Cavity resonances of metal-dielectric-metal nanoantennas.

We propose a new design of optical nanoantennas and numerically study their optical properties. The nanoantennas are composed of two cylindrical metal nanorods stacked vertically with a circular dielectric disk spacer. Simulation results show that when the dielectric disk is less than 5nm in thickness, such nanoantennas exhibit two types of resonances: one corresponding to antenna resonance, the other corresponding to cavity resonances. The antenna resonance generates a peak in scattering spectra, while the cavity resonances lead to multiple dips in the scattering spectra. The cavity resonant frequency can be tuned by varying the size of the dielectric disk. The local field enhancement inside the cavity is maximized when the diameter of the dielectric disk is roughly half that of the rod and when the cavity and antenna resonant frequencies coincide with each other. This new nanoantenna promises applications in single molecule surface enhanced Raman spectroscopy (SERS) owing to its high local field enhancements and large scale manufacturability.