Resonant transmission of microwaves through subwavelength fractal slits in a metallic plate

We demonstrate through both experiment and theory that in the microwave regime one can have high transmittance through narrow fractal-patterned slits in thick metallic plates, whereby the aperture is subwavelength in all cross sectional dimensions. In contrast to the recently discovered extraordinary transmissions via surface plasmon excitations and Fabry-Perot-like resonances, the transmission in the present case is independent of the incident angle, plate thickness, or array periodicity. We show the physics to be governed by the transversal shape resonance localized in the metallic fractal slots. In particular, for the lowest transmission mode the EM field experiences no phase change when transmitting through the metallic plate. Simulation demonstrates the viability of the observed phenomenon as a subwavelength $k=0$ waveguide mode, where k is the axial wave number.