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EPJ Web of Conferences

Theory of propagation of spectrum and correlations of radiation in optically dense gas in the case of the closed excitation contour

К. А. Баранцев, A. N. Litvinov, Е. Н. Попов

Published Dec 13, 2016• 0 citations
Publisher

Abstract

This work is devoted to generalization of the semi-classical theory of interaction of broadband laser radiation with the atomic gas at the room temperature in the cell in the case of the closed excitation contour. The atomic density matrix equations and spectrum and correlations transport equations have been derived for excitation by fluctuating field with Gaussian statistics. It is shown that the spatial oscillations of radiation intensity and atomic density matrix can be excited. It was found that such medium can serve as a filter of incoherent part of the radiation.

DOI
10.1051/epjconf/201713202003

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This work is devoted to generalization of the semi-classical theory of interaction of broadband laser radiation with the atomic gas at the room temperature in the cell in the case of the closed excitation contour. The atomic density matrix equations and spectrum and correlations transport equations have been derived for excitation by fluctuating field with Gaussian statistics. It is shown that the spatial oscillations of radiation intensity and atomic density matrix can be excited. It was found that such medium can serve as a filter of incoherent part of the radiation.
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The paper presents a theory describing the formation of the coherent population trapping resonance for the finite laser bandwidth in an optically-dense medium of atoms inside a buffer gas cell. The equations of the atomic density matrix are established, as well as the transfer equations of laser emission spectrum inside a cell with working and buffer gas at a determined temperature. The dependence of the quality of the coherent population trapping resonance on the laser bandwidth has been studied in the case of detecting the signal of the transmitted emission and the fluorescence signal.
A theory of excitation of a coherent population trapping resonance in hot alkali-metal atoms in a cell with buffer gas is constructed, taking into account the finite spectral width of two-component radiation, its vector properties, and the hyperfine and Zeeman structures of atoms. The propagation of intermode correlations in an optically dense gas cell is studied. It is found that in the case of partially correlated input modes, their correlation degree at the output can increase. This effect can be used for filtering the incoherent part of radiation.
The theory of coherent population trapping resonance is developed for the finite linewidth of the laser radiation spectrum in an optically dense medium of Λ atoms in a cell with a buffer gas. Equations are derived for the atomic density matrix and laser emission spectrum transfer in a cell with working and buffer gases at a finite temperature. The dependence of the quality factor of coherent population trapping resonance on the linewidth of the laser radiation spectrum is studied by measuring transmitted radiation and fluorescence signals.
We generalize the method of deriving a kinetic equation for a gas laser developed in the first paper of this series to include radiation-matter correlations. As a consequence, we are able to show that the first Born approximation with asymptotic conditions which contain radiation-matter correlations is sufficient to explain the nonthermal photon distribution observed in photon-counting experiments. Our derivation includes the spatial and velocity effects of the motion of the gas atoms. After adiabatically eliminating the matter variables, we obtain a master equation for the radiation density matrix RRR with nonlinear coefficients. Our radiation master equation for stationary atoms without spatial effects is the same as the generalized Fokker-Planck equation of the Langevin-noise-equation treatment of lasers. We show that the generalized Fokker-Planck equation is the result of the first-Born-approximation treatment of the j\ifmmode\cdot\else\textperiodcentered\fi{}A interaction and is not dependent upon the microscopic structure of the reservoirs. In the first Born approximation, the diagonal and off-diagonal matrix elements of RRR do not interact with each other. Consequently, an initially diagonal radiation density matrix remains diagonal. However, we show that even though the stationary state is diagonal it is necessary to know the propagator of the off-diagonal part of RRR to answer time-dependent questions about quantities, such as the line shape, that depend on phase.
This work is devoted to nonlinear effects in the atomic medium excited by multifrequency laser radiation. We have considered collective light scattering in the optically dense medium in conditions of coherent population trapping. Propagation of intermode correlation coefficient was calculated and it was found that correlation can increase in the medium. Light shifts of the coherent population trapping resonances and ways of their suppression were analyzed. Propagation of radiation in the medium with closed excitation scheme was investigated.