LAUSR

We propose a thermally tunable terahertz narrow band filter by combining two different periodic waveguides. When the terahertz tube wall is corrugated periodically, the transparent spectrum presents a very complex structure as various pass and stopbands emerging. The terahertz wave cannot propagate through the waveguide when its frequency falls into the stopbands. When we connected two tubes with different types of stopbands, it is very intriguing that we have found an unexpected ultra-narrow-band transparency in the frequency gap. Our findings provide a more feasible and effective terahertz waveguide filter beyond these existing ones. By using the finite-element method, we have obtained the filtering characteristics of the proposed structures. The filter bandwidth could be narrowed from 2.3 GHz to 0.29 MHz by increasing the number of waveguide segments and the highest $Q$ -factor achieved is about $3.5 \times 10^{6}$ . Moreover, the tunability on temperature is obtained in a waveguide device of Au based on the combination of opaque structures.