Sagnac-enhanced impulsive stimulated Raman scattering for highly sensitive low-frequency Raman spectroscopy

The "fingerprint" (500-1800  cm^-1) and "high-frequency" (2400-4000  cm^-1) regions in Raman spectroscopy are commonly used for label-free chemical analysis, while the "low-frequency" region (<200  cm^-1) is often overlooked, despite containing rich information. This is largely due to the challenge of measuring weak Raman signals that are obscured by strong Rayleigh scattering. Here we propose and experimentally demonstrate Sagnac-enhanced impulsive stimulated Raman scattering (SE-ISRS), a filter-free method for time-domain Raman spectroscopy that overcomes the challenge and provides low-frequency Raman spectra at all probe frequencies. Using SE-ISRS for simultaneous low-frequency and fingerprint region measurements, we demonstrate a >5× enhancement of the signal-to-noise ratio compared to conventional ISRS spectroscopy.