  We present a systematic analysis of the optical properties of slit arrays in
metal films. An exhaustive investigation of geometrical and dispersive
properties reveals the resonance features of these structures, including the
role of surface waves and their relationship with features in the transmission
spectrum. Although enhanced transmission windows are significantly dominated by
the longitudinal resonances localized inside the slits, the periodicity
introduces transverse resonances that can either enhance or inhibit light
transmission. We thus illustrate the intriguing interaction regime between
longitudinal and transverse resonances, where the two modes hybridize leading
to the formation of a photonic band gap spectrum.
