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Soviet Journal of Quantum Electronics

Saturation of light absorption in molecular gases

A O Markano, Viktor T Platonenko

Published May 31, 1979• 4 citations
Publisher

Abstract

The saturation of light absorption in a molecular gas is analyzed theoretically allowing for the redistribution of the molecules in terms of the orientations of their moments. It is shown that the amount of energy absorbed by the gas generally depends on the polarization of the incident radiation. It is noted that when the polarizations of the saturating and probe radiations are different, deviations from the usual saturation effect may be observed.

DOI
10.1070/qe1979v009n05abeh009026

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The saturation of light absorption in a molecular gas is analyzed theoretically allowing for the redistribution of the molecules in terms of the orientations of their moments. It is shown that the amount of energy absorbed by the gas generally depends on the polarization of the incident radiation. It is noted that when the polarizations of the saturating and probe radiations are different, deviations from the usual saturation effect may be observed.
The phenomenon of stimulated Rayleigh-wing scattering in anisotropic molecular liquids is analyzed. Representative combinations of polarization for the incident and scattered light are considered. It is shown that in regions where the focused laser beam is intense, saturation occurs in the molecular response to external optical fields, in that perfect alignment of the individual liquid molecules in the optical fields is approached. Accordingly, the nonlinear amplification associated with the stimulated radiation is reduced. The results of the analysis qualitatively explain several important recent observations which are in conflict with previous theroretical predictions. For example, it is shown that the nonlinear gain is considerably greater when the incident light is circularly polarized (the stimulated radiation being circularly polarized in the reverse sense) than it is when the incident light is linearly polarized. Despite saturation, sufficient amplification exists in the former case to allow the efficient conversion of incident to stimulated Rayleigh light at moderate power levels.
This paper presents a calculation of the depth of the saturation dip observed when laser radiation of a Gaussian TEM00{\mathrm{TEM}}_{00}TEM00​ mode is absorbed in a thermal molecular beam or in a very-low-pressure gas. Transit-time broadening is assumed to be the only broadening mechanism, with neglect of collisions and other relaxations. There is no restriction to small saturation parameters. The absorption experienced by one molecule has been calculated via the optical Bloch equations. Averaging over all relevant parameters of the molecular trajectory and the laser field followed. To get a reference for the saturation, from which to measure the dip depth, simply twice the absorption from a running-wave field has been taken, which therefore also had to be calculated in addition to the calculation of the absorption in a standing wave. Preliminary experiments have been carried out for comparison, and the data agree very well with the calculated shape of the curve and the absolute values of the calculated dip depth. From the laser-power scaling factor, necessary to fit the experimental data to the universal theoretical curve, the dipole moment can be derived without having to know such hard-to-determine quantities as absorber density, partition function, collision relaxation rate, or even the diameter of the laser beam.
The optical absorption of several substances has been found to depend in a reversible way on the incident light flux [1]-[5] of a high intensity laser. Moreover, it has been observed [6]-[7] that the absorption decreases not only at the laser wavelength but simultaneously within a large band around this wavelength. These effects may be explained by a time-dependant population distribution among the different excited states of absorbing molecules during the strong laser excitation [8]. The following experimental results enable us to determine the different time constants of importance in this phenomenon.
A general theoretical description is derived for the resonance fluorescence of dilute gases and molecular single crystals with special emphasis on optical rotation spectra. The validity of previous theoretical work on dilute gases is confirmed by proving that the damping constants connecting excited states on different molecules are zero. It is shown that the intensities of optical rotation spectra of molecular crystals are much larger than the corresponding quantities for gases. An important theoretical prediction is that the natural line breadth of a resonance fluorescence emission band belonging to an exciton state in a molecular crystal should be larger than the width of the corresponding optical absorption line. The dependence of the intensity of the optical rotation spectrum on the relative orientation of the crystal with respect to the incident beam of light is discussed.
Saturated absorption resonances in a gas 13CH3F at the vibration-rotation transition R(4,3) 0–1 of vibration mode ν3 detected in a probe radiation wave transmission in a presence of unidirectional saturating wave are investigated. The width of saturated absorption resonances is increasing with the growth of saturating radiation intensity. In the limit of strong saturation, the resonance shape differs from Lorentzian one by the sharper decrease in the wings, and the resonance width allows to estimate the dipole transition moment d = 0.045 D in good agreement with literature data for this transition of 13CH3F
The saturation behavior of molecular absorbers characterized by absorption coefficients expressible as the ratio of two polynomials in the intensity is analyzed. Two cases are considered in detail. In the first, the absorption coefficient is the ratio of a linear equation in I to a quadratic equation in I. In the second, the highest order of the intensity occurring in both numerator and denominator is increased by two. The absorption coefficients are derived from two specific five-level models which are pertinent to the interpretation of the absorption of CO2 laser radiation at 937 cm−1 by SF6, but the analysis of the saturation curves is independent of the models from which they are derived. The saturation curves display as many as three extrema and provide considerable flexibility for fitting complicated experimental data. Both five-level models include excited-state absorption with the terminal level of the ground-state absorption distinct from the initial level of the excited-state absorption. Each of the five levels can be considered as a composite of many rotational states which interact in a like manner with the incident radiation. In one of the models the ratio of the excited-state and ground-state absorption cross sections is taken to be intensity dependent.