Issue |
A&A
Volume 384, Number 1, MarchII 2002
|
|
---|---|---|
Page(s) | 350 - 363 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361:20020051 | |
Published online | 15 March 2002 |
Lasers without inversion (LWI) in Space: A possible explanation for intense, narrow-band, emissions that dominate the visible and/or far-UV (FUV) spectra of certain astronomical objects
IBM Research Division, PO Box 218, Yorktown Heights, NY 10598, USA
Corresponding author: P. P. Sorokin, sorokin@us.ibm.com
Received:
3
December
2001
Accepted:
20
December
2001
The optical or far-UV (FUV) spectra of certain objects in Space are
completely dominated by one or two spectrally narrow emission lines,
strongly suggesting that laser action of some kind occurs in these objects.
However, the electronic level structures of the atoms/ions producing these
emissions preclude the possibility of maintaining population inversions on
the electronic transitions involved. In lasers, gain is normally produced on
an optical transition that is inverted, i.e. one that has more atoms
maintained in the upper than in the lower state, so that stimulated emission
can exceed stimulated absorption. However, as a result of discoveries made
in quantum electronics over the past 30 years or so, one now knows that
there are several ways to make stimulated emission occur on a transition
that is not inverted, i.e. to realize a “laser without inversion (LWI)”.
This requires first making the atoms non-absorbing at the lasing frequency,
i.e. setting up a condition of “electromagnetically induced transparency
(EIT)”. Some recently developed EIT techniques for three-level atoms are
first reviewed. A simple model for a space LWI based upon a gas of two-level
atoms is then proposed. In this model, transparency results from a form of
EIT induced by the presence of an intense, monochromatic, continuous-wave,
laser beam tuned to the frequency of the two-level-atom transition.
Amplification of light at this same frequency occurs via resonant stimulated
hyper-Raman scattering (SHRS) and four-wave mixing (FWM), with pumping
energy provided by continuum starlight spectrally overlapping the two outer
absorption sidebands (“Mollow bands”) induced by the presence of the beam at
. Two specific examples of superintense line emission from
Space are here considered. These are (a) the H(α) emission line
appearing as a dominant singularity in certain reddened, early-type stars,
and (b) the powerful O VI (1032 Å, 1038 Å) emission doublet that
dominates the FUV emission spectra of symbiotic stars such as RR Tel.
Key words: atomic processes / radiation mechanisms: non-thermal / stars: early type / stars: individual: HD 44179 / stars: individual: RR Tel / ISM: lines and bands
© ESO, 2002
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