Revealing Light-Driven Dynamics at Nanostructured Solid-Liquid Interfaces with In-Situ SHG

Light and heat drive interfacial chemistry at solid-liquid interfaces, underpinning processes central to sustainable energy conversion, including photoelectrochemical and hydrovoltaic systems. Yet, non-invasive probing of light-induced interfacial dynamics remains challenging due to the weak and spatially complex nature of optical signals. Here, we introduce a nanophotonic platform that enhances second harmonic generation (SHG) from nanostructured interfaces by over two orders of magnitude, enabling real-time, all-optical access to interfacial processes. We develop a rigorous overlap-integral formalism that provides a general quantitative framework for SHG in nanostructured geometries. By accounting for spatially inhomogeneous electromagnetic fields, this approach links the nonlinear response to geometry-dependent near-field and reveals new degrees of freedom, namely independent control of attenuation and phase, which are absent in planar systems. This enables deterministic tuning of surface and electric-field-induced contributions through nanophotonic design. Using in situ SHG at silicon-oxide-electrolyte interfaces, we resolve subtle spectral shifts of ~1.3 nm with electrolyte concentration, indicating coupling between electrical double layer potential and semiconductor polarizability. Under controlled optical excitation, we observe reversible, intensity-dependent modulation of interfacial susceptibility, with a decrease at low intensities consistent with photocharging and an increase at higher intensities due to photothermal effects. These results establish nanophotonic-enhanced SHG as a quantitative and tunable probe of interfacial phenomena, providing a unified framework linking optical response, electrostatics, and geometry, and opening new avenues for controlling interfacial charge and potential with light for applications in energy conversion, catalysis, and nanophotonic devices.