We compare the design of two 0.85 THz SIS mixers fed with a radial probe antenna aligned to the E-Plane of the input full-height rectangular waveguide connected to a drilled smooth-walled horn. Both designs employ the same 0.5 µm2 hybrid Nb/AlN/NbN tunnel junction technology, sandwiched between a NbTiN ground and aluminium wiring layer fabricated on top of a 40 µm quartz substrate. The two designs is differed by how we tune out the unwanted junction capacitance for broadband performance. The first design uses the commonly-used twin-junction tuning scheme; whilst the second design utilises an end-loaded scheme. We successfully achieve close to 2× the double sideband quantum noise performance for both schemes, but the twin-junction design is less sensitive to fabrication accuracy of planar circuit components utilised. However, the end-loaded design offers a much better IF bandwidth performance, almost twice wider than the twin-junction design. The need for an ultra-wide IF bandwidth mixer is becoming more pressing and important for the future and up-coming upgrades of various millimetre (mm) and sub-mm astronomical instruments, hence we conclude that the end-loaded design is a better solution for the THz heterodyne mixing applications.
We compare the design of two 0.85 THz SIS mixers fed with a radial probe antenna aligned to the E-Plane of the input full-height rectangular waveguide connected to a drilled smooth-walled horn. Both designs employ the same 0.5 µm2 hybrid Nb/AlN/NbN tunnel junction technology, sandwiched between a NbTiN ground and aluminium wiring layer fabricated on top of a 40 µm quartz substrate. The two designs is differed by how we tune out the unwanted junction capacitance for broadband performance. The first design uses the commonly-used twin-junction tuning scheme; whilst the second design utilises an end-loaded scheme. We successfully achieve close to 2× the double sideband quantum noise performance for both schemes, but the twin-junction design is less sensitive to fabrication accuracy of planar circuit components utilised. However, the end-loaded design offers a much better IF bandwidth performance, almost twice wider than the twin-junction design. The need for an ultra-wide IF bandwidth mixer is becoming more pressing and important for the future and up-coming upgrades of various millimetre (mm) and sub-mm astronomical instruments, hence we conclude that the end-loaded design is a better solution for the THz heterodyne mixing applications.
Abstract We present and compare the design and performance of two 850 GHz radial probe fed superconductor-insulator-superconductor mixers, where the antenna is aligned perpendicular to the E-Plane of the input full-height rectangular waveguide connected to a multiple flare-angles smooth-walled horn. Both designs are comprised of 0.5 µm2 hybrid niobium/aluminium-nitride/niobium-nitride tunnel junction, fabricated on top of a niobium titanium nitride ground plane with an Al wiring layer. The entire superconducting circuit is supported with a 40 µm thick quartz substrate. The major difference between the two designs is the method used to cancel out the parasitic junction capacitance for broadband performance. The first design utilises two identical junctions connected in parallel with a short transmission line to convert the capacitance of one junction into the equivalent inductance of the other junction, commonly known as the twin-junction tuning scheme; whilst the second design employs an end-loaded scheme with only one tunnel junction. We found that both methods offer similar radio frequency performances, with close to 2× the double sideband quantum noise temperature, but the twin-junction design is more resilient to fabrication tolerances. However, the end-loaded design offers a much better intermediate frequency (IF) bandwidth performance, made possible by the sub-micron and high current density tunnel junction technology. The improved IF performance is important for many millimetre (mm) and sub-mm observatories, such as future upgrades of Atacama Large Millimetre/sub-mm Array receivers, as well as forthcoming space-borne far-infrared missions. Therefore, we conclude that the single-junction mixer design is the preferred option for THz applications, as long as the fabrication error can be minimised within a certain limit.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
In THz regime, NbN Superconductor - Insulator - Superconductor (SIS) mixers are potentially applicable to astronomy and atmospheric research. We have developed a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and NbN/MgO/NbN tuning circuit. In this paper, we compare it with a 0.5-THz Nb SIS mixer, which has been successfully used on POST telescope. The design and fabrication parameters of the two mixers are introduced, and the noise temperatures are measured. In addition, the direct detection performance is analyzed, such as the shot noise, the current responsivity and the noise equivalent power. The detailed results are presented.
With a gap frequency of 1.4 THz, all-NbN Superconductor-Insulator-Superconductor (SIS) junctions are potentially applicable to sensitive heterodyne mixers for THz astronomy and atmospheric research. In this paper, we report the design, fabrication, and performance characterization of a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and an NbN/MgO/NbN tuning circuit with no impedance transformers. The noise performance of two NbN SIS junctions with different tuning inductances is measured, compared with the simulated results without and with the multiple Andreev reflection (MAR) effect taken into account, and also compared with the video detection response. In addition, the IF-output-power stability and aging effect of fabricated NbN SIS junctions are investigated.
For the optimum design of integrated receivers operating above the gap frequency of Nb, we have designed, fabricated and tested NbN-based quasi-optical superconductor/insulator/superconductor (SIS) mixers. The mixer chip incorporates a resonant half-wavelength epitaxial NbN/AlN/NbN junction, a twin-slot antenna and their coupling circuits. We adopted two kinds of coupling circuit between the antenna and the SIS junction: one is an in-phase feed with a length of 95 µm and the other is an anti-phase feed of 30 µm length. It was found that the anti-phase mixer reveals a 3 dB bandwidth of 43% of the centre frequency; the uncorrected double-sideband receiver noise temperature TRX = 691 K at 0.91 THz and TRX = 844 K at 0.80 THz, while 17% and TRX = 1250 K at 0.79 THz for the in-phase version. Possible reasons for this difference are discussed, which could be transmission loss and its robustness with respect to the variation of junction parameters. These experimental results suggest the NbN-based distributed mixer with the anti-phase feed is a better candidate for wide-band integrated receivers operating above 0.7 THz.
We are developing improved versions of a slot antenna SIS mixer which we have previously described. The initial work demonstrated a double sideband noise temperature of 420 K for a 500 GHz quasi-optical SIS mixer employing a twin-slot antenna on a quartz dielectric substrate. A quartz hyperhemispherical lens is used to focus the incoming radiation onto the twin-slot antenna. The advantages of a twin-slot antenna include a low impedance (35 omega) and a clean, symmetric beam pattern into the dielectric with a 70 percent efficiency. In our original mixer, the radiation was coupled from the two slot antennas onto superconducting microstrip lines which fed the SIS junction. By performing an impedance transformation using tapered lines and by feeding the radiation from the two slots to the junction in parallel, the effective (real) impedance seen by the junction was reduced to just 4 omega. This very low impedance allowed a junction area of 2.3 sq micron to be used at 500 GHz, which was manufactured using optical lithography. However, no attempt was made to tune out the junction capacitance. We estimate that this capacitance reduces the impedance coupling efficiency to eta(sub Z) approx. equals 0.23, for our junction with omega R(sub N) C = 5.3 at 500 GHz. The recent development of techniques using electron-beam lithography to manufacture junctions with very small areas (approx. equals 0.1 sq microns) now allows considerably more flexibility in the design of SIS mixer circuits. We have redesigned the slot-antenna mixer to take advantage of this possibility. In particular, we have included a novel circuit which allows the junction capacitance to be tune out over a broad bandwidth. For instance, mixers designed for 800 GHz using NbN/MgO/NbN junctions with realistic parameters achieve a 3 dB impedance bandwidth of nearly 400 GHz. Furthermore, our circuit uses only short lengths of microstrip and should be less sensitive to RF losses than other designs. The improved impedance match should give a large reduction in noise temperature as compared to our previous mixer. The new devices are currently under fabrication. Further details of the design and any available experimental results are discussed.