Sum-frequency generation (SFG) spectroscopy is a highly versatile tool for surface analysis. Improving the SFG intensity per molecule is important for observing low concentrations of surface species and intermediates in dynamic systems. Herein, Shell-Isolated-Nanoparticle-Enhanced SFG (SHINE-SFG) was used to probe a model substrate. The model substrate, $p$-mercaptobenzonitrile adsorbed on a Au film with SHINs deposited on top, provided an enhancement factor of up to ${10}^5$. Through wavelength- and polarization-dependent SHINE-SFG spectroscopy, the majority of the signal enhancement was found to come from both plasmon enhanced emission and chemical enhancement mechanisms. A new enhancement regime, i.e., the nonlinear coupling of SHINE-SFG with difference frequency generation, was also identified. This novel mechanism provides insight into the enhancement of nonlinear coherent spectroscopies and a possible strategy for the rational design of enhancing substrates utilizing coupling processes.

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• Laser spectroscopy
• Plasmonics
• Second order nonlinear optical processes
• Nanoparticles
• Femtosecond laser spectroscopy