Strong Influence of Hole Shape on Extraordinary Transmission through Periodic Arrays of Subwavelength Holes

We show that extraordinary light transmission of periodic subwavelength hole arrays, generally attributed to surface-plasmon resonances, is strongly influenced by the hole shape. Both experiments and calculations, based on a Fourier modal method, demonstrate that a shape change from circular to rectangular increases the normalized transmission by an order of magnitude while the hole area decreases. Moreover, the spectra exhibit large redshifts ($\sim 2500{\mathrm{cm}}^{-1}$). A comparison with the transmission of isolated holes shows that shape resonances of the rectangular holes play a dominant role.