Realization of a three-dimensional photonic higher-order topological insulator

TL;DR

This paper reports the first experimental realization of a three-dimensional photonic higher-order topological insulator using a metal-cage photonic crystal, demonstrating coexisting boundary states and potential for robust 3D photonic devices.

Contribution

It introduces the first 3D photonic HOTI with experimental validation, expanding topological photonics into three dimensions with practical boundary state applications.

Findings

Observation of topological surface, hinge, and corner states in 3D photonic HOTI

Boundary states are self-guided even in the light cone continuum

Boundary states can be exposed to air without cladding

Abstract

The discovery of photonic higher-order topological insulators (HOTIs) has significantly expanded our understanding of band topology and provided unprecedented lower-dimensional topological boundary states for robust photonic devices. However, due to the vectorial and leaky nature of electromagnetic waves, it is challenging to discover three-dimensional (3D) topological photonic systems and photonic HOTIs have so far still been limited to two dimensions (2D). Here, we report on the first experimental realization of a 3D Wannier-type photonic HOTI in a tight-binding-like metal-cage photonic crystal, whose band structure matches well with that of a 3D tight-binding model due to the confined Mie resonances. By microwave near-field measurements, we directly observe coexisting topological surface, hinge, and corner states in a single 3D photonic HOTI, as predicted by the tight-binding model and simulation results. Moreover, we demonstrate that all-order topological boundary states are self-guided even in the light cone continuum and can be exposed to air without ancillary cladding, making them well-suited for practical applications. Our work thus opens routes to the multi-dimensional robust manipulation of electromagnetic waves at the outer surfaces of 3D cladding-free photonic bandgap materials and may find novel applications in 3D topological integrated photonics devices.