IMPURITY-RELATED OPTICAL NONLINEARITY AND BISTABILITY IN THE BAND-GAP REGION OF CDS-IN

Indium-doped bulk CdS exhibits strong broad-band adsorption in the energy regime below the band gap which can be probed with a weak laser. This absorption is easily bleached if simultaneously a second, narrow-band laser beam of high intensity impinges on the crystal. This effect is explained by a two-level model taking into account transitions from and into an In-related shallow-acceptor band created due to the strong self-compensation of CdS if doped with any donor element. The same uncoated CdS : In samples show absorptive optical bistability in a wide wavelength range of the incident laser. The switching time (as short as 700 ps minimum) as well as the switching contrast depend strongly on the impurity concentration and the excitation density. Filling of the acceptor band with holes combined with a positive feedback by light reflection from the perfect crystal surfaces explains this fast bistable phenomenon. Detailed investigations of the bistability data in dependence of the dopant's concentration, excitation density and wavelength, as well as of temperature strongly support our interpretation. The possibility of influencing the optical bistability by changing the impurity content is suggested as an apparent advantage for application. © 1990.