Photonic amorphous topological insulator

Photonics: finding one-way pathways in unlikely places Optical circuits that provide high-speed data traffic even in the presence of unexpected defects can now be fabricated from materials with less than perfect crystallinity. In photonic topological insulators, interactions between crystal energy bands produce unique 'edge states' that enable photons to move in one direction. Yidong Chong and Baile Zhang from Nanyang Technological University in Singapore and colleagues have investigated how disorder impacts the formation of these edge states. The team placed gyromagnetic cylindrical rods into a waveguide and arranged them in patterns ranging from periodic to completely unstructured. Probing the waveguide with microwave radiation revealed that short-range interactions enabled the one-way states to persist in amorphous structures until the lattice transformed into a liquid-like structure. The amorphous edge states proved robust enough to pass photons around rectangular obstacles and across large air gaps. The current understanding of topological insulators and their classical wave analogs, such as photonic topological insulators, is mainly based on topological band theory. However, standard band theory does not apply to amorphous phases of matter, which are formed by non-crystalline lattices with no long-range positional order but only short-range order, exhibiting unique phenomena such as the glass-to-liquid transition. Here, we experimentally investigate amorphous variants of a Chern number-based photonic topological insulator. By tuning the disorder strength in the lattice, we demonstrate that photonic topological edge states can persist into the amorphous regime prior to the glass-to-liquid transition. After the transition to a liquid-like lattice configuration, the signatures of topological edge states disappear. This interplay between topology and short-range order in amorphous lattices paves the way for new classes of non-crystalline topological photonic bandgap materials.