We suggest a novel Raman spectroscopy method to suppress the nonresonant background below the shot-noise limit (ideally to zero) using nonclassical nonlinear interference.

Coherent anti-Stokes Raman Spectroscopy (CARS) has become one of the most useful label-free spectroscopic techniques whereby chemical composition can be determined based on molecular vibrational spectra. While providing greatly enhanced gain over standard Raman spectroscopic methods, a major technical difficulty in this method has been the usual presence of a strong non-resonant background spectrum from surrounding molecules, such as a solvent or a background lattice, which greatly outnumber the molecules of interest and thus can completely mask the vibrationally resonant spectra.

Here we present the theoretical study of a new method for Raman spectroscopy by measuring the nonlinear phase shift introduced by the resonant Four-Wave mixing (FWM) Raman interaction, using an SU(1,1) interferometer. We show that addition of a non-linear medium (such as a Raman sample) in-between the two OPAs of the interferometer couples between the gain of the amplifiers and the Raman sample, resulting in enhanced, sub-shot noise signal detection. This configuration benefits from the squeezed light of the interferometer for complete rejection of the stimulating light and the non-resonant background, as well as the noise associated with them, down to the vacuum level.

We also show that seeding of the interferometer with coherent idler input further increases the measured Raman signal, similar to how the standard CARS and SRS techniques enhance the spontaneous Raman signal. Thus, our suggested scheme effectively behaves as "enhanced spontaneous gain", and aims to supplement the previously developed Raman techniques.