High-sensitivity sensing based on plasmon-induced transparency (PIT) in a rectangular resonator has been investigated in detail. Multimode theory is introduced to explain the redshift and blueshift of the transmission spectrum by adjusting a structural parameter (w or h). In sensing applications, the sensitivity of the proposed structure is about 800 nm/RIU, and its figure of merit (FOM) is as high as 17280. In addition, the influences of structural parameters on FOM are researched in detail. The results indicate that structural parameters play important roles in optimizing the sensing performance, and the length ðhÞ is more sensitive than the width ðw Þ for FOM. The plasmonic configuration has the advantages of easy fabrication and compactness, which may find important applications in highly integrated optics devices, optical communication, and sensitive nanometerscale refractive index sensors.

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An inverse T-shape structure, consisting of a bus waveguide coupled with two perpendicular rectangular cavities, has been investigated numerically and theoretically. The position of the transparency window can be manipulated by adjusting the lateral displacement between the two perpendicular cavities. The effects of changing different structural parameters on the transmission features are investigated in detail. The results indicate that the length of two cavities play important roles in optimizing optical response. Finally, two simple applications based on the inverse T-shape structure are briefly discussed. The findings demonstrate that the first- and second-order modes can be separated without interference, and the sensitivity of the inverse T-shape is as high as 1750 nm per refractive index unit (RIU); the corresponding figure of merit (FOM) reaches up to 77.1  RIU, which is higher than in previous reports. The plasmonic configuration possesses the advantages of easy fabrication, compactness, and higher sensitivity as well as higher FOM, which will greatly benefit the compact plasmonic filter and high-sensitivity nanosensor in highly integrated optical devices.

We propose and investigate a graphene periodic subsurface structure consisting of a coplanar pair of ring resonators and a ribbon. The Fano-like interference can be actively regulated by the applied magnetic field, incident angle and Fermi energy. Since the excited charges of graphene monolayer have cyclotron properties in the external magnetic field, the transmittance and line-shape can be effectively controlled. At a certain frequency, different magnetic fields have different effects on the conductivity tensor of graphene, which affects the metallic properties of graphene, leading to changes in the transmittance. The Fano-like line-shape can be regulated by adjusting the incident angle to delay the phase between adjacent graphene sheets. In addition, the resonance frequency can be electrically regulated through Fermi energy. Finite element method (FEM) is introduced to analyze the graphene periodic structure and the results are demonstrated by multimode interference coupled mode theory (MICMT). The Faraday rotation angle exceeding 85o are observed in a small magnetic flux density B of about 0.5T. Moreover, a high ON/OFF ratio plasma-optic switching is designed, and the ON/OFF ratio (η) of the switch is 19.921dB. Notably, combining the influence of magnetic flux density on transmittance and the modulation of resonance frequency by Fermi energy, optical switching can be implemented at any frequency within the frequency range studied. These results provide methods for active regulation of electromagnetic waves in the terahertz field and have potential applications in optical switching and integrated photonic circuit.

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