Polarization effects of picosecond CARS in liquids

Coherent Raman scattering of delayed probing pulses after ultrafast excitation is investigated under generalized polarization conditions. Three factors are shown to contribute to the scattering signal: (i) scattering off the isotropic part of the resonant material excitation via the isotropic component of the Raman polarizability (ii) scattering via the anisotropic part of the Raman polarizability from a second, oriented component of the resonant material excitation (iii) four-wave mixing via the non-resonant part χ_NR of the third-order susceptibility. We demonstrate theoretically and experimentally that different polarization conditions lead to drastic changes of the signal transients in liquids. For the ring breathing mode of C₆H₅Br the ratio of non-resonant to resonant contributions is measured to be χ_NR/χ_res=0.037±0.015.