Theory of coupled plasmon modes and Fano-like resonances in subwavelength metal structures

In this paper, we develop a quasielectrostatic theory describing the coupling of plasmon modes and we discuss its implications for the analysis and the design of the scattering and absorption spectra of complex metal nanostructures. In particular, we show that the interaction of bright plasmon modes determines the onset of zeros in the scattering spectra of nanoscale coupled systems. Under well-defined conditions, these zeros give rise to asymmetric scattering line shapes similar to the spectral signatures described by Ugo Fano in the context of atomic physics. We provide rigorous conditions in which Fano-like resonances occur, and we introduce a method for the direct calculation of their spectral position. In addition, we investigate the role of dark and bright modes in the power absorption near a Fano-like resonance. Our analysis demonstrates the quasielectrostatic origin of Fano-like resonances in subwavelength plasmonic structures.