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A general approach to the light-induced anisotropy (LIA) effects in initially isotropic media subjected to a biharmonic laser field (pump-probe), that is based on a tensor operator formalism, is presented. Using such a theoretical treatment, we propose novel nonlinear spectroscopic techniques. We show new results dealing with advantageous measurement of the 3rd-order nondegenerate nonlinear susceptibility (3NNS), including the 3NNS phase (3NNSP) and the real and imaginary parts of the effective 3NNS. Figure 1 is the schematic illustration of a method for measuring 3NNSP in the framework of optically heterodyned polarization spectroscopy (OHPS) when the components of 3NNS are connected by the Kleinman relationships. In the standard OHPS arrangement with the slightly tilted analyzer and the linearly polarized probe beam at 45 to the major axis of the pump polarization ellipse, under tuning of the pump intensity W) or ellipticity (tans) we can find the minimum OHPS signal (AI which is determined by 3NNSP. Another possibility for measuring 3NNSP occurs when the linearly polarized pump and circularly polarized probe beams are used. In this case, the tangent of double azimuth of the output probe polarization is exactly equal to 3NNSP. The validity of the theoretical treatment presented is supported by the experimental data. Furthermore, we point out that some previous theoretical studies devoted to the similar problems are not quite correct and should be revised. In Fig. 2 the nonlinear polarization spectroscopy signal normalized to the maximum value is plotted as a function of c. The line 1 is fitted with experimental data for malachite green in water.1 The experimental results for crystal violet, methylene-blue and rhodamine-B in aqueous solutions are shown by circles, triangles and squares, respectively. They are in rather good agreement with our experimental curve 2.
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