Tip-enhanced sum frequency generation spectroscopy using temporally asymmetric pulse for detecting weak vibrational signals

Abstract

Vibrational sum frequency generation (SFG) spectroscopy is a powerful technique for investigating molecular structures, orientations, and dynamics at surfaces. However, its spatial resolution is fundamentally restricted to the micrometer scale by the optical diffraction limit. Tip-enhanced SFG (TE-SFG) using a scanning tunneling microscope has been developed to overcome this limitation. The acquired spectra exhibit characteristic dips originating from vibrational responses located within the strong broadband non-resonant background (NRB), which distorts and obscures the molecular signals. By making the second pulse temporally asymmetric and introducing a controlled delay between the first and second laser pulses, the NRB was effectively suppressed, which led to an optimized ratio between the resonant and non-resonant signals, thereby maximizing the contrast of an interferometric signal and improving the detectability of weak vibrational signals. This interference also made it possible to determine absolute molecular orientations. Furthermore, forward- and backward-scattered signals were simultaneously detected, conclusively confirming that the observed signals originated from tip enhancement rather than far-field contributions. Finally, the signal enhancement factor in TE-SFG was estimated to be 6.3 × 106 − 1.3 × 107, based on the experimental data. This TE-SFG technique overcomes the optical diffraction limit and enables the investigation of molecular vibrations at surfaces with unprecedented detail.