Efficient unidirectional light propagation realized via asymmetrically exciting and transmitting Tamm plasmon-polaritons in a metal-dielectric-metal waveguide

An efficient nanoscale unidirectional transmission device at telecommunication wavelength is proposed, which is made up of a metal-dielectric-metal (MDM) waveguide. The waveguide core is asymmetric and comprises of a one-dimensional photonic crystal (PhC), a thin Ag film and a dielectric layer in a sequential order. In this waveguide structure, the PhC acts as a finite potential barrier and the unidirectional propagation is realized by combining the photonic bandgap effect and the asymmetric excitations of Tamm plasmon polaritons (TPPs) on two sides of the PhC. With respect to asymmetric excitation, the TPPs are only excited at the interface between the PhC and the Ag core film, and not excited on the other facet of the PhC. Further, for the forward transmission, the TPPs can penetrate the thin Ag core film and is then coupled to the surface plasmon-polaritons (SPPs) for the following propagation, which is also enhanced in this work by utilizing the F-P resonance in the dielectric layer. Differently, for the backward transmission, the TPPs cannot tunnel through the PhC because it is trapped in the stopband of the PhC. The numerical simulations demonstrate that the maximum forward transmittance of 87% and a high transmission contrast ratio of 1250 are achieved.