Realization of Z$_2$ topological photonic insulators made from multilayer transition metal dichalcogenides

TL;DR

This paper demonstrates the creation of Z2 topological photonic insulators using multilayer transition metal dichalcogenides, specifically WS2, showing topological edge states and unidirectional light propagation at room temperature.

Contribution

It introduces a novel approach to realize topological photonic lattices with layered TMDs, combining nanofabrication and optical characterization for room-temperature topological photonics.

Findings

Photonic gap observed in near-infrared in trivial and topological phases

Unidirectional edge states demonstrated via circularly polarized excitation

High-quality nanostructures achieved through anisotropic dry etching

Abstract

Monolayers of semiconducting transition metal dichalcogenides (TMDs) have long attracted interest for their intriguing optical and electronic properties. Recently TMDs in their quasi-bulk form have started to show considerable promise for nanophotonics thanks to their high refractive indices, large optical anisotropy, wide transparency windows reaching to the visible, and robust room temperature excitons promising for nonlinear optics. Adherence of TMD layers to any substrate via van der Waals forces is a further key enabler for nanofabrication of sophisticated photonic structures requiring heterointegration. Here, we capitalize on these attractive properties and realize topological spin-Hall photonic lattices made of arrays of triangular nanoholes in 50 to 100 nm thick WS$_2$ flakes exfoliated on SiO$_2$/Si substrates. High quality structures are achieved taking advantage of anisotropic dry etching dictated by the crystal axes of WS$_2$. Reflectance measurements at room temperature show a photonic gap opening in the near-infrared in trivial and topological phases. Unidirectional propagation along the domain interface is demonstrated in real space via circularly polarized laser excitation in samples with both zigzag and armchair domain boundaries. Finite-difference time-domain simulations are used to interpret optical spectroscopy results. Our work opens the way for future sophisticated nanophotonic devices based on the layered (van der Waals) materials platform.