Deep subwavelength electromagnetic transparence through dual metallic gratings with ultranarrow slits

We study the transmission response of microwaves through two identical metallic plates perforated with periodical ultranarrow slits. The experimental and numerical transmission spectra consistently display a striking transmission peak at wavelength much larger than any characteristic length of the microstructure (e.g., about 20-fold of the lattice constant), which can not be directly interpreted 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.