Suppression of forward bulk signal for buried-interface nonlinear spectroscopy

Nonlinear optical spectroscopy has been widely established as a powerful tool for probing interfaces with molecular specificity. However, for buried interfaces, the interfacial response is often obscured by strong bulk radiation when the adjoining media on both sides also exhibit nonlinear optical activity. Using sum-frequency generation (SFG) spectroscopy as an example, we demonstrate a general and experimentally simple strategy to suppress forward-generated bulk contributions from a second-order nonlinear crystal and isolate the buried interfacial response. The key idea is to tune the emission geometry near the Brewster condition at the exit interface, where the [Formula: see text]-polarized Fresnel reflection sharply decreases, strongly attenuating the reflected forward-generated bulk field. The residual forward field then interferes destructively with the backward (BW)-generated field, providing additional suppression of the nonresonant bulk background. Experiments on an octadecyltrichlorosilane (OTS) monolayer at a quartz buried interface show that this Brewster-angle suppression reduces the bulk background by [Formula: see text]two orders of magnitude and enables reliable retrieval of monolayer vibrational features in the C–H stretch region. Angle-dependent analysis further shows that the optimal signal-to-noise ratio (SNR) occurs slightly detuned from the bulk-intensity minimum, consistent with interference-based contrast. This approach enables high-fidelity in situ interrogation of buried interfaces across a broad range of systems and provides a general framework for quantitative analysis of interfaces embedded in complex environments.