On-Chip Diffraction-Free Beam Guiding beyond the Light Cone

Abstract

On-chip channelless diffraction-free beam guiding enables dense integration of optical circuits in a reconfigurable manner, where total internal reflection, which is considered the cornerstone of guided-wave optics, is utilized to confine light in the out-of-plane direction. Here, we theoretically propose a physical mechanism to achieve on-chip channelless diffraction-free beam guiding beyond the light cone, utilizing the physics of bound states in the continuum. A bound state in the continuum with a tailored spatial dispersion plays an important role in cancelling both the in-plane diffraction and the out-of-plane scattering when the condition for total internal reflection is not satisfied. As a proof-of-concept verification, we experimentally demonstrate such an effect based on an all-dielectric platform using microwaves. The on-chip channelless diffraction-free beam guiding beyond the light cone also allows direct free-space coupling to such self-collimation modes. Our results may open up an avenue for exploring the physics and applications of guided-wave optics.