Phase-sensitive tip-enhanced sum frequency generation spectroscopy using temporally asymmetric pulse for detecting weak vibrational signals

TL;DR

This paper introduces a phase-sensitive tip-enhanced sum frequency generation spectroscopy method that uses temporally asymmetric pulses to suppress non-resonant background, enhancing weak vibrational signals and allowing detailed surface molecular analysis beyond the diffraction limit.

Contribution

The study develops a novel phase-sensitive TE-SFG technique with temporally asymmetric pulses, significantly improving detection of weak vibrational signals and enabling molecular orientation determination.

Findings

Achieved signal enhancement factor of $6.3 imes 10^6$ to $1.3 imes 10^7$.

Effectively suppressed non-resonant background using pulse asymmetry.

Confirmed tip enhancement origin of signals through forward and backward detection.

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 in turn amplified the vibrational response through interference and facilitated the detection of weak vibrational signals. This interference also made the technique phase-sensitive, enabling the determination of 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 $6.3\times 10^6-1.3\times 10^7$, based on the experimental data. This phase-sensitive TE-SFG technique overcomes the optical diffraction limit and enables the investigation of molecular vibrations at surfaces with unprecedented detail.