Role of surface plasmon polaritons on optical transmission through double layer metallic hole arrays

We study analytically and numerically the far-field extraordinary optical transmission (EOT) through double-layer metallic grating structures patterned with subwavelength hole arrays. In addition to EOT phenomena due to the well-known surface plasmon polaritons (SPPs) on the outer surfaces such as those on a single layer hole array, further EOT peaks are observed. The separation between the metallic layers is small enough to allow SPPs propagating through the inner interfaces to couple and form an internal SPP, with a different dispersion relation from the outer one and so giving rise to EOT peaks at different frequencies. We propose a relatively simple model to predict the frequencies of those EOT peaks. Internal SPPs show certain unique properties different from the external SPPs: they can give rise to a magnetic response and a negative effective permeability, and the transmission of their EOT peaks increases when no direct line of sight is allowed through the structure. All these findings may be utilized in wavelength tuning of extraordinary optical transmission in subwavelength optics.