Raman sensing is a powerful technique for detecting chemical signatures, especially when combined with optical enhancement techniques such as using substrates containing plasmonic nanostructures. In this work, we successfully demonstrated surface-enhanced Raman spectroscopy (SERS) of two analytes adsorbed onto gold nanosphere metasurfaces with tunable subnanometer gap widths. These metasurfaces, which push the bounds of previously studied SERS nanostructure feature sizes, were fabricated with precise control of the intersphere gap width to within 1 nm for gaps close to and below 1 nm. Analyte Raman spectra were measured for samples for a range of gap widths, and the surfaceaffected signal enhancement was found to increase with decreasing gap width, as expected and corroborated via electromagnetic field modeling. Interestingly, an enhancement quenching effect was observed below gaps of around 1 nm. We believe this to be one of the few studies of gap-width-dependent SERS for the subnanometer range, and the results suggest the potential of such methods as a probe of subnanometer scale effects at the interface between plasmonic nanostructures. With further study, we believe that tunable sub-nanometer gap metasurfaces could be a useful tool for the study of nonlocal and quantum enhancement-quenching effects. This could aid the development of optimized Raman-based sensors for a variety of applications.

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We investigate an interdigitated nanograting structure on a GaAs substrate for plasmonic metal-semiconductormetal photodetector applications. This computational work has studied the effects that the taper angle of the nanograting sidewall and the light wave angle of incidence have on the optical and current enhancements in the device. The study, involving two types of taper angle structures-positive and negative-showed that both taper angle directions can generate more optical and electrical enhancements than perfectly vertical wall structures for light incident at both the Brewster angle and the normal incident angle. The enhanced electric field value at the optimum positive taper angle is ∼22% and ∼120% greater than the negative taper angle and vertical wall structure, respectively. In addition, the total weighted optical enhancement value for the optimal positive taper angle structure is ∼65% and ∼120% greater than the optimal negative taper angle and vertical wall structure, respectively. This work demonstrates that the increased enhancements are due to the nanoscale focusing of light and impedance matching. The incident wave angle along with the taper angle can significantly promote these enhancements, especially at the Brewster angle.