Ultra-narrowband terahertz filtering based on transverse standing wave interaction

We propose an ultra-narrowband terahertz (THz) filter based on a parallel plate waveguide with staggered structures. The aligned and staggered periodic corrugations on the plates can result in the Bragg and non-Bragg gaps in the THz band, respectively. It is common to achieve an additional narrow transmission when introducing a defect into the periodic corrugations, which can be a good candidate for THz narrowband filtering. It is found that the transmission in the non-Bragg gap is over one order of magnitude narrower than that in the Bragg gap. The further numerical analyses reveal that the narrowband response grows out of an antisymmetric localized defect mode in the staggered periodic structures, which is created by the coupling of the first and second-order transverse standing waves. These findings pave the way for developing highly frequency-selective, low-loss, and ultra-narrowband filters in integrated environments, with potential applications in advanced THz communication and sensing systems.