Probing the Local Near-Field Intensity of Plasmonic Nanoparticles in the Mid-infrared Spectral Region.

The enhanced local field of gold nanoparticles (AuNPs) in mid-infrared spectral regions is essential for improving the detection sensitivity of vibrational spectroscopy and mediating photochemical reactions. However, it is still challenging to measure its intensity at subnanometer scales. Here, using the NO2 symmetric stretching mode (nuNO2) of self-assembled 4-nitrothiophenol (4-NTP) monolayers on AuNPs as a model, we demonstrated that the percentage of excited nuNO2 mode, determined by femtosecond time-resolved sum-frequency generation vibrational spectroscopy, allows us to directly detect the local field intensity of the AuNP surface in subnanometer ranges. The local-field intensity is tuned by AuNP diameters. An approximate 17-fold enhancement was observed for the local field on 80 nm AuNPs compared to the Au film. Additionally, the local field can regulate the anharmonicity of the nuNO2 mode by synergistic effect with molecular orientation. This work offers a promising approach to probe the local field intensity distribution around plasmonic NP surfaces at subnanometer scales.