Coupled surface plasmons and resonant optical tunnelling in symmetric optical microcavities

This study presents an analytical model for light transmission through a symmetric optical microcavity. The structure comprises two metallic films separated by a thin layer of a low refractive index dielectric, and embedded within a semi-infinite dielectric of higher refractive index. This configuration supports both volume and surface resonances. The surface resonances result from synchronised collective electronic oscillations at the inner surfaces of the two thin metallic films, called coupled surface plasmons. For clarity, surface resonances are first defined and analysed considering the Drude model for ideal, lossless metal films. The model is then extended using the generalised complex Drude model, which accounts for losses. The results depict that high transmittance is possible even when the thickness of the inner dielectric layer spans several wavelengths. This phenomenon is an enhanced form of frustrated total reflection between dielectrics analogous to the quantum tunnelling effect. The phenomenon is more pronounced because of the two absorbing metal films. Under resonant conditions, these films enable unusually high transmission despite their inherent losses. Experimental results are presented in excellent agreement with the theoretical predictions. This proposed transmission model in a plasmonic microcavity provides a foundation for future theoretical research and potential applications of plasmonic microcavity devices.