The influence of hole shape on the nonlinear optical properties of metallic subwavelength hole arrays is investigated. It is found that the amount of second harmonics generated can be enhanced by changing the hole shape. In part this increase is a direct result of the effect of hole shape on the linear transmission properties. Remarkably, in addition to enhancements that follow directly from the linear properties of the array, we find a hot hole shape. For rectangular holes the effective nonlinear response is enhanced by more than 1 order of magnitude for one particular aspect ratio. This enhancement can be attributed to slow propagation of the fundamental wavelength through the holes which occurs close to the hole cutoff.
Phys. Rev. Lett.

van Nieuwstadt, J. A. H., Sandtke, M., Harmsen, R. H., Segerink, F. B., Prangsma, J. C., Enoch, S., & Kuipers, K. (2006). Strong modification of the nonlinear optical response of metallic subwavelength hole arrays. Phys.Rev.Lett., 97(Article number: 146102), 1–4. doi:10.1103/physrevlett.97.146102