Magnetoplasmonic isolators based on graphene waveguide ring resonators

Nanoscale optical isolators are crucial for nanophotonic, quantum-optical, and optoelectronic applications and have attracted considerable attention recently. The application of external magnetic fields is the standard way for creating nonreciprocity as the basis for optical isolation. However, the magneto-optical response is usually small, thus it should be enhanced to create strong enough nonreciprocity for high-contrast isolation. We demonstrate that a graphene waveguide ring resonator allows a nanoscale platform for a high-contrast optical isolator. Graphene provides strong subwavelength confinement and low loss, therefore large nonreciprocity can be realized for ultracompact high-contrast isolators by plasmon resonance enhancement combined with resonator resonance enhancement. The resonance frequency is widely tunable by controlling the gate voltages of the rolling graphene sheet of a graphene resonator. Manipulation of the waveguide-resonator coupling can be achieved by controlling the gate voltage of the planar graphene sheet. The concept of magnetically biased ultracompact graphene waveguide ring resonators for one-way plasmon flow allows one to tune the operation band over a one-octave-spanning broad frequency range, keeping the contrast higher than 99% and a moderate insertion loss lower than 50%. High performance is achievable by only adjusting both gate voltages of the planar and rolling graphene sheets.