<title>Development of SIS mixers for 1 THz</title>

Abstract

SIS heterodyne mixer technology based on niobium tunnel junctions has now been pushed to frequencies over 1 THz, clearly demonstrating that the SIS junctions are capable of mixing at frequencies up to twice the energy gap frequency (4∆/h). However, the performance degrades rapidly above the gap frequency of niobium (2∆/h ≈ 700 GHz) due to substantial ohmic losses in the on-chip tuning circuit. To solve this problem, the tuning circuit should be fabricated using a superconducting film with a larger energy gap, such as NbN; unfortunately, NbN films often have a substantial excess surface resistance in the submillimeter band. In contrast, the SIS mixer measurements we present in this paper indicate that the losses for NbTiN thin films can be quite low.

Sensor Technology at Submillimeter Wavelengths for Space Applications

“…In the following section we evaluate the Superconductor insulator superconductor (SIS) mixers, Hot Electron Bolometer (HEB) mixers, and Schottky mixers for their usage at submillimeter wavelengths. SIS mixers are the most sensitive mixers available today in the 100-1200 GHz frequency range [33]. These mixers are less sensitive at frequencies beyond the superconductor bandgap (2Δ for NbTin ≈ 1500 GHz) where reverse tunnelling becomes a factor and mixer performance is dominated by circuit losses.…”

Superconducting detectors and mixers for millimeter and submillimeter astrophysics

“…However, above 700 GHz, tuning inductors made with the most commonly used superconductor (niobium) become lossy [225] because photons have enough energy to break Cooper pairs , and the mixer performance degrades rapidly. Various approaches have been used to mitigate this problem [226], including the use of normal [190], [220], [222], [227], [231], [232], [234], [235], [237], [238], [276]- [284]. The HEB data are from [236], [285]- [296].…”

Superconductive hot-electron-bolometer mixer receiver for 800-GHz operation

“…By using materials with gap frequencies higher than that of Nb to form the junction circuitry, such as NbN or NbTiN, it should be possible to extend quantum-limited noise performance using SIS mixers above 700 GHz. Recent progress with NbN-based [2] and NbTiN-based [3] mixers shows promise that this might be achieved. Nevertheless, for either material, the high frequency limit for low-noise operation is about 1.2 THz.…”