1,065 research outputs found
Possibility of Measuring the Width of Narrow Fe II Astrophysical Laser Lines in the Vicinity of Eta Carinae by means of Brown-Twiss-Townes Heterodyne Correlation Interferometry
We consider the possibility of measuring the true width of the narrow Fe II
optical lines observed in spectra of the Weigelt blobs in the vicinity of Eta
Carinae. The lines originate as a result of stimulated amplification of
spontaneous emission of radiation in quantum transitions between energy levels
showing inverted population (Johansson & Letokhov, 2002, 2003, 2004). The lines
should have a subDoppler spectral width of 30-100 MHz, depending on the
geometry of the lasing volume. To make measurements with a spectral resolution
of R>10^7 and an angular resolution better than 0.1 arcsec, we suggest the use
of the Brown-Twiss-Townes optical heterodyne intensity correlation
interferometry. The estimates made of the S/N ratio for the optical heterodyne
astrophysical laser experiment imply that it is feasible.Comment: Accepted for publication in New Astronom
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
Successsive resonance-enhanced two-photon ionization of elements abundant in nebulae. I. Atoms and ions of C, N, and O
We discuss resonance-enhanced two-photon ionization (RETPI) and presentschemes of successive RETPI of the elements C, N, and O in nebulae.RETPI is activated by intrinsic radiation stored in the form of trappedspectral lines of HI, HeI, and HeII in the optically thick nebula. Therate of this two-step photoionization is comparable with or exceeds thelow recombination rate of the photoions formed in the process. Thisleads to an accumulation of photoions and subsequent RETPI until suchhighly charged ions are formed that they cannot further be ionized inthis way by the intrinsic radiation from the strong spectral lines ofHI, HeI, and HeII. Erratum: http://lu-research.lub.lu.se/php/gateway.php?who=lr&method=getfile&file=archive/00017447
Anomalous Fe II Spectral Effects and High H I Lyα Temperature in Gas Blobs Near η Carinae
We consider the origin of the anomalously high intensity of theultraviolet Fe II lambda 2507/2509 A lines observed with high spatialand spectral resolutions from gas blobs (GBs) near Carinae. This rareeffect in stellar spectra is attributable to a combination of severalfactors: (1) the high hydrogen density (>10^{8} cm^{-3}) that ensuresthe blocking of the Lyman continuum by GBs and, accordingly, theformation of a cold H I region with completely ionized Fe atoms; (2) thesmall distance between the GBs and the central star that ensures a high(>8000-10 000 K) Ly_alpha H spectral temperature, which photoexcitesFe II selectively; and (3) the population of Fe II levels and,accordingly, the opening of a stimulated emission channel, whichtogether with spontaneous transitions creates a radiative cycle where asingle Fe II ion can multiply absorb Ly_alpha emission
Laser action in a gas condensation in the vicinity of a hot star
In the region near 1 µm, we have found laser action in a quantum transition between highly excited states of Fe II, with its higher levels being optically pumped by the intense H Ly radiation (1215 Å) formed in the ionized HII region of a gas condensation (blob B) in the close vicinity of the central star in Carinae
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
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
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
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