Polarization‐Resolved Approach to the Origin of the Nonresonant Background in Coherent Anti‐Stokes Hyper‐Raman Scattering Spectroscopy

Higher-order spectroscopy is a powerful tool for investigating molecular structures and dynamics. Coherent anti-Stokes hyper-Raman scattering (CAHRS) spectroscopy based on a fifth-order nonlinear vibrational spectroscopy is a new spectroscopic technique that provides hyper-Raman spectra at high speed. Even though CAHRS spectroscopy has the potential for applications in various fields, the nonresonant background signals, which contain no information on vibrational resonances, degrade the signal-to-background ratios of CAHRS spectra. Detailed information on the origin of the nonresonant background signals in CAHRS spectroscopy is necessary to extract information on vibrational resonances efficiently. We have demonstrated polarization-resolved CAHRS measurements for eight liquids under the vibrationally nonresonant conditions. The polarization dependences of CAHRS intensities have revealed that cascading third-order processes, which involve third-harmonic generation followed by the third-order difference-frequency generation, significantly contribute to the vibrationally nonresonant CAHRS signals.