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We propose and experimentally validate a technique for four-wave mixing background suppression in coherent anti-Stokes Raman spectroscopy. It is based on the interaction of the signals generated from the Kerr third-order nonlinearity and the cascaded quadratic process in a nonlinear crystal. Theoretical analysis agrees well with the experimental results, which provide a quantitative assessment of different contributions and allow extraction of the nonlinearity parameters.

We study interference between a local oscillator and coherent anti-Stokes Raman scattering signal fields by controlling their relative phase and amplitude. This control allows direct observation of the real and imaginary components of the third-order nonlinear susceptibility (chi((3))) of the sample. In addition, we demonstrate that the heterodyne method can be used to amplify the signal.

We demonstrate broadband light generation by focusing two-color ultrashort laser pulses into a Raman-active crystal, lead tungstate (PbWO(4)). As many as 20 anti-Stokes and 2 Stokes fields are generated due to strong near-resonant excitation of a Raman transition. The generated spectrum extends from the infrared, through the visible region, to the ultraviolet, and it consists of discrete spatially separated sidebands. Our measurements confirm good mutual spatial and temporal coherence among the generated fields and open possibilities for synthesis of subfemtosecond light waveforms.