Evanescent resonant mode for a T-shaped cavity in a terahertz parallel-plate waveguide

The evanescent resonant modes are proposed for a T-shaped cavity in a terahertz (THz) parallel-plate waveguide (PPWG) to understand the essence of the resonant phenomenon. The theoretical modal field and the resonant frequency of the TE₂-like evanescent resonant mode show agreement with the simulated ones through the whole THz region. The modal field consists of three evanescent fields outside the cross region and one propagation field inside the cross region. When the TE₂-like mode is excited, the input THz wave is reflected. The resonance frequency of the TE₂-like mode depends closely on the geometrical parameters of the T-shaped cavity. In particular, the separation of the PPWG must be twice larger than the width of the groove to avoid the non-evanescent electric field in the groove. In addition, the electric field of the TE₁-like mode is localized in the T-shaped cavity. These results have potential applications in field enhancement, sensors, and filters.