Plasmonic- and Electronic-Enhancement-Free Coherent Raman Detection of Ångström-Scale Molecular Layers at Metal Interfaces.

Coherent Raman scattering provides highly sensitive vibrational analysis through nonlinear light-matter interactions. However, its application to metal interfaces remains challenging because the intrinsically large nonresonant background (NRB) of metals overwhelms weak interfacial molecular vibrational signals. Here, we report a time-frequency hybrid coherent Raman spectroscopy approach that overcomes this limitation and enables the sensitive detection of ångström-thick molecular systems even on atomically flat metal surfaces. Our method combines a femtosecond pump and Stokes pulses with a time-delayed, asymmetrically shaped picosecond probe pulse. By exploiting the instantaneous temporal response of the metal NRB, this scheme effectively filters out the dominant NRB while retaining a controlled residual NRB that acts as an internal local oscillator, enabling the interferometric amplification of weak interfacial vibrational signatures. This all-optical coherent enhancement strategy establishes a route for direct, noninvasive Raman detection of interfacial molecular systems across diverse surfaces without requiring structure- and material-specific plasmonic and electronic enhancement mechanisms.