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In order to understand the details of the extraordinary enhancement of the optical transmission through subwavelength holes in metal films, we have fabricated perforated free-standing metal films where the nature of the bulk and surfaces can be controlled independently. We find that the transmission enhancement depends only on the dielectric properties of the metal within a skin depth of the two in-plane surfaces and not on those of the film core, nor on the metal constituting the hole walls. This provides direct and conclusive evidence that the phenomenon is mediated by surface plasmons.
Grupp et al. (2000) studied this question.