Transverse structures in resonatorless absorptive switching in bulk ZnSe

Absorptive switching and transverse structures in resonatorless optical bistability in bulk ZnSe is observed upon exciting the sample at the Urbach edge. The switching is due to nonlinear absorption induced by a temperature tuning of the band gap. We have modeled such behavior by numerically solving for the intensity, carrier density, and temperature, including both longitudinal and transverse fluctuations. Simulations show that both longitudinal and transverse heat diffusion are necessary in producing specific structures, as well as in determining the time of switching. During switching, the experiment and theory illustrate for the intensity profile the formation of a local minima on axis (hole) followed by a local maxima on axis and local minima off axis (rings). Such beam distortions and bistable switching are the result of the spatial and temporal dynamics and positive feedback existing among the absorption and heating.