Light propagation is usually reciprocal, meaning that the trajectory of light travelling in one direction is identical to that of light travelling in the opposite direction. Breaking reciprocity can make light propagate only in one direction. Optical components that support such unidirectional flow of light, for example isolators and circulators, are indispensable building blocks in many modern laser and communication systems. They are currently almost exclusively based on the magneto-optic effect, making the devices bulky and difficult for integration. A magnetic-free route to achieve nonreciprocal light propagation in many optical applications is therefore in great demand.
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Source: Phys.org