Spatial modification of a Gaussian beam on reflection from a saturable absorber

We calculate the reflection coefficient of a dielectric-gas interface including the nonlinear local field, saturation, and self-reflected wave effects. We specialize to collisionally broadened gases with absorption lengths that are necessarily small compared with an optical wavelength and preclude the use of the slowly varying amplitude approximation. The dependence of the reflectivity on the intensity of the incoming field is revealed through the changes that it induces in the transverse profile of a reflected Gaussian beam. These changes derive principally from effective aperturing effects in amplitude and phase.