EP restoration and fast-light edge states in photonic crystal waveguide with glide and time reversal symmetry

TL;DR

This paper proposes a novel photonic crystal waveguide design with glide and time reversal symmetry to realize and restore exceptional points, enabling ultra-fast light propagation with group velocities up to 25 times the original.

Contribution

It introduces a systematic method to generate and restore exceptional points in photonic crystal waveguides using non-Hermitian perturbations and symmetry considerations.

Findings

Achieved group velocity up to 3.3c near EPs

Demonstrated EP restoration by permittivity adjustment

Reduced group velocity contrast significantly

Abstract

Exceptional points (EPs) in the propagation states give rise to the emergence of intriguing properties with the divergence of the group velocity. However, there have been no experimental reports due to the necessity of maintaining high levels of fabrication precision and the requisite high group velocity contrast. In our study, we propose a design of photonic crystal waveguide with glide and time reversal symmetry, and derive an effective Hamiltonian for edge states to realize fast-light edge states. We adopt a systematic method to generate EPs in edge states by introducing non-Hermitian perturbations to Dirac points guaranteed by glide symmetry, which ensures that EP modes are free from out-of-plane radiation losses. Then, our study reveals the conditions for the exact EP restoration and provides an analytical solution to offset the EP smoothing due to symmetry breaking, which drastically reduces the group velocity contrast. A good symmetry property of the photonic crystal waveguide allows us to derive the effective Hamiltonian as a simple form, and the EPs can be restored by adjusting the real part of the permittivity. Furthermore, we design a feasible photonic crystal slab waveguide incorporating graphene as the absorbing material, and numerically demonstrate a group velocity reaching $v_g = 3.3c$ near the EP, which is up to 25 times that of the original structure. Thanks to the short periodicity of photonic crystals, it's possible to reach the speed of light in vacuum with group velocity contrasts on the order of one digit. Our study paves an innovative way to manipulate the group velocity of light.