Plasmonic mode-gap waveguides using hetero-metal films.

We propose a novel waveguide structure using a hetero-metal film which is composed of two metals of different plasma frequencies. In the proposed waveguide, a long-range surface plasmon-polariton (LR-SPP) mode in the central metal film of a higher plasma frequency are horizontally confined since no propagation mode is allowed in the outer films of a lower plasma frequency for a certain frequency range which is dubbed a plasmonic mode-gap (PMG). The propagation characteristics of the proposed PMG waveguide are numerically analyzed. The proposed waveguide shows tight horizontal confinement close to the diffraction limit and notable suppression of radiation loss in bendings due to the PMG effect. It seems that the PMG guiding can improve integration densities of optical devices based on the LR-SPPs without exacerbating their propagation losses.