Enhanced microwave transmission and focusing effect through step structure with subwavelength slit arrays

We explore the transmission and focusing properties of a step structure with subwavelength slit arrays in metallic aluminum plates. We show that the microcavities formed between adjacent step can greatly enhance the coupling of the incident wave into the slit by improving the impedance matching between the incident wave and the waveguide mode, leading to X-band (8-12 GHz) transmission enhancement and efficient focusing. In addition, the electric field is localized and enhanced at the slit exits, in contrast with non-step slit gratings. By putting the time gate in the time domain of transmission spectra, we are able to accurately calculate the improved coupling efficiency to illustrate the enhancement at both of the resonant and nonresonant frequencies. The simulation by a finite element code agrees well with the experimental results. Potential applications of this effect can be anticipated in microwave devices.