"Dressed-Atom" Lasers in Space

It is shown that a model recently proposed for two-level-atom lasers without inversion
(LWI) in Space (Sorokin & Glownia 2002) gains significant credibility when the model is
instead constructed on the basis of atoms simultaneously saturated on two atomic transitions
sharing a common level. The principal advantage attained by considering the atoms
in the model to have three active levels rather than two is that the efficiency of the pumping
mechanism no longer rapidly decreases with increasing power of the coherently generated
light beam(s). The powerful "dressed-atom" approach for analyzing the physics of atoms
strongly driven by resonant light beams (Cohen-Tannoudji & Reynaud 1977) provides a
direct and insightful way both to understand this effect and to deduce exactly which optical
pumping mechanisms should be most effective in converting intense broadband continuum
light into coherent radiation at the bare-atom resonance frequencies z_o and z'_o in three-level-
atom gases situated near bright continuum-emitting sources.

For three-level dressed-atom lasers, linear optical pumping by the continuum light is
shown to be generally ineffective, leaving two nonlinear processes, broadband stimulated
Raman scattering (SRS) and stimulated hyper-Raman scattering (SHRS) to be considered
as excitation mechanisms. However, only SHRS can account for the narrow-band emission
intensity being significantly greater than the continuum background level. In space objects
having very high electron densities, such as symbiotic stars, an additional nonlinear mechanism,
stimulated hyper-Compton scattering (SHCS), is postulated to be operative. SHCS
would enable efficient transfer of continuum photon energy to occur over very large spectral
intervals. One can account for the very large peak-to-background intensity ratio observed
for the O VI (1032 Å, 1038 Å) doublet emission in symbiotic stars only by postulating that
both SHRS and SHCS occur in these systems.

On the basis of Doppler width considerations, it is further explained that a three-level
atomic system has a very much greater chance of operating as a dressed-atom laser in Space
if its bare-atom frequencies happen to lie very close together. This may help explain why
the O VI doublet emission is so dominant in symbiotic stars.