Even 35 years after the discovery of surface-enhanced Raman spectroscopy (SERS) much remains to be learned about the phenomenon.1-3 Despite broad consensus on the mechanism of SERS, many features remain poorly understood and in particular much less effort has been put into understanding the continuum emission called the "background" observed in SERS spectra. Here the SERS background is studied systematically on sphere segment void (SSV) plasmonic substrates. We establish the physicochemical dependence of the background on plasmons, the identity of the adsorbate, adsorbate coverage and electrochemical potential. In particular, by exchanging electron-donating and electron-withdrawing adsorbates, we demonstrate predictable modulation of the SERS background. Using these observations, we propose a model for the origin of the SERS background. Finally, we test the proposed model against its predictions for anti-Stokes SERS spectra.

Placing metallic nanoparticles inside cavities, rather than in dimers, greatly improves their plasmonic response. Such particle-in-cavity (PIC) hybrid architectures are shown to produce extremely strong field enhancement at the particle-cavity junctions, arising from the cascaded focusing of large optical cross sections into small gaps. These simply constructed PIC structures produce the strongest field enhancement for coupled nanoparticles, up to 90% stronger than for a dimer. The coupling is found to follow a universal power law with particle-surface separation, both for field enhancements and resonant wavelength shifts. Significantly enhanced Raman signals are experimentally observed for molecules adsorbed in such PIC structures, in quantitive agreement with theoretical calculations. PIC architectures may have important implications in many applications, such as reliable single molecule sensing and light harvesting in plasmonic photovoltaic devices.