Coherent active spectroscopy of optically isotropic media formed by randomly oriented elements

The technique of irreducible tensor operators is used to obtain an accurate expression for the macroscopic cubic susceptibility tensor of a homogeneous isotropic medium formed by randomly oriented molecules (liquids and gases). The result is expressed in terms of the invariants of the molecular hyperpolarizability tensor. Purely rotational and vibrational-rotational molecular transitions are considered. The conditions for maximum intensity of the coherent active Raman (anti-Stokes) spectroscopy (CARS) signal are determined for a finely dispersed medium (crystalline powder). A separate analysis is made of CARS of a noncentrosymmetric crystal powder in which suppression of the cascade process by the quadratic nonlinearity is possible. The first experimental observation of an ARS signal from a finely dispersed sample is reported.