Pioneering second-order nonlinear vibrational nanoscopy for interfacial molecular systems beyond the diffraction limit

Sum-frequency generation (SFG) is a powerful vibrational spectroscopy that can selectively probe molecular structures at surfaces and interfaces, but its spatial resolution has been limited to the micrometer scale by the diffraction limit of light. Here, we overcame this limitation by utilizing a highly confined near field within a plasmonic nanogap and successfully extended the SFG spectroscopy into nanoscopic regime with ~10-nm spatial resolution. We also established a comprehensive theoretical framework that accurately describes the microscopic mechanisms of this near-field SFG process. These experimental and theoretical achievements collectively represent a groundbreaking advancement in near-field second-order nonlinear nanospectroscopy, enabling direct access to correlated chemical and topographic information of interfacial molecular systems at the nanoscale.

Sum-frequency generation (SFG) is a second-order nonlinear optical process in which two photons with different frequencies interact with a material and are converted into a single photon with sum frequency of the original photons. When one of the incident light beams is mid-infrared light that can resonantly excite molecular vibrations, the SFG intensity is significantly amplified through vibrational resonance effects. This property enables SFG spectroscopy to serve as a powerful molecular vibrational spectroscopic tool in the infrared wavelength region. A more notable feature of SFG spectroscopy is its surface specificity and absolute up/down molecular orientation sensitivity, allowing discrimination between up- and down-oriented molecular configurations at surfaces and interfaces. Owing to these unique capabilities, which are inaccessible via linear vibrational spectroscopies such as infrared absorption and Raman scattering, SFG has been extensively utilized to investigate the structure and vibrational properties of molecules at a variety of surface and interface systems.

Despite these advantages, a major drawback of SFG spectroscopy is its intrinsically limited spatial resolution, which is restricted to the micrometer scale by the diffraction limit of light. This limitation has prevented direct visualization of nanoscale site-specific variations in absolute up/down molecular orientation, adsorption geometries, and dynamics in complex inhomogeneous interfacial molecular systems.

In this study, researchers at Institute for Molecular Science, SOKENDAI, and Tohoku University successfully overcame this challenge by implementing a tip-enhanced SFG (TE-SFG) scheme that exploits the plasmonic near-field confinement within the tip-substrate nanogap in a scanning tunneling microscope (STM). Their newly developed TE-SFG technique achieved molecular vibrational nanoscopy with ~10-nm spatial resolution, which is beyond the diffraction limit of light, and directly visualized nanoscale orientation heterogeneity within aggregated interfacial molecular domains, previously inaccessible by conventional far-field SFG (Figure 1).

For the quantitative analysis and rigorous interpretation of the TE-SFG spectra, they further established a comprehensive theoretical framework describing the mechanisms underlying the TE-SFG process, incorporating both dipole and higher-order multipole light-matter interactions. Precise numerical calculations based on this theory accurately reproduce key spectral features and enable reliable extraction of molecular aggregation structures and absolute up/down orientation at the nanoscale. This theoretical-experimental synergy establishes TE-SFG as a robust tool for spatially resolved molecular analysis at complex interfaces beyond the diffraction limit.

At surfaces and interfaces with broken inversion symmetry, even chemically identical molecules can exhibit entirely different functionalities or reactions depending on their absolute up/down orientation. Thus, the capability to resolve such up/down orientations at the nanoscale is essential for elucidating molecular mechanisms in a variety of functional interfaces. This work establishes a fundamentally new class of up/down orientation-sensitive second-order nonlinear near-field nanoscopy beyond the existing near-field spectroscopic methods based on linear optical schemes such as tip-enhanced Raman or scattering-type near-field infrared microscopy, which inherently lack sensitivity to absolute up/down molecular orientation. Therefore, our method is powerful and can be broadly applicable to elucidating the nanoscale polarity of interfacial molecular layers, an essential descriptor in chemical processes on biological, electrochemical, and catalytic surfaces and interfaces