A trapped mode by higher-order Fano-like interference in a symmetric plasmonic structure

In this work, we have proposed a trapped mode arising from higher-order Fano-like interference in a symmetric plasmonic structure, which results in an electromagnetically induced transparency-like spectrum. The current oscillation of this trapped mode is rigorously anti-symmetric about the mirror plane parallel to the current oscillation and symmetric about the mirror plane perpendicular to the current oscillation, so the radiation loss is extremely weak. A modified model of two-driven coupled oscillators with frequency-dependent dissipation coefficients has been employed to describe the Fano-like interference. The Fano-like interference between two localized modes will evolve into the interference between a surface lattice mode and a localized mode on condition that the lattice mode forms through diffractive coupling. The resulting trapped mode simultaneously possesses both characteristics of localized and collective oscillation. Slow light with a group index as large as 90 could be realized by this interference.