Generation of Hankel-type surface plasmon polaritons in the vicinity of a metallic nanohole

It is shown that the electromagnetic fields of surface plasmon polaritons (SPPs) generated around a nanohole, milled in a metal film, can be described by Hankel functions. These SPPs are dipole active and can be excited by a linearly polarized electromagnetic plane wave under normal incidence (with respect to the metal surface). Two kinds of Hankel-type SPPs are generated simultaneously around a nanohole: inward and outward propagating with respect to the nanohole. The wave fields of the Hankel-type SPPs increase anomalously in the close vicinity of the nanohole and exceed considerably that of the incident wave. It is shown analytically that the excitation cross section of Hankel-type SPPs, around a nanohole with excitation area limited by circular boundary, is significantly higher compared to the case of an infinite metal surface. We conclude that the unusually high transmission of light through arrays of nanoholes milled in a metal film can be ascribed to the excitation of inward Hankel-type SPPs, which transfer the energy toward the nanohole region.