Theoretical study of mechanisms of non-linear optical response in liquid crystals

Here, we carry out a theoretical analysis of the z scan experiments used to measure the non-linear optical properties of liquid crystals, with the aim of clarifying the mechanism responsible for the observed non-linearity in the nanosecond regime. We consider various orders of non-linear absorption and non-linear refraction in order to illustrate their effects on the resulting z scan curve. The shape of the z scan curve, in principle, contains information about the order of the non-linearity. Qualitative features of the experimental z scan curves are consistent with a fifth order absorption and a third order refraction. However, given the level of disagreement between the calculated and experimental curves, it is not possible to make quantitative statements about the coeffients of the various orders. The magnitude of the refractive part of the non-linearity is consistent with laser heating due to linear absorption. The temporal profile of a two pulse experiment is also consistent with this mechanism. We suggest that the temperature dependence of the non-linear refraction might be explained in terms of a thermal mechanism by accounting for the temperature dependence of the rate of relaxation of the orientational order.