Experimental investigations of non-reciprocal wave and plasma interactions in circulators, antennas and metasurfaces*

A circulator is a simple non-reciprocal system that demonstrates time-reversal symmetry breaking. It can be used as a unit cell to design structures with different topological orders. We present an experimental proof-of-concept of such a system, utilizing a gaseous plasma column as the circulator load instead of the conventional ferrite rod. By leveraging the gyrotropic property of magnetized plasma, we achieved wide-band strong isolation (40% bandwidth for effective isolation of 10 dB) in the transmission spectrum within the 4 to 6 GHz range. The performance and frequency band of the circulator can be readily adjusted by varying the plasma density (plasma frequency ranging from 1 to 8 GHz) and magnetic field strength (cyclotron frequency ranging from 0 to 6 GHz). Further simulations show potential to extend these plasma columns into a plasma photonic crystal with a defect-resistant unidirectional edge mode. This work inspired us to further study other modes of non-reciprocal wave-plasma interactions, such as those involving antennas or metasurfaces. Innovative plasma-coated high impedance surfaces or frequency selective surfaces were proposed and experimentally realized. By excitating surface waves traveling at the interface between magnetized bulk or periodic plasma and dielectrics, we demonstrated a new platform to study the topological phase of wave travelling in magnetized plasma.