Valley degree of freedom in topological metamaterials: from microwaves in meta-waveguides to nanoscale surface graphene plasmons (Conference Presentation)

Abstract

Topological photonics enable us to design novel devices that exploit counter-intuitive propagation of electromagnetic waves. The key ingredient of topological photonics is a photonic topological insulator (PTI): a periodic structure that, in its bulk form, exhibits a propagation bandgap for a range of frequencies, yet supports localized edge states when interfaced with a different photonic structure exhibiting a bandgap for the same frequency range. Several types of PTIs emulating their respective condensed matter counterparts have already been proposed and experimentally demonstrated. One of the simplest PTIs exploits the valley degree of freedom in photonic crystals with a C_3 spatial symmetry. I will describe two examples of such structures: one designed and experimentally demonstrated at microwave frequencies and another designed for the mid-IR spectral range. We show that the microwave PTI structure, which is based on a metallic waveguide with an embedded array of specially designed metal rods, exhibits the previously unknown phenomenon of valley-protected “perfect” refraction: when interfaced with another waveguide, the edge states refract from the PTI metamaterial into the waveguide without any reflection. For the nanoscale topological metamaterial, we utilize graphene surface plasmons (GSPs) that propagate through a sheet of graphene with nano-patterned landscape of chemical potential. The chemical potential landscaping is achieved using an electrically biased metagate placed in close proximity of the graphene sheet. The advantage of this scheme is that the topological properties of the GSPs can be rapidly turned on and off, thus heralding the new era of active topological photonics on a nanoscale