Dipole Radiation-Induced Extraordinary Optical Transmission for Silver Nanorod-Covered Silver Nanohole Arrays

A new extraordinary optical transmission (EOT) mode is discovered when a hexagonal Ag nanohole array is covered by a tilted Ag nanorod array. The resonant wavelength of this EOT mode red-shifts with respect to the normal EOT mode predicted by plasmon-grating coupling theory or dynamic scattering theory and increases linearly with the nanorod length. The structures also show strong polarization dependence; i.e., when the electric field (E-field) direction of the incident light is perpendicular to the long axis of the nanorods, only normal EOT spectrum is visible, but when the E-field is parallel to nanorod axis, the new mode appears. Our finite-difference-time-domain calculations confirm this finding and show that this new mode is due to the dipole radiation of the nanorods on top of the nanoholes. It is expected that other complicated unit cells of nanohole arrays could generate other new EOT modes and can be used to design new optical devices.