Surface Enhanced Raman Spectroscopy in Molecular Junction by Remote Excitation

Surface enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonance (LSPR) in metallic nanostructures. The SERS enhancement factor can be as large as ~10¹⁰, which contributes to single molecule detection. When the SERS is excited by direct illumination of the nanostructure, the background heating of lattice and electrons can prevent further manipulation of the molecules. To overcome this flaw, we report SERS in electromigrated gold tunneling junctions through remote excitation: surface plasmon polaritons (SPPs) are excited at nearby gratings and then propagate to the junction and couple to the local nanogap plasmon modes. Like direct excitation of the nanogap, this remote excitation can generate both SERS emission and an open-circuit photovoltage (OCPV) from the junction. We compare the OCPV and SERS intensity in both direct and remote illumination configurations. Coupling efficiency for an ensemble of junctions is statistically analyzed to examine the consistency with propagation loss of SPPs and their coupling to LSPR modes, which we compare with numerical simulations.