Three-photon absorption in ZnO and ZnS crystals

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 ( 3E(photon)/ E-g - 1) (5/2)( 3E(photon)/ E-g) (-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/ cm(2). 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. (c) 2005 Optical Society of America.