Deep subwavelength electromagnetic transparency through dual metallic gratings with ultranarrow slits

We study the transmission response of microwaves through two identical metallic plates machined with ultranarrow slit arrays. The measured and calculated spectra consistently display a striking transmission peak at wavelength much larger than any characteristic length of the structure (e. g., about 20-fold of the lattice period), which cannot 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 transmission property can be anticipated, considering the merits of compact and excellent immunity to structural imperfections.