Plasmonic- and Electronic-Enhancement-Free Coherent Raman Detection of {\AA}ngstr\"om-Scale Molecular Layers at Metal Interfaces

Abstract

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