Chiral/directional mode transfer based on a tunable non-Hermitian system

Abstract

Exceptional points (EPs) of non-Hermitian systems are the degeneracies of both the eigenvalues and eigenvectors, which have led to a series of novel and counterintuitive physical effects. Adiabatically encircling the EP in parameter space could lead to chiral mode transfer in coupled waveguides. However, the fixed refractive index distribution in existing devices will confine the parametric loops and lay great limitations on the performances of the mode conversion. Herein, we theoretically propose a non-Hermitian system based on refractive index modulation of coupled liquid crystal (LC) waveguides, which allows for encircling the EP with tunable loops. As a result, chiral mode transfer is achieved with optimized performances at wide telecommunication wavelengths, including a high transfer efficiency (>80%). Moreover, the dynamic modulation of the refractive index enables a directional mode transfer which is solely dependent on the modulation direction, even with non-closing evolution routes. The tunable non-Hermitian system serves as a versatile platform for realizing tunable mode transfer processes with optimized performances, showing great promise for developing multifunctional non-Hermitian nanophotonic devices.