510 research outputs found
A Radiative Cycle with Stimulated Emission from Atoms (Ions) in an astrophysical Plasma
We propose that a radiative cycle operates in atoms (ions) located in a
rarefied gas in the vicinity of a hot star. Besides spontaneous transitions the
cycle includes a stimulated transition in one very weak intermediate channel.
This radiative "bottle neck" creates a population inversion, which for an
appropriate column density results in amplification and stimulated radiation in
the weak transition. The stimulated emission opens a fast decay channel leading
to a fast radiative cycle in the atom (or ion). We apply this model by
explaining two unusually bright Fe II lines at 250.7 and 250.9 nm in the UV
spectrum of gas blobs close to h Carinae, one of the most massive and luminous
stars in the Galaxy. The gas blobs are spatially resolved from the central star
by the Hubble Space Telescope (HST). We also suggest that in the frame of a
radiative cycle stimulated emission is a key phenomenon behind many spectral
lines showing anomalous intensities in spectra of gas blobs outside eruptive
stars.Comment: Accepted for publication in Phys. Rev. Letter
Matter-wave analog of an optical random laser
The accumulation of atoms in the lowest energy level of a trap and the
subsequent out-coupling of these atoms is a realization of a matter-wave analog
of a conventional optical laser. Optical random lasers require materials that
provide optical gain but, contrary to conventional lasers, the modes are
determined by multiple scattering and not a cavity. We show that a
Bose-Einstein condensate can be loaded in a spatially correlated disorder
potential prepared in such a way that the Anderson localization phenomenon
operates as a band-pass filter. A multiple scattering process selects atoms
with certain momenta and determines laser modes which represents a matter-wave
analog of an optical random laser.Comment: 4 pages, 3 figures version accepted for publication in Phys. Rev. A;
minor changes, the present title substituted for "Atom Random Laser
Statistical regimes of random laser fluctuations
Statistical fluctuations of the light emitted from amplifying random media
are studied theoretically and numerically. The characteristic scales of the
diffusive motion of light lead to Gaussian or power-law (Levy) distributed
fluctuations depending on external control parameters. In the Levy regime, the
output pulse is highly irregular leading to huge deviations from a mean--field
description. Monte Carlo simulations of a simplified model which includes the
population of the medium, demonstrate the two statistical regimes and provide a
comparison with dynamical rate equations. Different statistics of the
fluctuations helps to explain recent experimental observations reported in the
literature.Comment: Revised version, resubmitted to Physical Review
Fluctuations in a diffusive medium with gain
We present a stochastic model for amplifying, diffusive media like, for
instance, random lasers. Starting from a simple random-walk model, we derive a
stochastic partial differential equation for the energy field with contains a
multiplicative random-advection term yielding intermittency and power-law
distributions of the field itself. Dimensional analysis indicate that such
features are more likely to be observed for small enough samples and in lower
spatial dimensions
Optofluidic random laser
An active disordered medium able to lase is called a random laser (RL). We
demonstrate random lasing due to inherent disorder in a dye circulated
structured microfluidic channel. We consistently observe RL modes which are
varied by changing the pumping conditions. Potential applications for on-chip
sources and sensors are discussed.Comment: 3 pages, 4 figure
A study of random laser modes in disordered photonic crystals
We studied lasing modes in a disordered photonic crystal. The scaling of the
lasing threshold with the system size depends on the strength of disorder. For
sufficiently large size, the minimum of the lasing threshold occurs at some
finite value of disorder strength. The highest random cavity quality factor was
comparable to that of an intentionally introduced single defect. At the
minimum, the lasing threshold showed a super-exponential decrease with the size
of the system. We explain it through a migration of the lasing mode frequencies
toward the photonic bandgap center, where the localization length takes the
minimum value. Random lasers with exponentially low thresholds are predicted.Comment: 4 pages, 4 figure
Photon scattering from strongly driven atomic ensembles
The second order correlation function for light emitted from a strongly and
near-resonantly driven dilute cloud of atoms is discussed. Because of the
strong driving, the fluorescence spectrum separates into distinct peaks, for
which the spectral properties can be defined individually. It is shown that the
second-order correlations for various combinations of photons from different
spectral lines exhibit bunching together with super- or sub-Poissonian photon
statistics, tunable by the choice of the detector positions. Additionally, a
Cauchy-Schwarz inequality is violated for photons emitted from particular
spectral bands. The emitted light intensity is proportional to the square of
the number of particles, and thus can potentially be intense. Three different
averaging procedures to model ensemble disorder are compared.Comment: 7 pages, 4 figure
Steady-state signatures of radiation trapping by cold multilevel atoms
In this paper, we use steady-state measurements to obtain evidence of
radiation trapping in an optically thick a cloud of cold rubidium atoms. We
investigate the fluorescence properties of our sample, pumped on opened
transitions. The intensity of fluorescence exhibits a non trivial dependence on
the optical thickness of the media. A simplified model, based on rate equations
self-consistently coupled to a diffusive model of light transport, is used to
explain the experimental observations in terms of incoherent radiation trapping
on one spectral line. Measurements of atomic populations and fluorescence
spectrum qualitatively agree with this interpretation.Comment: 8 pages, 5 figure
Non-Markovian Decay and Lasing Condition in an Optical Microcavity Coupled to a Structured Reservoir
The decay dynamics of the classical electromagnetic field in a leaky optical
resonator supporting a single mode coupled to a structured continuum of modes
(reservoir) is theoretically investigated, and the issue of threshold condition
for lasing in presence of an inverted medium is comprehensively addressed.
Specific analytical results are given for a single-mode microcavity resonantly
coupled to a coupled resonator optical waveguide (CROW), which supports a band
of continuous modes acting as decay channels. For weak coupling, the usual
exponential Weisskopf-Wigner (Markovian) decay of the field in the bare
resonator is found, and the threshold for lasing increases linearly with the
coupling strength. As the coupling between the microcavity and the structured
reservoir increases, the field decay in the passive cavity shows non
exponential features, and correspondingly the threshold for lasing ceases to
increase, reaching a maximum and then starting to decrease as the coupling
strength is further increased. A singular behavior for the "laser phase
transition", which is a clear signature of strong non-Markovian dynamics, is
found at critical values of the coupling between the microcavity and the
reservoir.Comment: to appear in Phys. Rev. A (December 2006 issue
Nuclear quantum optics with x-ray laser pulses
The direct interaction of nuclei with super-intense laser fields is studied.
We show that present and upcoming high-frequency laser facilities, especially
together with a moderate acceleration of the target nuclei, do allow for
resonant laser-nucleus interaction. These direct interactions may be utilized
for the optical measurement of nuclear properties such as the transition
frequency and the dipole moment, thus opening the field of nuclear quantum
optics. As ultimate goal, one may hope that direct laser-nucleus interactions
could become a versatile tool to enhance preparation, control and detection in
nuclear physics.Comment: 5 pages, 3 eps figures, revised versio
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