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.