Non-reciprocal Effects in Waveguides Based on Spin-Momentum Locking

Abstract

Optical isolators traditionally rely on magneto-optic effects induced by externally applied magnetic fields. This study investigates the effective index contrasts induced by spin-momentum locking, which links the transverse photonic spin with the directionality of a guided mode, in non-reciprocal waveguides. These effects can be exploited in order to design all-optical isolators based on 2D semiconductors, and specifically on transition metal dichalcogenides (TMDs) integrated on a silicon nitride platform. We explore different hybrid integrated configurations, identifying the geometrical and physical conditions for a non-reciprocal behavior. Furthermore, we simulate the absorption contrast and non-reciprocal phase shift for a TE0 mode in a waveguide covered by a TMD monolayer, investigating their dependence on the material parameters. The results discussed in this work allow the design of all-optical devices for integrated isolation, based either on a direct contrast in the transmission at both propagation directions, or through the implementation of a Mach–Zehnder interferometer that takes advantage of the non-reciprocal phase shift effect. Our analysis contributes to the development of integrated optical isolators based on spin-momentum locking without external magnetic fields.