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 (~2500 cm-1). A comparison with the transmission of isolated holes shows that shape resonances of the rectangular holes play a dominant role.
Klein Koerkamp, K. J., Enoch, S., Segerink, F. B., van Hulst, N. F., & Kuipers, L. (2004). Strong influence of hole shape on extraordinary transmission through periodic arrays. Physical review letters, 92(18), 183901-1-183901-4. https://doi.org/10.1103/PhysRevLett.92.183901