Coherent anti-Stokes Raman spectroscopy in the presence of strong resonant signal from background molecules.

ABSTRACT

Spectroscopy of molecular mixtures with broadband femtosecond laser pulses often involves simultaneous excitation of multiple molecular species with close resonance frequencies. Interpreting the collective optical response typically requires Fourier analysis of time-resolved signals. We propose and demonstrate an alternative method of separating coherent Raman scattering signals from two molecular components with neighboring vibrational modes. We exploit "silence windows," which arise from coupling of the vibrational and rotational degrees of freedom in molecular dynamics. In silence windows, the detected signal stems solely from the minority species (here, CO2), while the strong resonant background from the dominant species (O2) is greatly suppressed.