Gap plasmon-polariton nanoresonators: Scattering enhancement and launching of surface plasmon polaritons

Gap plasmon-polariton (GPP) nanoresonators based on a metal nanostrip separated with a small gap from a metal surface or metal block are considered. Scattering resonances and field enhancements are studied for two-dimensional structures using the Green's-function surface integral equation method (GFSIEM). For small gaps, we show that the scattering resonances occur due to the constructive interference of counterpropagating GPPs, forming standing waves. By varying the gap size we find that the resonance wavelength can be tuned over a wide range of wavelengths, which makes the resonators interesting for spectroscopic and sensing applications, and observe the transition between GPP-based resonators (for narrow gaps) and slow surface plasmon-polariton (SPP) strip resonators (for wide gaps). Considering the resonant field distributions, we find that, for an insulator thickness of 10 nm, the maximum field enhancement (with respect to the incident field) can reach values close to 50 along the line passing through the gap center. For the case of a strip placed close to a metal surface, two scattering channels, viz., the out-of-plane scattering and the scattering into SPPs (propagating along the surface) are evaluated separately using a generalized version of the GFSIEM. We find that, even though the out-of-plane scattering is in general dominating in the considered range of parameters, scattering into SPPs can be very efficient for smaller gaps featuring a cross section that at resonance even exceeds the strip width. The considered properties of GPP nanoresonators, i.e., resonant scattering and local-field enhancements along with efficient scattering into SPPs, hold promises for their useful applications within plasmonic sensing devices.