Degenerate four-wave mixing in the isotropic phase of liquid crystals: The influence of molecular structure

We review the ways in which the molecular characteristics of nematogenic fluids influence the efficiency of degenerate four-wave mixing and illustrate them in experiments on several materials. The nematic pretransitional effect makes the nonlinear susceptibility very large, but also slows the response times. This limits the maximum effective nonlinearity for a given laser pulse length. Although all nematogens show a universal pretransitional behavior, the rate at which the susceptibility and response time diverge, on approaching the transition point, is very dependent on the chemical structure of the molecule involved and may also be influenced by adding solvents or forming mixtures with other nematogenic molecules. We identify the influences on our results of molecular shape and polarizability, the viscosity of the fluid, and of specific intermolecular interactions, such as those leading to local smectic ordering.