Design, fabrication, and measurement of terahertz mode converter

In this report, two mode converters are designed and fabricated to extract TE₄₁ and TE₀₁ modes for the generations of 0.4 THz and 1 THz signal by high-harmonic interaction with gyrating electron beam, respectively. The purities of the desired modes in these two converters are both higher than 90% with remarkably broad bandwidth (380 GHz to 420 GHz for TE₄₁ mode and 910GHz to 995 GHz for TE₀₁ mode). Since the geometrical aspect ratios of the designed mode converters are both larger than 5, it is too difficult to realize them by the traditional machining processes and UV lithography. We therefore employed X-ray lithography to fabricate these designs, which will be characterized by THz time-domain spectroscopy.