Characterization system for terahertz-band transmission properties of high-temperature superconductors

The terahertz (THz) frequency range is a vital window for modern astronomy, rich in atomic and molecular transitions that reveal the physical conditions of the interstellar medium and trace key stages of galaxy evolution. Superconducting detectors offer the sensitivity required in this band, but current systems rely mainly on low-temperature superconductors (LTS) such as aluminum, niobium, and niobium nitride. Their required operating temperatures below 4 K necessitate bulky and power-intensive cryogenic systems, greatly limiting the scalability and deployment of large detector arrays, especially for space missions or remote observatories.To overcome these limitations, this work investigates copper-based high-temperature superconductors (HTS), focusing on yttrium barium copper oxide (YBCO). With a critical temperature over 90 K, YBCO devices could, in principle, operate with simpler liquid-nitrogen cooling, reducing system complexity and cost. Here, we evaluate the conductive loss characteristics of YBCO thin films in the W-band (75–110 GHz), an essential step toward developing practical HTS THz detectors.We fabricated half-wavelength microstrip resonators from YBCO thin films grown on magnesium oxide (MgO) substrates and developed a dedicated waveguide-coupled measurement system operating from 4.5 K to 120 K. The setup combines a terahertz vector network analyzer with a closed-cycle 4 K cryocooler. Stainless-steel waveguides with gold-plated interiors were employed and thermally anchored to balance transmission loss against heat conduction. A custom vacuum-seal flange with a thin Mylar membrane maintained the cryogenic vacuum while permitting efficient THz transmission. The measured insertion loss of the complete system was approximately 12 dB across 90–105 GHz.Initial measurements at 4.5 K showed clear resonance peaks whose frequencies agreed well with simulations. The extracted unloaded quality factor (Q ≈ 140) was comparable to gold resonators tested under identical conditions, confirming the basic operability of YBCO in the W-band. A pronounced temperature dependence was observed: the Q-factor remained stable between 4 K and 10 K but degraded rapidly above 30 K, indicating strong high-frequency loss mechanisms that differ from microwave behaviors and are not explained by the YBCO energy gap alone.In summary, this work establishes a robust methodology for characterizing HTS thin films in the millimeter-wave regime and demonstrates the initial feasibility of YBCO resonators at W-band frequencies. The results provide essential insight into temperature-dependent losses and lay the foundation for future optimization of YBCO film quality, fabrication techniques, and resonator design, with the long-term goal of enabling practical, high-performance HTS THz detectors.