Coherent spectroscopy of isotropic noncentrosymmetric media based on measurement of the frequency dispersion of a nonlinear optical activity

A phenomenological analysis (in terms of the nonlinear optical susceptibility) is given of a novel form of nonlinear optical spectroscopy based on measurement of a nonlinear optical activity (NOA) under the conditions of coherent anti-Stokes Raman scattering (CARS) in noncentrosymmetric media (nonracemic solutions and mixtures of mirror-asymmetric molecules). The dependence of the NOA on the state of polarisation and directions of propagation of the interacting waves is determined. The conditions are identified for discrimination of a quadrupole NOA–CARS signal from the background of the much stronger dipole CARS and optical-Kerr-effect signals. An estimate is obtained of the role of cascade processes due to lower-order (quadratic) nonlinearities of the investigated medium. A comparison is made of the NOA–CARS and Raman optical activity spectroscopy methods in the case of spontaneous Raman scattering. The intensities of the expected signals are estimated. The proposed nonlinear spectroscopy method is shown to be promising for the study of bioorganic molecules in aqueous solutions.