TRANSVERSE BEAM PROFILE VARIATIONS IN AN INDUCED ABSORBER.

For a system showing intrinsic optical bistability due to an absorption which increases with increasing excitation density, the transverse beam profile variations are investigated assuming that longitudinal diffusion over the sample length is large enough to exclude formation of longitudinal kinks. The general features of the results apply to induced absorption in semiconductors and to other diffusive nonlinear mechanisms. The numerical results show that deviations from the plane wave results become quite pronounced for increasing electron-hole-pair diffusion. Development of a distinct ring structure is clearly seen. Near the center of the beam the density is large enough that the absorption has switched to a high value, causing a significant decrease of the central light intensity along the medium. The outer part of the beam is less absorbed than the center part, leading to development of a transverse ring structure. The transverse density profile is quite smooth and no kinks develop. There is a sharp edge in the transmitted intensity, somewhat reminiscent of a stationary switching wave. To the left of this edge the system is in the high absorbing phase; to the right it is in the low absorbing phase. The sharp edge provides a connection between the two phases.