Photonic quadrupole topological insulator using orbital-induced synthetic flux

• 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
• Journal Title: Nature Communications
• Volume: Volume 13
• Publisher: Nature Research (part of Springer Nature) | 2022-11-03
• Type of Work: Article | Final Publisher PDF
• Publisher DOI: http://doi.org/10.1038/s41467-022-33894-6
• Subjects: Engineering, Mechanical; Physics, General
• Keywords: Optical materials and structures; Condensed-matter physics

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.