Direct visualization of topological transitions and higher-order topological states in photonic metasurfaces

TL;DR

This paper reports the design and experimental visualization of a photonic kagome metasurface that exhibits higher-order topological phases, demonstrating topological transitions and boundary states of different dimensions for robust optical device applications.

Contribution

The work introduces a novel photonic metasurface supporting higher-order topological states and visualizes topological transitions and boundary modes directly.

Findings

Demonstrated topological transition and Dirac cone opening.

Visualized 1D edge states and 0D corner modes.

Showed potential for robust optical device applications.

Abstract

Topological photonic systems represent a new class of optical materials supporting boundary modes with unique properties, not found in conventional photonics. While the early research on topological photonics has focused on edge and surface modes in 2D and 3D systems, respectively, recently higher-order topological insulators (HOTIs) supporting lower-dimensional boundary states have been introduced. In this work we design and experimentally realize a photonic kagome metasurface exhibiting a Wannier-type higher-order topological phase. We demonstrate and visualize the emergence of a topological transition and opening of a Dirac cone by directly exciting the bulk modes of the HOTI metasurface via solid-state immersion spectroscopy. The open nature of the metasurface is then utilized to directly image topological boundary states. We show that, while the domain walls host 1D edge states, their bending induces 0D higher-order topological modes confined to the corners. The demonstrated metasurface hosting topological boundary modes of different dimensionality paves the way to a new generation of universal and resilient optical devices which can controllably scatter, trap and guide optical fields in a robust way.