An international research team led by Aalto University has found a new and easy way to break the law of reciprocity in the electromagnetic world, by periodically changing the properties of materials over time. The discovery could help create efficient non-reciprocal devices, such as compact insulators and circulators, that are needed for the next generation of microwave and optical communication systems.
When we look out the window and see our neighbor on the street, the neighbor can also see us. This is called reciprocity and it is the most common physical phenomenon in nature. Electromagnetic signals propagating between two sources are always regulated by the law of reciprocity: if the signal from source A can receive source B, then the signal from source B can also receive source A with equal efficiency.
Researchers from the University of Aalto, Stanford University and the Swiss Federal Institute of Technology in Lausanne (EPFL) have successfully shown that the law of reciprocity can be violated if the nature of the breeding medium changes periodically over time. A propagation medium refers to a material in which light and electromagnetic waves survive and propagate from one point to another.
The team theoretically showed that if a medium is shaped into an asymmetric structure and if its physical property varies globally over time, the signal generated from source A can receive source B, but not vice versa. This creates a strong non-reciprocal effect, since source source A cannot receive signal A.
‘This is an important milestone in both the physical and engineering community. We need one-way light transmission for a variety of applications, such as stabilizing laser work or designing future communication systems, such as full-duplex systems with increased channel capacity, ‘says postdoctoral researcher Xuchen Wang of Aalto University.
Previously, the creation of a non-reciprocal effect required the bias of external magnets, which makes devices bulky, temperature unstable and sometimes incompatible with other components. The new discoveries provide the simplest and most compact way to break up electromagnetic reciprocity, without the need for bulky and heavy magnets.
‘Such variations only over time allow us to design simple and compact platforms from materials capable of one-way light transmission and even amplification,’ explains Xuchen.
The results were published in Letters of physical examination On December 22, 2020, the study was funded by the Finnish Academy, the European Union call for Horizon 2020 Future Emerging Technologies (FETOPEN – RIA) within the VISORSURF project, the Finnish Foundation for the Promotion of Technology and the MURI project. FA9550-18-1-0379).
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