Photonic quadrupole topological insulator using orbital-induced synthetic flux

• Type of Material: Article
• Authors: Julian Schulz, TU Kaiserslautern; Jiho Noh, University of Illinois at Urbana-Champaign; Wladimir Benalcazar, Emory University; Gaurav Bahl, University of Illinois at Urbana-Champaign; Georg von Freymann, TU Kaiserslautern
• Language: English
• Date: 2022-11-03
• Publisher: Nature Research (part of Springer Nature)
• Publication Version: Final Published Version
• Copyright Statement: © The Author(s) 2022
• License: Creative Commons Attribution 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.1038/s41467-022-33894-6
• Title of Journal or Parent Work: Nature Communications
• Volume: 13
• Grant/Funding Information: Open Access funding enabled and organized by Projekt DEAL.
• Keywords: Condensed-matter physics; Optical materials and structures
• Research Categories: Engineering, Mechanical; Physics, General

Abstract

The rich physical properties of multiatomic crystals are determined, to a significant extent, by the underlying geometry and connectivity of atomic orbitals. The mixing of orbitals with distinct parity representations, such as s and p orbitals, has been shown to be useful for generating systems that require alternating phase patterns, as with the sign of couplings within a lattice. Here we show that by breaking the symmetries of such mixed-orbital lattices, it is possible to generate synthetic magnetic flux threading the lattice. We use this insight to experimentally demonstrate quadrupole topological insulators in two-dimensional photonic lattices, leveraging both s and p orbital-type modes. We confirm the nontrivial quadrupole topology by observing the presence of protected zero-dimensional states, which are spatially confined to the corners, and by confirming that these states sit at mid-gap. Our approach is also applicable to a broader range of time-reversal-invariant synthetic materials that do not allow for tailored connectivity, and in which synthetic fluxes are essential.