The dispersion and symmetry of optical nonlinearities in semiconductors.

摘要

We report a new time-resolved 2-color Z-scan technique for measuring the nondegenerate nonlinear optical absorption and refraction at a frequency {dollar}omegasb1{dollar} due to the presence of light at frequency {dollar}omegasb2{dollar} in solids. For semiconductors with excitation pulsewidth in the picosecond range, we use this technique to time resolve the "instantaneous" bound-electronic effects and the cumulative free-carrier effects (decay time longer than the pulsewidth). We also use this technique to separately determine the sign and magnitude of nonlinear refraction, and the value of nonlinear absorption. The measured electronic nonlinear refractive index {dollar}nsb2{dollar} is compared with calculations using the newly developed theory based on a two-band model and the nonlinear Kramers-Kronig relation. This theory predicts the magnitude and dispersion of {dollar}nsb2{dollar} from the calculated spectrum of nonlinear absorption which depends only on the bandgap energy and the linear refractive index. Good agreement is achieved.; Time-resolved, polarization dependent, degenerate four-wave mixing experiments have been used to study the symmetry of independent susceptibility elements in semiconductors. We have observed that for ZnSe, where the photon energy is close to the bandgap {dollar}Esb{lcub}g{rcub}{dollar}, the effective susceptibility nearly vanishes when the probe polarization is perpendicular to both the forward pump and backward pump. By contrast, the effective susceptibility for large bandgap materials such as NaCl, PbF{dollar}sb2{dollar} and BK-7 glass under the identical experimental condition is approximately one-third of the susceptibility when all input beams are parallel polarized. Based on a three band model, a qualitative explanation is given in terms of the cancellation of two nonlinearities when two photon absorption is present. We also point out that the susceptibility including time ordering is more appropriate for describing the nonlinear processes in semiconductors when the excitation is close to the bandgap energy of the material.