STIMULATED ORIENTATIONAL SCATTERING AND 3RD-ORDER NONLINEAR-OPTICAL PROCESSES IN NEMATIC LIQUID-CRYSTALS

We study two different effects intimately related to the existence of a third-order nonlinear optical susceptibility chi((3)) in a nematic l iquid crystal. We first present data and calculations on stimulated or ientational scattering and show that it can be accounted for by a four -wave mixing process in which noise is coherently amplified to the ext ent 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 struct ures, giving rise to a suitable formalism for the study of different e xperimental situations. It is suggested that the photonic crystal appr oach to four-wave mixing can be used to evaluate the diffraction effic iencies of the probe beam for different polarizations and scattering d irections in a relatively easy manner. Finally, we predict by using th is formalism the existence of umklapp four-wave mixing events in which the phase matching of the waves is achieved by a reciprocal lattice v ector of the grating itself.