The optical cubic susceptibility dispersion of some transparent thin films.

Abstract

The aim of this dissertation has been to investigate the third order nonlinear susceptibility dispersion of molecules and polymers in order to estimate their purely electronic nonlinear response and in particular their optical Kerr nonlinear susceptibility in the 1 to 2 μm infrared spectral region. We have been successful in modeling with a four level system the near resonant cubic susceptibility of the polydiacetylene, poly(4-BCMU). In that case tunable Third-Harmonic-Generation and Two-Photon-Absorption measurements both agreed with the result of a Near-Infrared-Three-Wave-Mixing measurement. In the case of β-carotene and polythiophenes the four level model also fits the Third-Harmonic-Generation data well. In the previously mentioned cases the spectral behavior of the Two-Photon figure of merit derived by Mizrahi et al. is calculated. The four level model, when extrapolated far off resonance predicts that an all-optical switching device constructed with these materials will be dominated by Two-Photon-Absorption. A promising new class of side-chain substituted polymers with large microscopic second order nonlinearities was also investigated. Third-Harmonic-Generation measurements indicate that no forbidden two-photon transition is present in this case and that the magnitude of the nonlinear third order susceptibility is dominated by the charge-transfer nature of the nonlinear moieties combined with cascading of second order effects at a microscopic level. In the case of Sol-Gel thin films of varying TiO₂ concentration in SiO₂, the formula derived by Boling et al., based also on a three level model, predicts successfully the magnitude of the third order susceptibility. THG is in that case an invaluable technique used for the first time to measure relatively small nonlinear susceptibilities of glass-like thin films.