An ultralow crosstalk and broadband subwavelength grating-assisted chiral mode converter by encircling exceptional points

Abstract

Exceptional points (EPs) are degeneracies of two or more eigenstates and eigenvalues in non-Hermitian systems, promising applications in optoelectronics. In particular, chiral state conversion can be achieved by dynamic encircling an EP to enable backward-scattering light isolation and asymmetric mode switching. However, critical bottlenecks have plagued most mainstream EP-based chiral mode converters, since they mainly use the traditional dual-coupled waveguide systems for parametric tuning as the essential part of the chiral mode converter, which induce mode mismatch, and bandwidth-limited EP encircling path and, therefore, cause deficiencies in crosstalk and bandwidth. To overcome this challenge, we propose a chiral mode converter adding customized subwavelength gratings (SWGs) in dual-coupled waveguide systems to enhance parametric tuning. Indeed, the SWG structure decreases crosstalk and enhances bandwidth by using its refractive index control characteristics to mitigate mode mismatch and weaken the wavelength correlation of the EP encircling path. The designed device has expanded the available working band, demonstrating favorable performance in both the optical communication band (1.26–1.675 μm) and 2 μm (1.85–2.05 μm) band. At the same time, the crosstalk reduces to below −20 and −13 dB, respectively, superior to most of the previously reported devices. Furthermore, the transmission efficiency remains above 90% in the full operating bands, which is at the advanced level as the reported optimal performance of chiral mode converters. This study paves the way for developing efficient chiral transmission devices (such as optical switches, isolators, and logic gates), inspiring fascinating opportunities in future optical communication and topological quantum computing technologies.