Design of a broadband reciprocal optical diode in a silicon waveguide assisted by silver surface plasmonic splitter.

In this paper, we propose a new scheme for a reciprocal optical diode integrated in a multimode silicon waveguide. The compact 4μm long functional region consists of a tapered coupler, a narrow single-mode waveguide, and a half-elliptical silver surface plasmonic splitter with refractive index modification of silicon. This spatial asymmetric design achieves even-to-odd mode conversion in the forward direction and blocks propagation of the even mode in the backward direction. The maximum contrast ratio and forward transmission efficiency reach approximately 0.99 and 87% while the values respectively keep higher than 0.96 and 80% within a 100nm operational bandwidth in a two-dimensional design. Both freestanding and SOI-based three-dimensional devices are simulated and at least a 0.94 contrast ratio is observed. Moreover, the robustness is demonstrated by introducing deviations to the surface plasmonic splitter. The proposed scheme brings together advantages including a high contrast ratio (>0.94), a large operational bandwidth (100nm) and a small footprint (4μm long).