Extremely nondegenerate two-photon absorption in direct-gap semiconductors [Invited].

Two-photon absorption (2PA) spectra with pairs of extremely nondegenerate photons are measured in several direct-gap semiconductors (GaAs, CdTe, ZnO, ZnS and ZnSe) using picosecond or femtosecond pulses. In ZnSe, using photons with a ratio of energies of ~12, we obtain a 270-fold enhancement of 2PA when comparing to the corresponding degenerate 2PA coefficient at the average photon energy (ηω1 + ηω2)/2. This corresponds to a pump photon energy of 8% of the bandgap. 2PA coefficients as large as 1 cm/MW are measured. Thus, by using two widely different wavelengths we are able to access the large 2PA observed previously only in narrow gap semiconductors. We also calculate the corresponding enhancement of nonlinear refraction, consisting of two-photon, AC-Stark and Raman contributions. The net effect is a smaller enhancement, but exhibits very large dispersion within the 2PA regime.