Halting a wave in its tracks

Topological ideas have recently taken the center stage of modern electromagnetics. Typical topological photonic systems are based on nonreciprocal materials, a class of materials that enables asymmetric light-matter interactions. In particular, nonreciprocal platforms may support unidirectional channels that allow propagation in a given direction of space--let's say from left to right, but not the other way around. Such unidirectional guides are of key importance in optical systems because their...