dynamics reveal the analogy
with the AC Josephson effect. We have introduced a theoretical two-mode model
to describe the observed effects, which allows us to explain how the different
realizations of the pulsed experiment have a similar phase relation
2s exciton-polariton revealed in an external magnetic field
We demonstrate the existence of the excited state of an exciton-polariton in
a semiconductor microcavity. The strong coupling of the quantum well heavy-hole
exciton in an excited 2s state to the cavity photon is observed in non-zero
magnetic field due to surprisingly fast increase of Rabi energy of the 2s
exciton-polariton in magnetic field. This effect is explained by a strong
modification of the wave-function of the relative electron-hole motion for the
2s exciton state.Comment: 5 pages, 5 figure
Dynamics of long-range order in an exciton-polariton condensate
We report on time resolved measurements of the first order spatial coherence
in an exciton polariton Bose-Einstein condensate. Long range spatial coherence
is found to set in right at the onset of stimulated scattering, on a picosecond
time scale. The coherence reaches its maximum value after the population and
decays slower, staying up to a few hundreds of picoseconds. This behavior can
be qualitatively reproduced, using a stochastic classical field model
describing interaction between the polariton condensate and the exciton
reservoir within a disordered potential.Comment: 7 pages, 4 figure
Data analysis of gravitational-wave signals from spinning neutron stars. V. A narrow-band all-sky search
We present theory and algorithms to perform an all-sky coherent search for
periodic signals of gravitational waves in narrow-band data of a detector. Our
search is based on a statistic, commonly called the -statistic,
derived from the maximum-likelihood principle in Paper I of this series. We
briefly review the response of a ground-based detector to the
gravitational-wave signal from a rotating neuron star and the derivation of the
-statistic. We present several algorithms to calculate efficiently
this statistic. In particular our algorithms are such that one can take
advantage of the speed of fast Fourier transform (FFT) in calculation of the
-statistic. We construct a grid in the parameter space such that
the nodes of the grid coincide with the Fourier frequencies. We present
interpolation methods that approximately convert the two integrals in the
-statistic into Fourier transforms so that the FFT algorithm can
be applied in their evaluation. We have implemented our methods and algorithms
into computer codes and we present results of the Monte Carlo simulations
performed to test these codes.Comment: REVTeX, 20 pages, 8 figure
Polariton Condensation in a One-Dimensional Disordered Potential
We study the coherence and density modulation of a non-equilibrium
exciton-polariton condensate in a one-dimensional valley with disorder. By
means of interferometric measurements we evidence a modulation of the
first-order coherence function and we relate it to a disorder-induced
modulation of the condensate density, that increases as the pump power is
increased. The non-monotonous spatial coherence function is found to be the
result of the strong non-equilibrium character of the one-dimensional system,
in the presence of disorder
Probing the Dynamics of Spontaneous Quantum Vortices in Polariton Superfluids
The experimental investigation of spontaneously created vortices is of utmost
importance for the understanding of quantum phase transitions towards a
superfluid phase, especially for two dimensional systems that are expected to
be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time
resolved near-field interferometry we track the path of such vortices, created
at random locations in an exciton-polariton condensate under pulsed
non-resonant excitation, to their final pinning positions imposed by the
stationary disorder. We formulate a theoretical model that successfully
reproduces the experimental observations
Optical manipulation of the wave function of quasiparticles in a solid
Polaritons in semiconductor microcavities are hybrid quasiparticles
consisting of a superposition of photons and excitons. Due to the photon
component, polaritons are characterized by a quantum coherence length in the
several micron range. Owing to their exciton content, they display sizeable
interactions, both mutual and with other electronic degrees of freedom. These
unique features have produced striking matter wave phenomena, such as
Bose-Einstein condensation, or parametric processes able to generate quantum
entangled polariton states. Recently, several paradigms for spatial confinement
of polaritons in semiconductor devices have been established. This opens the
way to quantum devices in which polaritons can be used as a vector of quantum
information. An essential element of each quantum device is the quantum state
control. Here we demonstrate control of the wave function of confined
polaritons, by means of tailored resonant optical excitation. By tuning the
energy and momentum of the laser, we achieve precise control of the momentum
pattern of the polariton wave function. A theoretical model supports
unambiguously our observations
Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR
LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz
frequencies each orbital cycle of 26.5 d and a superorbital
modulation with a period of 4.6 yr. We have performed a detailed
study of the low-frequency radio emission of LS I +61 303 by analysing all the
archival GMRT data at 150, 235 and 610 MHz, and conducting regular LOFAR
observations within the Radio Sky Monitor (RSM) at 150 MHz. We have detected
the source for the first time at 150 MHz, which is also the first detection of
a gamma-ray binary at such a low frequency. We have obtained the light-curves
of the source at 150, 235 and 610 MHz, all of them showing orbital modulation.
The light-curves at 235 and 610 MHz also show the existence of superorbital
variability. A comparison with contemporaneous 15-GHz data shows remarkable
differences with these light-curves. At 15 GHz we see clear outbursts, whereas
at low frequencies we see variability with wide maxima. The light-curve at 235
MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of
0.5 orbital phases in the maxima. We model the shifts between the maxima
at different frequencies as due to changes in the physical parameters of the
emitting region assuming either free-free absorption or synchrotron
self-absorption, obtaining expansion velocities for this region close to the
stellar wind velocity with both mechanisms.Comment: 12 pages, 10 figures, accepted for publication in MNRA
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