Abstract

A rigorous study of degenerate four-wave mixing in two- and three-photon absorbing semiconductors with picosecond pulses has been performed. In the two-photon absorbing regions, we found two different nonlinearities to be present. The first is due to excitation of a free-carrier grating by two-photon absorption which scatters a third beam to produce a phase conjugate. Thus this is a sequential ${\chi }^{(3)}-{\chi }^{(1)}$ process which appears as a fifth-order nonlinearity. The second nonlinearity is found to be fast, independent of grating period, and described by a third-order susceptibility which in ZnSe at 532 nm is 2 orders of magnitude greater than that of CS₂. These results were replicated at 1.06μm in CdTe and thus seem to be generally applicable to two-photon absorbing semiconductors. Polarization selection has shown that this nonlinearity is of the grating type. Despite the two-photon resonance, the two-photon coherence term was found to be absent. This behavior was found to be replicated in ZnSe at 1.06μm, now in the three-photon absorption regime. Here a fast χ⁽⁵⁾process is accompanied by a slow χ⁽⁷⁾process [explained by sequential three-photon absorption followed by linear refraction ${\chi }^{(5)}-{\chi }^{(1)}$. The fast nonlinearities seem to be intimately linked to the two- and three-photon absorption. By variation of pump-probe angles and input irradiances, the density dependence of the effective diffusion coefficient has also been determined.

© 1988 Optical Society of America