  We report a systematic investigation of both three-photon absorption
(3PA)spectra and wavelength dispersions of Kerr-type nonlinear refraction in
wide-gap semiconductors. The Z-scan measurements are recorded for both ZnO and
ZnS with femtosecond laser pulses. While the wavelength dispersions of the Kerr
nonlinearity are in agreement with a two-band model, the wavelength dependences
of the 3PA are found to be given by (3Ephoton/Eg-1)5/2(3Ephoton/Eg)-9. We also
evaluate higher-order nonlinear optical effects including the fifth-order
instantaneous nonlinear refraction associated with virtual three-photon
transitions, and effectively seventh-order nonlinear processes induced by
three-photon-excited free charge carriers. These higher-order nonlinear effects
are insignificant with laser excitation irradiances up to 40 GW/cm2. Both
pump-probe measurements and three-photon figures of merits demonstrate that ZnO
and ZnS should be a promising candidate for optical switching applications at
telecommunication wavelengths.
