Frequency modulation coherent anti-Stokes Raman spectroscopy (FM-CARS): A novel sensitive nonlinear optical method

A frequency modulation coherent anti-Stokes Raman spectroscopy (FM-CARS) is proposed. Using phase-modulated Stokes input field, a frequency modulated CARS output is generated. Raman resonances are detected by a CARS heterodyne signal generated in a square-law photodetector. Theoretical estimations show that a high signal-to-noise ratio which is shot-noise limited and effective background suppression are expected with the proposed method independent of the lasers polarization. In typical solvents the sensitivity estimated is χ(3)R=4–8×10−18 esu, which corresponds to the detection of 1–2×10−4 M benzene in cyclohexane.