Broadband Metamaterial Termination for Planar Superconducting Transmission Line Circuits

The broadband metamaterial termination for use in planar superconducting transmission lines has been successfully demonstrated in CLASS circuit structures as an effective termination. This metamaterial implementation is fully compatible with microfabrication techniques commonly used for microwave circuitry, and its response is insensitive to geometric tolerances, material properties, and interface details of conductive elements in device fabrication. In the context of far-infrared imaging, polarimetric, and superconducting integral field unit (IFU) spectrometer arrays for astrophysics, this strategy leads to higher performance, increased device yield, and greater overall circuit density. The metamaterial termination achieves a broadband absorption response with lower reflectance in a smaller physical footprint compared to existing adiabatic structures. This absorption response demonstrates significantly lower sensitivity to fabrication tolerances, material properties, and modeling assumptions than previous designs. These characteristics are critical for cryogenic applications, but the termination can also enhance the performance of room-temperature planar transmission line structures used in microwave engineering. The termination is realized as a lossy stepped impedance transition between Nb and PdAu, which reduces the total meander length, device footprint, and sensitivity to detailed implementation. This broadband metamaterial termination is applicable in superconducting technologies, including quantum communications, computing, and sensors. It has reached Technology Readiness Level (TRL) 7 (technology demonstrated in an operational environment) and is now available for patent licensing. NASA scientists at Goddard Space Flight Center have developed a broadband metamaterial termination for planar superconducting transmission line circuits. This technology was created as part of a collaborative effort with researchers on the Cosmology Large Angular Scale Surveyor (CLASS) microwave telescope array. Key improvements included adding an adiabatic absorber at the magic tee and vialess crossovers, which significantly reduces reflectance and sensitivity to fabrication variations. These changes ensured better impedance matching across the system, enhancing the fidelity and performance of the microwave circuits. The redesign aimed to address the limitations of earlier narrowband, resonant-tuned terminations that were highly sensitive to manufacturing tolerances, particularly at cryogenic temperatures. These traditional designs required strict control over surface resistance to meet transmission line impedance targets, which proved challenging and resulted in poor device yield at lower temperatures. This approach also provides better adaptability for high-performance microwave circuits used in astrophysical sensors and superconducting applications, where precision and reliability are paramount.