Resonant cavity modes of circular plasmonic patch nanoantennas

We present theoretical analysis and numerical studies of cavity modes in circular plasmonic patch nanoantennas. There exist both even and odd cavity modes, while the even cavity modes were often missed in the literature because they can only be excited by oblique illumination. The cavity resonance frequencies are affected by near-field coupling at small periods and by coupling with surface plasmon outside the cavity at large periods. For intermediate periods with non-coupling effects, a simple resonant condition is obtained and validated by numerical simulations to relate the gap plasmon wave number and the effective patch size.