Stimulated orientational scattering and third-order nonlinear optical processes in nematic liquid crystals

Abstract

We study two different effects intimately related to the existence of a third-order nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$ in a nematic liquid crystal. We first present data and calculations on stimulated orientational scattering and show that it can be accounted for by a four-wave mixing process in which noise is coherently amplified to the extent that it can deplete the input pump wave almost completely. On the other hand, we show the formal relation between four-wave mixing (as a laser-induced grating process) and the theory of photonic band structures, giving rise to a suitable formalism for the study of different experimental situations. It is suggested that the photonic crystal approach to four-wave mixing can be used to evaluate the diffraction efficiencies of the probe beam for different polarizations and scattering directions in a relatively easy manner. Finally, we predict by using this formalism the existence of umklapp four-wave mixing events in which the phase matching of the waves is achieved by a reciprocal lattice vector of the grating itself.