Design of terahertz ultra-wide band coupling circuit based on superconducting hot electron bolometer mixer

Superconducting hot-electron-bolometer (HEB) mixer is the most sensitive detector in the terahertz (THz) frequencies. The design of RF coupling circuit is very crucial to the mixer´s sensitivity. In this paper, we present a theoretical model for the calculation of the feed point matching impedance of the coupling circuit, and demonstrate the simulation results with the aid of 3D electromagnetic simulation software (HFSS) and a lumped-gap source method similar to quasi optical antennas in the frequency range of 0.9-1.3 THz. The influences of the slice tolerance of substrate thickness, the high-order modes, and the feed-point size on the embedding impedance at 0.9-1.3 THz have been studied in detail. The mixer´s embedding impedance is simulated to be around 35-j10Ω in the whole working band whose bandwidth can reach 36%. The simulation results are in good agreement with the theoretical results, which provide helpful instructions for the future developments of ultra-wide band and highly sensitive superconducting hot-electron-bolometer (HEB) mixer.