  Using the Landau-de Gennes theory, the temperature, pressure and frequency
dependence of the non-linear effect in the isotropic phase above the
isotropic-cholesteric phase transition is calculated. The influence of pressure
on the isotropic-cholesteric phase transition is discussed by varying the
coupling between the orientational order parameter and the macroscopic
polarization of polar cholesterics. Comparing the results of the calculations
with existing data, we finally conclude that the model provides a description
of the isotropic-cholesteric transition that takes all experimentally known
features of the unusual negative and positive pretransitional effect in the
isotropic phase of the system into account in a qualitatively correct way.
