Self-focusing and spatial ringing of intense cw light propagating through a strong absorbing medium

This paper deals with the strongly nonlinear regime of the atom and field interaction which arises for both high cw input intensities and large absorption. The intensity of the light inside the medium is then highly nonlinear with respect to the incident one, the order of magnitude of the nonlinearity being approximately $\alpha 1$, for a cell of length $l$ and an absorption length ${\alpha }^{-1\mathrm{}}$. The transverse effects related to such a regime are analytically treated. They are shown to induce self-focusing, blooming, and also spatial ringing. These features strongly depend on the distance of the screen from the cell, on the detuning between the incident wavelength and that of the atomic transition, and on the input intensity. For self-focusing, analytical expressions of the focal point, of the waist, and of the maximum intensity are given. This model for transverse effects in strong nonlinear propagation of pulses would contribute to explain recent anomalous observations.