In this paper, we propose a plasma structure that can effectively enhance surface plasmon resonance and achieve significant local field enhancement. For specific incident wave frequencies, two plasma rings and a vacuum layer between them can form a metal-insulator-metal (MIM) waveguide, which can resonant as a Fabry–Perot cavity and couple the incident wave energy to the vicinity of the plasma ring slit, and thus effectively enhance the localized surface plasmon resonance inside the plasma ring. The simulation results show that, by adjusting the thickness and angle of the outer plasma ring, the average electric field of the incident wave inside the structure can be increased by a factor of 9. Moreover, at the same plasma frequency and incident wave band, the local field enhancement of the double-ring structure is better than that of a circular ring structure or a circular ring structure with a slit. We have also analyzed the physical mechanism of field enhancement and calculated the dispersion relation of surface plasmon polaritons in the Fabry–Perot cavity. The results are in good agreement with the theory of the MIM waveguide cavity.
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