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.