Impact of Plasmon-Induced Optically Rectified Electric Fields on Second Harmonic Generation

Abstract

Plasmon associated optically rectified fields arising from electron tunneling between metal nanojunctions have been shown to impact photocatalytic reactions on the surface of plasmonic nanostructures. However, temporal differences have been shown between quantum regime electron tunneling events and optically rectified surface charge buildup. Computational work shows that electron tunneling across a nanometer-sized gap between two plasmonic entities can be induced from a single ultra-fast laser pulse and occurs on a femtosecond timescale. Our group has shown that charge buildup from optically rectified fields occurs over 10s to 100s of seconds through light-induced redox events in cyclic voltammetry experiments using a CuSO4 solution. Utilizing a femtosecond laser to generate second harmonic signals along with widefield excitation from a continuous light-emitting diode (LED) source, it is shown that surface potentials that average −400 mV are only generated upon continuous illumination of a gold nanoisland plasmonic surface with a widefield LED source. Negligible changes in second harmonic generation (SHG) signals are seen from surface excitation with the femtosecond laser alone, showing the importance of continuous plasmon excitation for sustained surface potentials. It is also shown, through Stark spectroscopy and spectroelectrochemical measurements, that optically rectified fields impact SHG signals in a manner similar to an externally applied bias on pristine plasmonic surfaces. This result shows the potential for utilizing SHG signals to better understand the impact surface heterogeneity has on the optically rectified direct current field and enables the optimization of plasmonic surfaces for future use as photocatalysts.

From SERS to TERS and Beyond: Molecules as Probes of Nanoscopic Optical Fields

“…Beyond Stark shifts observed from molecules adsorbed to plasmonic structures, there is additional evidence of the plasmon associated rectified fields. As noted earlier, SHG measurements, performed in the absence of Stark reporters, show the same change in surface potential upon excitation of the plasmon resonance . Calculations report that a femtosecond pulsed laser can excite electrons to tunnel across a nanometer sized gap on a femtosecond time scale, thereby inducing an electric field across the gap.…”

“… Others have used this relationship to explore electric field dependencies on the optical processes; , however, the plasmon-induced electric fields also drive the electric field observed on plasmonic surfaces. Indeed, plasmonic excitation has shown significant changes in surface potential, as much as −400 mV, as measured by the SHG response …”

“…The inset shows how the CN stretching mode evolves with applied electrochemical potential. Figure adapted with permission from ref . Copyright 2019 American Chemical Society.…”

Suppressing Molecular Charging, Nanochemistry, and Optical Rectification in the Tip-Enhanced Raman Geometry

“…The switch between contact-mode and tapping-mode feedback provides a mechanical perturbation analogous to the time-dependent changes in optical rectification noted by Nelson et al The previous study demonstrated that optically rectified surface potential was observed when plasmonic nanostructures were illuminated with CW illumination, but not with pulsed excitation. The 13 ns time window between pulses enabled short-lived plasmons to relax prior to the arrival of the next optical pulse.…”

Power-Dependent Dual Analyte Tip-Enhanced Raman Spectral Imaging

“…37−39 It is important to recall that, in ensemble-averaged SERS measurements, vibrational Stark shifts were found to level off with applied electric field strengths, which was attributed to electrochemical changes in surface chemistry. 35,36,40 In this work, the more confined probing volume leads to a quantized frequency shift that depends on the specifics of the molecular junction and the energy of the tunneling plasmon. Although the tip is corrugated (sputtered), the makeup of the junction is more or less preserved across the images, as the substrate itself is uniform.…”