Broadband multiplex coherent anti-Stokes hyper-Raman scattering (CAHRS) spectroscopy with a white-light continuum

Hyper-Raman (HR) spectroscopy provides complementary vibrational information to infrared and Raman spectroscopy, yet its application is limited by extremely weak signals. To overcome the limitation, we demonstrate broadband multiplex coherent anti-Stokes hyper-Raman scattering (CAHRS) spectroscopy using a white-light continuum as the Stokes pulse. This approach expands the measurable spectral range to 500-2000 cm-1, enabling simultaneous detection across the entire molecular fingerprint region. By applying the maximum entropy method to the intensity-corrected CAHRS spectra of an all-trans-retinal solution, we successfully extracted the imaginary part of the fifth-order non-linear susceptibility {Im[χ(5)]}. The retrieved spectrum exhibits nice agreement with its spontaneous HR spectrum, validating the accurate recovery of HR resonant phase information. We applied this technique to a fluorescent laser dye, Coumarin 540A, whose spontaneous HR spectrum is completely obscured by intense multi-photon fluorescence. Due to the directional nature of the CAHRS process, we spatially isolated the signal from the isotropic fluorescence background. The retrieved Im[χ(5)] spectrum demonstrates that broadband multiplex CAHRS is a powerful and viable tool for elucidating the vibrational modes of fluorescent molecules that are otherwise inaccessible.