  We provide rigorous numerical calculations (using the rigorous coupled-wave
analysis) of transmission spectra of various metallic and metamaterial
one-dimensional gratings and identify plasmonic resonances responsible for
enhanced transmission. We argue that the most important mechanism which
influences the resonant transmission is the coupling of incident
electromagnetic wave with two plasmonic waves: lengthwise plasmons, which
propagates along the grating, and crosswise plasmons excited in air gaps.
