Infrared saturable absorbers. Part I: Five-level rate-equation analysis

Abstract

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.