Shift and Broadening of Absorption and Dispersion Resonance by Saturation Parameter

Within the framework of the classical model, which describes electrons as damped harmonic oscillators in an external electromagnetic field, expressions for the absorption and dispersion coefficients in the second order of smallness in the saturation parameter are found for a uniformly broadened line at a low concentration of oscillators, when the amplitude of the field produced by neighboring electrons is significantly less amplitude of the external field. It is shown that with increasing saturation parameter, the shape of the dispersion resonance becomes asymmetric, that at small saturation parameter leads to its linear shift to the "red" region. At the same time, as the saturation parameter increases, the dispersion resonance broadens (increases the interval between the maximum and minimum of the dispersion curve) and, in addition, there is a relative increase in the low-frequency amplitude compared to the high-frequency one. As for the shape of the absorption line, just as for the dispersion resonance, as saturation parameter increases, an asymmetry of the line shape appears which is accompanied by a shift to the "red" region. Because the low-frequency wing of the absorption line shifts toward the resonance center faster than the high-frequency wing moves away from the center, a narrowing of the resonance from the saturation parameter is observed. The dependence of this narrowing is nonlinear. In this case, the maximum of the absorption resonance linearly shifts to the "blue" region at the same speed as the dispersion line shifts to the "red" one.