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Nanoscale optical isolators are crucial for nanophotonic, quantum-optical, and optoelectronic applications and have attracted considerable attention recently. Magnetoplasmonics combines magnetic and plasmonic properties in subwavelength structures for the realization of the active control of surface plasmon polaritons (SPPs) by external magnetic fields and enables the realization of nanoscale devices for new functionalities. Research on this topic has resulted in a considerable number of experimental and theoretical papers.
Magnetoplasmonics is also particularly promising because of its nonreciprocal properties for the development of optical isolators. The unidirectional propagation of SPPs is a highly important topic for information processes as an optical analog of electrical diodes, integrated nanophotonic circuits, and other nanophotonic and optoeletronic applications.
Many isolators have been studied based on the magneto-optical effect in different integrating materials. However, the previously demonstrated nonreciprocity was in general weak and required a high magnetic field to achieve a high-contrast isolation. Also, a larger frequency bandwidth and better tunability of the operation frequencies have been requested. For this purpose graphene supporting SPPs with widely tunable frequencies is a very promising material. In the present paper we explore an ultracompact graphene-based magnetoplasmonic configuration that supports broadly frequency-tunable one-way plasmon flow based on a tunable conductivity and the ability to extremely confine light in graphene.
Recently, we demonstrated an ultracompact high-contrast magneto-optical resonator. Here, a similar mechanism for creating strong nonreciprocity and a high-contrast one-way plasmon flow by using a magnetically controlled graphene waveguide ring resonator (WRR) is studied. The graphene WRR consists of a graphene sheet side-coupled to a magneto-optical cylindrical semiconductor covered with a rolling graphene sheet. The tight confinement of surface plasmons can be attained by supporting a strong light-matter interaction in an ultracompact graphene-based structure. The low loss in a graphene waveguide allows for a high-quality factor ring resonator, which further enhances the light-matter interaction. The electrically tunable conductivity of grapheme allows for a broadly tunable optical isolation.
In this paper, we considered the transmission of a magnetically controlled graphene WRR, as shown in Fig. 1. The resonance frequency is sensitive to the graphene ring radius and is almost independent on the other geometric parameters such as the distance between the planar graphene sheet and the rolling graphene sheet. is the external magnetic field and and are gate voltages of grapheme sheets.
Figures 2(a) and 2(b) show the distributions of the magnetic field component of the grapheme plasmons with an external magnetic field strength B = 2 T for forward incidence and backward incidence, respectively. One can see that the transmission vanishes for forward incidence and a transmission of more than half for backward incidence.
The concept of magnetically controlled ultracompact grapheme waveguide ring resonators for one-way plasmon flow allows one to tune the operation band over a one-octave-spanning broad frequency range.
Our results were published in the journal of "Physical Review B"(100, 041405(R) (2019)) with the title of "Magnetoplasmonic isolators based on graphene waveguidering resonators"(https://doi.org/10.1103/PhysRevB.100.041405).