| Summary: | Transmission of light through periodic metal films with intensity considerably exceeding that predicted by aperture theory is referred to as extraordinary optical transmission (EOT). The mechanisms responsible for this effect have been investigated intensively during the past decade. Here, we show an elegant method of visualizing the operative physical mechanisms for model resonance systems. By numerically mapping the resonance loci, modal and plasmonic mechanisms emerge clearly with delineated regions of dominance. Thus, the photonic transmission resonances are parametrically correlated with localized electromagnetic fields forming pure surface-plasmon polaritons (SPPs), coexisting plasmonic and cavity-mode (CM) states, and pure CMs. This mapping method renders a consistent picture of the transitions between photonic states in terms of key parameters. It shows how the TM<sub>1</sub> CM seamlessly morphs into the odd SPP mode as the film thickness diminishes. Similarly, the TM<sub>0</sub> mode converts to the even SPP mode. At the intersection of these mode curves, an EOT-free gap forms due to their interaction. On account of a reflection phase shift of a slit-guided mode, an abrupt transition of the resonance loci in the SPP/CM region is observed.
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