Near-field observation of mid-infrared edge modes in topological photonic crystals

Abstract

Topological photonic crystals inherit the unique properties of topological insulators, including topologically protected energy transfer and unidirectional propagation, which offer an excellent platform for exploring exotic physics and developing photonic devices. However, topological photonic crystals possessing mid-infrared edge modes that have potential applications in infrared imaging, biosensing, thermal radiation energy transfer, etc., are seldom brought into focus. In this work, we study the topological properties of a photonic crystal slab (PCS) consisting of silicon square veins in the mid-infrared, which is intended to mimic the two-dimensional Su–Schrieffer–Heeger model. By interfacing topologically trivial and nontrivial PCSs, mid-infrared edge modes can appear at domain wall, according to the principle of bulk-edge correspondence. It is also demonstrated high-efficiency mid-infrared light transport can be achieved by these edge modes. In addition, adjusting the vertical offset near the interface can manipulate the bandwidth for various applications and turns the connected PCS structure to a photonic realization of Rice–Mele model. We further fabricate the PCS and provide an experimental observation of transverse-electric-like edge modes in mid-infrared by using the scattering-type scanning near-field optical microscope. Additionally, we integrate it with phase change material of nanoscale thickness, Ge2Sb2Te5, to realize an ultrafast and switchable topological waveguide with zero static power. This work not only enriches the fundamental understanding of topological physics in mid-infrared optical settings, but also shows promising prospects in compact devices for energy transfer and information processing for light sources in these wavelengths, for instance, thermal radiation.