Negative refraction mediated by bound states in the continuum

Abstract

Negative refraction might occur at the interface between a two-dimensional photonic crystal (PhC) slab and a homogeneous medium, where the guiding of the electromagnetic wave along the third dimension is governed by total internal reflection. Herein, we report on the observation of negative refraction in the PhC slab where the vertical guiding is enabled by a bound state in the continuum and essentially beyond the light cone. Such abnormal refraction and guiding mechanism are based on the synchronous crafting of spatial dispersion and the radiative lifetime of Bloch modes within the radiative continuum. Microwave experiments are provided to further validate the numerical proposal in an all-dielectric PhC platform. It is envisioned that the negative refraction observed beyond the light cone might facilitate the development of optical devices in integrated optics, such as couplers, multiplexers, and demultiplexers.