2ND-HARMONIC GENERATION BY A GAUSSIAN-BEAM ON A 4TH-ORDER OPTICAL NONLINEARITY IN THE INTERIOR OF AN ISOTROPIC AND GYROTROPIC MEDIUM

A theoretical investigation is made of second-harmonic generation in t he interior of an isotropic and gyrotropic medium on a fourth-order op tical nonlinearity. It is shown that a correct description of this exp erimentally detected effect requires taking into account the spatially finite nature of the fundamental-frequency wave, giving rise to a sma ll longitudinal (directed along the beam axis) component of the electr ic field. Quadrature formulas are derived for the transverse distribut ion of the second-harmonic field and for the total power of the harmon ic. A detailed study is made of the special cases of tight focusing of the pump (fundamental-frequency) wave at the centre of a long medium and of exact phase matching. In these cases the quadrature expressions can be reduced to an analytic form. A study is reported of the charac teristic features of the transverse profile of the second-harmonic int ensity and of the dependence of the harmonic power on the state of pol arisation of the pump wave.