Efficient excitation of cavity resonances of subwavelength metallic gratings

One-dimensional rectangular metallic gratings enable enhanced transmission of light for specific resonance frequencies. Two kinds of modes participating in enhanced transmission have already been demonstrated: waveguide modes and surface plasmon polaritons. Since the original article by Hessel and Oliner [Appl. Opt. 4, 1275 (1965)] pointing out the existence of waveguide modes, little progress was made in their understanding. We present here a careful analysis, and show that the coupling between the light and such resonances can be tremendously improved using an evanescent wave. This leads to enhanced localization of light in cavities, yielding, in particular, a very selective light transmission through these gratings.