Static SERS with near-minus-one-epsilon substrate
A mechanism for additional enhancement of SERS in the nanoparticle-on-mirror scheme is proposed. This new mechanism is based on the use of a substrate made of material with a near-minus-one permittivity. The setup involves a plasmonic nanoparticle in the form of an oblate ellipsoid positioned above the substrate and a Raman-active molecule located between them. In the conventional nanoparticle-on-mirror scheme, the plasmonic dipole resonance frequency coincides with the Stokes frequency of the Raman-active molecule. Consequently, due to the Purcell effect, the molecule's near fields mostly excites a dipole mode in nanoparticle. This dipole moment is many times greater than the dipole moment of the molecule by itself. If the real part of substrate permittivity is near minus one, the image of the nanoparticle dipole moment in the mirror-substrate is a dipole moment pointed in the same direction but approximately $ 1/{\rm{Im}} \varepsilon_{_{\rm{ENZ}}}$ times larger in magnitude. The simultaneous radiation of these two dipoles additionally increases the SERS intensity in $ 10^4$ times.