Transverse effects in increasing absorption optical bistability

Experimental verification of the appearance of transverse signal rings in the transmitted intensity profiles of an incident Gaussian laser beam due to nonuniform switching of a thin semiconductor in response to the varying beam profile is presented. The appearance of new scattering orders in the scattering spectrum from degenerate four-wave mixing (DFWM) with a self-diffraction geometry, due to transverse spatial frequencies introduced by the nonlinearity, is also confirmed. Experiments were performed with an approximately 5-μm platelet CdS held at a temperature of 10 K. Increasing absorption optical bistability (IAOB) in this sample is attributed to photoinduced electron-hole pairs, which modify the optical absorption spectrum primarily through exciton broadening and bandgap renormalization. A single dimension of the spatial ring formed in the transmitted beam profile as a function of time is shown along with the input beam profile. The waist of the beam is 90 μm FWHM. Self-diffraction from the transient grating formed during DFWM shows the appearance of new subharmonic scattering orders if the nonlinearity is due to IAOB. The experimental development of the scattering spectrum (far field) with pulsed excitation is shown. The appearance of new subharmonic scattering orders is apparent.