Nonlinear Bragg structures based on ZnS/ZnSe superlattices

Nonlinear optical response of periodic ZnSe/ZnS heterostructures is reported using interband excitation of a ZnSe sublattice by nano-, pico- and femtosecond laser pulses. A considerable shift of the reflection spectrum and large relative reflection changes were observed in a wide spectral range corresponding to the transparency region of ZnSe far from the intrinsic absorption onset. Evaluated refraction-index change is about −0.05 with the relaxation time being about 3 ps. In the case of femtosecond excitation, a wide-band nonlinear response is observed for both one-photon near-UV and two-photon near-IR excitation. The nonlinear refraction is supposed to be controlled by population-induced absorption changes in ZnSe single crystals and relevant refraction-index modification via Kramers–Kronig relations. The nonlinearity relaxation time is supposed to trace a transition from a non-equilibrium to a quasi-equilibrium distribution of electrons and holes within ZnSe conduction and valence bands, respectively, rather than the electron–hole recombination time. The nonlinearity mechanism does not reduce to just population-dependent absorption saturation, but essentially results from the specific distribution function in the first instance after excitation.