Two-Photon Absorption and Photorefraction in Semiconductors and Ferroelectrics

Doktorsavhandling, 1994

Two-photon absorption spectra of Al0.4Ga0.6As/GaAs quantum wells in static electric fields are investigated experimentally. A drastic field-induced increase in the absorption peaks at energies close to half the bandgap energy is reported for the first time. In particular, we observed the rise and red-shift of a peak corresponding to halfthe lowest light-hole exciton energy not seen in non-biased quantum wells. For moderate fields, the results agree with a theory based on a zero-bias infinite- level model, whereas the large peak value at stronger fields is explained by a two-level model. Contrary to bulk GaAs, a large value of the third order optical susceptibility is predicted just below the two-photon absorption edge at which the biased quantum wells are highly transparent. This gives a large figure of merit for ultrafast optical nonlinearities. We have also theoretically and experimentally investigated time-resolved two-photon absorption in bulk GaAs and Ga-doped CdTe with picosecond pump-probe excitation. It is shown that the transmittance change of a weak probe by pump-induced two-photon absorption follows a temporal correlation between the pulses, provided that free carrier absorption and linear absorption are negligible. Contrary to GaAs, we show that CdTe:Ga exhibit a much larger pump-induced free-carrier absorption as well as a saturation of the linear absorption. The possibility of high-accuracy pulse width measurements through such pump-probe correlations evaluated at different probe delays is also demonstrated. Finally, the stoichiometric, photorefractive, dielectric and thermooptic properties of Cr-, Cu- and undoped potassium sodium strontium barium niobate are investigated. In particular, near-infrared photorefractivity in the Cr-doped sample is demonstrated with a two-beam coupling gain of 2 cm-1 at .lambda. = 8400 Å. Cr-doping is also found to increase the gain at .lambda. = 5145 Å by about 10 times while the response speed remains essentially unchanged. In the Cu-doped sample, we observed peculiar quasi-periodic gain fluctuations and they are analysed by means of nonlinear coupled wave equations with extra cross-phase modulation terms supposedly originating from a thermal grating. We also determined the thermooptic coefficients from thermal self-focusing experiments. Using these numerical values in the coupled wave equations, we could not, however, reproduce the observed quasi-periodic oscillations. On the other hand, chaotic solutions similar to the observed fluctuations were obtained when larger values of the thermooptic coefficients were used in the simulations.