Effect of Paired Apertures in a Periodic Hole Array on Higher Order Plasmon Modes

We demonstrate that the transmission of higher-order surface plasmon modes in the mid-infrared range can be enhanced through rectangular hole array on the basis of paired apertures. Experiments prove that enhanced high-order transmission can be generated by either identical shapes or combinations of different hole shapes in pairs. The structure factor is adopted to explain the observed intensity of enhanced transmission. Numerical simulations of the enhanced second-order mode verify a significant field enhancement in a unit cell of pairs. It is clarified that the separation between the paired apertures and the paired resonance is the key to determine certain higher-order plasmon modes to be enhanced.