  In this Letter, we study the transmission response of microwaves through two
identical metallic plates machined with ultranarrow slit arrays. The measured
and calculated transmission spectra consistently display a striking
transmission peak at wavelength much larger than any characteristic length of
the structure (e.g., about twenty-fold of the lattice period), which can not be
directly explained by the existing mechanisms. Both the LC-circuit-based
microscopic picture and the effective-medium-based macroscopic model are
established to capture the essential physics behind such unexpected resonance
at the deep subwavelength scale. Prospective applications of this novel
transmission property can be anticipated, considering the merits of compact and
excellent immunity to structural imperfections.
