559 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
A random laser tailored by directional stimulated emission
A disordered structure embedding an active gain material and able to lase is
called random laser (RL). The RL spectrum may appear either like a set of sharp
resonances or like a smooth line superimposed to the fluorescence. A recent
letter accounts for this duality with the onset of a mode locked regime in
which increasing the number of activated modes results in an increased inter
mode correlation and a pulse shortening ascribed to a synchronization
phenomenon. An extended discussion of our experimental approach together with
an original study of the spatial properties of the RL is reported here.Comment: 9 Pages; 16 Figure
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
Effects of spatial non-uniformity on laser dynamics
Semiclassical equations of lasing dynamics are re-derived for a lasing medium
in a cavity with a spatially non-uniform dielectric constant. It is shown that
the non-uniformity causes a radiative coupling between modes of the empty
cavity. This coupling results in a renormalization of self- and
cross-saturation coefficients, which acquire a non-trivial dependence on the
pumping intensity. Possible manifestations of these effects in random lasers
are discussed.Comment: 4 pages, 1 figure, LaTex. Introduction is significantly rewritten,
and the results is placed in the context of random lasin
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
- …