Observation of Unidirectional Bound States in the Continuum Enabled by Topological Defects

TL;DR

This paper introduces and demonstrates unidirectional bound states in the continuum in photonic crystal slabs, enabled by topological defects, achieving high radiation asymmetry without mirrors, with potential optoelectronic applications.

Contribution

It presents the first experimental realization of unidirectional bound states in the continuum driven by topological polarization defects in photonic crystals.

Findings

Achieved a radiation asymmetry ratio of 27.7 dB.

Demonstrated high-quality factors up to 1.6×10^5 in the telecommunication regime.

Validated the topological origin of unidirectional radiation in photonic structures.

Abstract

Unidirectional radiation is important for a variety of optoelectronic applications. Many unidirectional emitters exist, but they all rely on the use of materials or structures that forbid outgoing waves, i.e. mirrors. Here, we theoretically propose and experimentally demonstrate a class of resonances in photonic crystal slabs, which only radiate towards a single side with no mirror placed on the other side - we call them ``unidirectional bound states in the continuum". These resonances are found to emerge when a pair of half-integer topological charges in the polarization field bounce into each other in the momentum space. We experimentally demonstrate such resonances in the telecommunication regime, where we achieve single-sided quality factor as high as 1.6e5, equivalent to a radiation asymmetry ratio of 27.7 dB. Our work represents a vivid example of applying topological principles to improve optoelectronic devices. Possible applications of our work include grating couplers, photoniccrystal surface-emitting lasers, and antennas for light detection and ranging.