# Two-Photon Absorption and Photorefraction in Semiconductors and Ferroelectrics

[Doktorsavhandling]

Two-photon absorption spectra of Al_{0.4}Ga_{0.6}As/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 *half*the 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.

Denna post skapades 2006-09-19. Senast ändrad 2013-09-25.

CPL Pubid: 1381