Role of collision anisotropy in two-photon processes in a magnetic field. Phenomenological analysis

It is shown that the nonlinear susceptibility tensor of an isotropic medium is axially symmetric because of the anisotropy of collisions characterized by a symmetry axis along the field wave vectors. In the collinear geometry a two-photon process is described by six functions of the frequencies and wave vectors of the fields, whereas in the case of a mirror-symmetric medium it is described by three such functions. A new difference method is proposed for separating just the collision anisotropy contribution to the signal of a two-photon process in a magnetic field. The method makes it possible to obtain several types of difference signal. In a probe field variant the number of possible signals is four and eight for mirror-symmetric and mirror-asymmetric media, respectively. In a variant involving observation of fluorescence at right-angles to the magnetic field and also parallel to the direction of propagation of the exciting light, the number of possible difference signals is eight and sixteen for mirror-symmetric and mirror-asymmetric media, respectively, whereas in the case of observations along a magnetic field there is only one type of signal. An analysis is made of the parity of the difference resonances relative to reversal of the magnetic field direction.