Orientational effects in electric-field-induced second-harmonic generation

The theory of electric-field-induced second-harmonic generation is developed within the framework of quantum electrodynamics. Using a recently developed exact averaging procedure to account for the anisotropy introduced to a dipolar molecular fluid by the static electric field, the polarisation dependence of the process is investigated. A number of experimental arrangements are suggested which allow a discrimination to be made between the two mechanisms involved: the third-order optical route facilitated by the field-induced anisotropy, and the fourth-order electro optical route. This allows measurement of the corresponding non-linear optical and electro-optical tensor parameters. The full temperature and field-strength dependences are presented in detail, and it is shown that in two particular polarisation cases, that of helicity conversion, and where the static field and pump polarisation are perpendicular to the polarisation of the harmonic, the usual approximation for the orientational average is incomplete and produces misleading results.