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 modular designs involve one-way interactions that require the optical isolation of the different modules.
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Source: Phys.org