Excitonic light-absorption and amplification bands in the presence of laser radiation |
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Authors: | S A Moskalenko V G Pavlov |
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Institution: | (1) Institute of Applied Physics, Moldavian Academy of Sciences, MD-2028 Kishinev, Republic of Moldova |
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Abstract: | We examine the absorption and amplification bands of a weak probe signal in the presence of Bose-Einstein condensation of
excitons that emerges in nonequilibrium conditions in the field of coherent laser radiation with a wave vector k
0. We assume that the detuning
from resonance between the energy ħω
ex
(k
0)+L
0 of the exciton level, which is shifted because of exciton-exciton interaction, and the laser photon energy ħω
L
, is generally nonzero. The elementary excitation spectrum consisting of the quasiexcitonic and quasienergy branches determines
the optical properties of the system. When there is real induced Bose-Einstein condensation, at
the two branches touch, as they do in spontaneous Bose-Einstein condensation. In virtual induced Bose-Einstein condensation,
when
, instabilities emerge in the spectrum in certain regions of the k-space. These instabilities are caused by a real transformation of two laser photons into two extracondensate particles. Nonequilibrium
extracondensate excitons strongly affect the absorption and amplification of the probe light signal. We show that light absorption
is due to the quantum transition from the ground state of the crystal to the quasiexcitonic branch of the spectrum. On the
other hand, amplification of the signal is caused by the transition from the quasienergy branch to the ground state of the
crystal. The same transition can be explained by a real transformation of two laser photons into a vacuum photon of frequency
ħcq and a crystal exciton with a wave vector 2k
0−q. Finally, we show that the excitonic absorption and light-amplification bands are essentially anisotropic at
and depend on the orientation of the vectors q and k
0.
Zh. éksp. Teor. Fiz. 112, 167–179 (July 1997) |
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Keywords: | |
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