Surface mode with large field enhancement in dielectric-dimer-on-mirror structures

Plasmonic nanostructures with accessible and strongly enhanced fields are useful for a variety of applications related to surface-enhanced light-matter interaction. We describe a method to migrate localized fields from a metal to dielectric surface. By arranging low-index contrast dielectric dimers on an optically thick metal film, a narrow-linewidth resonant mode is formed through diffraction coupling, with accessible enhancement away from a metal surface. The enhancement in the electric field intensity is over 2000 by dielectric dimers with a 100 nm gap and 720 nm period, and the resonant linewidth is about 0.35 nm around the wavelength of 725 nm. The dispersion of this periodic structure allows resonant enhancement of not only emission but also excitation. The design principle provides a means to tune the narrow-linewidth resonance over a wide wavelength range from ultraviolet to near-infrared.