Asymmetric surface plasmon resonances revisited as Fano resonances

Abstract

Attenuated total reflection spectra of planar metal-dielectric structures obtained in the Otto and Kretschmann configurations exhibit resonance dips due to excitation of surface-plasmon polaritons. Although the dips have asymmetric line shapes, the asymmetry has not received much attention and the Lorentzian symmetric line shape has commonly been used in the spectral analyses. We analyze the reflectivity spectra of two- and three-layer plasmonic systems based on the electromagnetic and coupled-mode theories, and demonstrate that the asymmetric line shapes can be regarded as the Fano line shapes. In the resonance region, the spectra calculated as Fano line shapes using our approximate expressions are in excellent agreement with those obtained by rigorous electromagnetic calculations. The present formulations of the reflectivity allow us to gain insight into physical origins of characteristic behavior of the plasmonic resonances.