The influence of the air gap on the response of transmission for a transverse-electric mode parallel plate waveguide with a single deep groove has been experimentally studied. As the air gap is larger than the resonant wavelength of a high-order cavity mode in a single deep grooved waveguide, only the fundamental cavity mode can be excited and the single resonance (band) can be observed in a transmission spectrum. The decrease of the air gap can not only efficiently push the radiation of the fundamental cavity mode into the deep groove but also excite the high-order cavity modes, resulting in multiple resonances (multiband) in the corresponding spectrum. Based on the above observations, a tunable multiband terahertz notch filter has been proposed and the variation of the air gap has turned out to be an effective method to select band number. Experimental data and simulated results verify this band number tunability.
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