Terahertz HEB mixers with the Negative Electrothermal Feedback

Superconducting hot-electron bolometers (HEB) occupy a unique niche being the only suitable heterodyne detector technology for astrophysics in the 1.3-5 THz range. After their inception in 1990, the HEB mixers have become widespread and used by many research groups worldwide. The most noticeable application has been on the space-borne Herschel/HIFI instrument but also SOFIA/GREAT & upGREAT instruments, and on past and upcoming balloon instruments such as STO, STO-2, GUSTO, and ASTHROS. All these instruments use the same superconducting detector element, a tiny HEB device made from NbN film. This material allows operation at 4K with the bandwidth of a few GHz which is uniquely broad compared to other conventional low-Tc superconductors. An emerging MgB2-based HEB mixer offers a higher operating temperature (up to 20K) and even broader bandwidth (up to ~ 10 GHz). The main drawback of all HEB mixers is an inferior sensitivity to that of the Superconductor-Insulator-Superconductor (SIS) junction mixers which are nearly perfect detectors in many ways except their frequency range is limited by the value of superconducting gap in technologically feasible materials (≈ 1.3 THz). For example, a state-of-the-art SIS mixer demonstrates the double-sideband noise temperature Tm ≈ 25 K @ 580 GHz (optical loss removed) that is very close to the quantum-limited sensitivity (hf/2kB ≈ 14K). In contrast, the HEB mixers typically have Tm ≈ 1,000 K in the 2-5 THz range, which is much greater than hf/2kB ~ 60 K. This large sensitivity gap is poorly understood; the intrinsic detector thermal noise should amount to a ~ 20-25 K in addition to the quantum limit. It is important to address this issue which plagues practically all current HEB receivers. We believe that the performance of the existing HEB mixers can be significantly improved if a different way of biasing is adopted. A close analogy is the situation with the superconducting transition-edge sensors (TES) direct detectors, which have improved substantially after the negative Electrothermal Feedback (nETF) was implemented in 1995 through the voltage bias. nETF increases the detector signal bandwidth and reduces the contribution of the Johnson noise and the readout amplifier noise in the resistive bolometer. In contrast, HEB mixers always operate with the positive ETF (pETF). This is because of instabilities arising in the dc bias circuit and/or the GHz IF circuit when the HEB is biased in a way that its current-voltage characteristic is N-shaped (negative differential resistance). If the electrical stability were ensured the conversion gain would increase greatly, thus the noise temperature would decrease. We propose to investigate this operating regime and develop specialized circuits preventing unwanted oscillations and enabling the nETF in the GHz range. Existing mixer devices (both NbN and MgB2) will be investigated. The work will include the development of an accurate thermal-electrical model of the HEB mixer, the electromagnetic design of the specialized dc and IF circuits, and the experimental comparison of the THz HEB mixer performance in the traditional and pETF and new nETF modes. We anticipate a strong impact of this research on the entire field of heterodyne instruments. As a result, there will be no need to pursue better HEB materials or undertake other expensive development steps. Only modification of the IF circuitry in the existing mixer blocks will be needed. The payoffs will be the IF bandwidth increase and the noise temperature reduction by several times which would lead to an order of magnitude increase in the scientific return on the heterodyne missions. The near-term application of this approach could be on the follow-up to the ASTHROS balloon instrument.

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