Science 365, 1163–1166 (2019)

A signal sent across a chip doesn’t arrive at its full strength because of losses. On optical chips these come mainly from defects, but the scattering-free optical modes propagating along an interface between two topologically different regions may solve this problem. Photonic topological properties are defined by the physical structure, making such waveguides rigid — like traces on a circuit board. Han Zhao and colleagues were able to steer topological modes along optically reconfigurable paths.

The team exploited the interplay of topological and non-Hermitian symmetries, where new topological states emerge for a specific contrast between gain and loss in the system. By optically pumping some regions of a microring lattice patterned onto an InGaAsP-based multiple quantum well, they generated an overlay structure of optical gain and intrinsic material loss in such a way that a topological mode was created on the boundary between the two regions, whose shape can be flexibly controlled by patterning the illumination.