Fano Interference Between Higher Localized and Propagating Surface Plasmon Modes in Nanovoid Arrays

Surface plasmon resonance-based optical properties for gold nanovoid arrays were analyzed using three-dimensional finite-difference time-domain method, revealing that two asymmetric Fano resonances, with extremely low reflectivity and narrow line width, are caused by the strong interference between localized quadrupolar and hexadecapolar modes and propagating surface plasmon mode. The two Fano resonances produce significant electromagnetic field enhancement, resulting in 7 orders of magnitude surface-enhanced Raman scattering (SERS) enhancement, with about 100 times more than that produced by single propagating surface plasmon polaritons (SPPs) or localized surface plasmon resonance (LSPR) mode, including dipolar mode. Their narrow line width and giant field strength also allow highly sensitive sensing of refractive index; the sensitivities and figure of merits for such Fano resonance-based sensor are predicted to be 789 and 642 nm/RIU, 39 and 31, respectively. These plasmonic characteristics, thus, would enable the nanovoid arrays for using in wide practical applications including surface-enhanced spectroscopy and surface plasmon sensor.