Direct observation of corner states in second-order topological photonic crystal slabs

TL;DR

This paper reports the direct observation of corner states in second-order topological photonic crystal slabs, demonstrating topological transitions and in-gap states using near-field measurements, advancing the understanding of higher-order topological phases in photonics.

Contribution

It provides the first direct experimental observation of corner states in 2D topological photonic crystals based on the SSH model, highlighting the role of edge polarization.

Findings

Observation of in-gap corner and edge states in microwave photonic crystals.

Demonstration of topological transition via tuning intra-cell distances.

Confirmation that corner states originate from edge dipolar polarization.

Abstract

Recently, higher-order topological phases that do not obey the usual bulk-edge correspondence principle have been introduced in electronic insulators and brought into classical systems, featuring with in-gap corner/hinge states. So far, second-order topological insulators have been realized in mechanical metamaterials, microwave circuit, topolectrical circuit and acoustic metamaterials. Here, using near-field scanning measurements, we show the direct observation of corner states in second-order topological photonic crystal (PC) slabs consisting of periodic dielectric rods on a perfect electric conductor (PEC). Based on the generalized two-dimensional (2D) Su-Schrieffer-Heeger (SSH) model, we show that the emergence of corner states roots in the nonzero edge dipolar polarization instead of the nonzero bulk quadrupole polarization. We demonstrate the topological transition of 2D Zak phases of PC slabs by tuning intra-cell distances between two neighboring rods. We also directly observe in-gap 1D edge states and 0D corner states in the microwave regime. Our work presents that the PC slab is a powerful platform to directly observe topological states, and paves the way to study higher-order photonic topological insulators.