Breaking the reciprocity law enables smaller microwave devices

December 30, 2020 // By Jean-Pierre Joosting
One-way light transmission. Image courtesy of Xuchen Wang / Aalto University.
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A research team lead by Aalto University has found a new and simple route to break the reciprocity law, by changing material properties periodically in time. The breakthrough could help to create efficient nonreciprocal devices, such as compact isolators and circulators, that are needed for the next generation of microwave and optical communications systems.

Electromagnetic signals propagating between two sources is always governed by the reciprocity law, which states,  if the signal from source A can be received by source B, then the signal from source B can also be received by source A with equal efficiency.

Researchers from Aalto University, Stanford University, and Swiss Federal Institute of Technology in Lausanne (EPFL) have successfully demonstrated that the reciprocity law can be broken if the property of the propagation medium periodically changes in time.

The team theoretically demonstrated that, if the medium is shaped into an asymmetric structure and its physical property varies globally in time, the signal generated by source A can be received by source B but not the other way around. This creates a strong nonreciprocal effect, since the signal from Source B cannot be received by source A.

Picture: 
One-way light transmission. Image courtesy of Xuchen Wang / Aalto University.

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