Electrically tunable graphene terahertz isolator assisted by nonreciprocal plasmon

Nonreciprocal isolators enable unidirectional light propagation without back reflection. Typical terahertz isolators require magnetic fields to break the time reversal symmetry. Here, we propose a nonmagnetic isolator in the terahertz range based on nonreciprocal graphene plasmon operated in reflection configuration. The bias voltage generates the drift current in graphene which breaks the time-reversal symmetry and induces the nonreciprocal reflection. The isolator device exhibits a high isolation beyond 20 dB with insertion loss less than 3 dB. Moreover, the bandwidth with isolation beyond 20 dB can be broadened by five times to 1.7 THz via tuning the carrier density. The indexes including isolation, insertion loss and bandwidth of the isolator show strong dependence on the drift velocity and mobility of graphene, as well as the air-gap thickness. Our work shows great potential in the burgeoning terahertz technology where nonmagnetic and electrically tunable isolators are still lacking.