Photonic Wannier-Stark Ladder from Coupled Electromagnetic Cavities
(a) The 8 stacked metallic plates on which a periodic array of fractal slits was generated. In the schematic picture, the plate has 2 × 2 unit cells, each of which is a five-level H-fractal slit. (b) The 8 plates are embedded, respectively, inside 8 dielectrics, which have the refractive index, , where takes the denoted numbers. The index gradient is along the propagation direction, as pointed by the arrow. The dash lines depict a single EM cavity cell composed of a structured plate and a homogeneous dielectric. (c) The simulated transmissions at normal incidence of microwave, where three cases, no, small, and large gradient of refractive index, are considered.