Abstract

Recently we reported large third-order nonlinear susceptibilities in semiconductors in the presence of two-photon absorption as measured on a picosecond time scale DFWM.^1,2 These nonlinearities have fast response (resolution limited by the laser pulse width ≃25-ps FWHM) and are distinguished from the slowly decaying carrier effects which appear as fifth-order effects. Using the newly developed Z-scan technique (a simple single beam method for n₂ measurements³), we have determined the sign and magnitude of these nonlinearities. These measurements show the changing sign of n₂ from positive to negative ZnSe and CdS when going from 1- to 0.5-μm radiation. Using a Kramers-Kronig transformation on the two-photon absorption spectrum of the direct gap semiconductors as calculated from second-order perturbation theory, we have been able to account not only for the dispersion but the magnitude of these nonlinearities. This simple formalism yields a general scaling formula for n₂ as where A is a nearly material independent constant and G₂ is the calculated dispersion function. The above expression also gives an estimate of the low frequency electronic n₂ (positive) for a large number of transparent materials (e.g., glasses) which possess nonlinearities that are 2-3 orders of magnitude smaller than the semiconductors.

© 1989 Optical Society of America