Nonlinear Energy Transfer in Semiconductors Yielding Optical Bistability

Abstract

Nonlinear energy transfer from a high intensity photon field to the electronic system of a semiconductor resulting in an absorptive optical bistability was investigated. The high intensity photon field creates a high density of free carriers forming an electron hole plasma. In the plasma density-dependent renormali-sations occur: the band gap is strongly shifted to lower energies giving rise to excitation dependent indicies of refraction and absorption. K. Bohnert explained how these effects were applied for a realization of absorptive optical bistability. For this purpose high intensity nanosecond laser pulses in the spectral range below the excitonic resonances were transmitted through a thin CdS sample. During the temporal development of the pulse the band gap shifts from its original position to energies below the laser energy and back again to its original value. When it passes the laser energy there are transitions from two to one photon absorption and from one to two photon absorption when the gap goes downward and upward, respectively. These transitions show up as jumps of the transmitted intensity. Because they occur at different intensities of the incident laser light, they give rise to an absorptive optical bistability. The switching times are in the subnanosecond range. The dependence of the bistability on photon energy was investigated and discussed.