Optical behaviour of thick gold and silver films with periodic circular nanohole arrays

To better understand the enhanced optical transmission observed with an array of nanoholes on optically thick metallic films, an extensive numerical calculation is presented in this paper to relate the scattering modes observed to transmittance spectral behaviour. Surface plasmon polariton Bloch waves, Wood's anomalies and localized surface plasmon resonances at the rim of the nanoholes are investigated based on scattering modes that form the features of the transmission spectrum. The finite difference time-domain method is applied to calculate the transmission intensity from films of gold and silver subjected to variations in geometrical parameters such as film thickness, spatial period of the structures and hole diameter. Analysis of transmission spectra showed that the cut-off frequency of the array of subwavelength holes is mostly defined by the thickness of the film and the diameter of the holes rather than the periodicity of the structures.