  We present a novel theoretical approach for modeling the resonant properties
of transmission through subwavelength apertures penetrating metal films. We
show that cavity mode theory applies to an effective resonant cavity whose
dimensions are determined by the aperture's geometry and the evanescent decay
lengths of the associated diffracted waves. This method suggests a concrete
physical mechanism for the enhanced transmission observed in periodic aperture
arrays, namely it is the evanescently scattered light, localized in the near
field of metal surface, which couples into the apertures. Furthermore, it
analytically predicts the frequencies of peaks in enhanced transmission, the
quality factor of the peaks, and explains their dependence on variation in the
hole radius, periodicity, and the film thickness over a wide range of
geometries. This model demonstrates strong correlation to simulation and
existing results with a high degree of accuracy.
