Near band-gap nonlinearities of ZnSe crystals

The origin and dynamics of optical nonlinearities in ZnSe crystals near band gap have been investigated by four-wave mixing technique. A modulation of refractive index due to localized states at excitation energies below the band-gap was observed. The ambipolar diffusion coefficient of nonequilibrium charge carriers and the longitudinal relaxation time were estimated to: D-a = (1.6+/-0.3) cm(2)/s and T-l=(4.5+/-0.5) ns, respectively. An observed increase of the intensity at the ''coherence pike'' has been explained by a photorefractive effect induced by the interference electric fields of pump beams. The space charge induced photorefractive effect causes a modulation of the refractive index 100-300 ps after excitation.