There are great interests in developing Nb SIS mixers because of the extremely low noise temperatures and because of the need of low local oscillator (LO) power. Several groups have demonstrated experimentally that Nb SIS mixers with integrated tuning elements can perform near the quantum noise limit up to the Nb gap frequency (/spl sim/700 GHz). Beyond this frequency, a major loss will take place in the tuning elements, which are Nb-SiO/sub 2/-Nb micro-striplines, because the incoming photons can break the Cooper-pairs, thus increasing the surface resistance of the Nb strips. Applying highly conductive Al instead of Nb for a stripline can reduce the loss since the former has a lower surface resistance at higher frequencies. In this work, we report the fabrication of waveguide Nb SIS mixers with Al striplines and heterodyne measurement results around 1 THz.
There are great interests in developing Nb SIS mixers because of the extremely low noise temperatures and because of the need of low local oscillator (LO) power. Several groups have demonstrated experimentally that Nb SIS mixers with integrated tuning elements can perform near the quantum noise limit up to the Nb gap frequency (/spl sim/700 GHz). Beyond this frequency, a major loss will take place in the tuning elements, which are Nb-SiO/sub 2/-Nb micro-striplines, because the incoming photons can break the Cooper-pairs, thus increasing the surface resistance of the Nb strips. Applying highly conductive Al instead of Nb for a stripline can reduce the loss since the former has a lower surface resistance at higher frequencies. In this work, we report the fabrication of waveguide Nb SIS mixers with Al striplines and heterodyne measurement results around 1 THz.
We have succeeded in developing a novel fabrication process for Nb SIS junctions integrated with an aluminium (Al) stripline for 1 THz waveguide mixers. In particular, a pure Al film, which is evaporated in an ultra-high vacuum system, is applied for the stripline. It can provide a lower surface resistance in comparison with a sputtered Al film, consequently reducing radio frequency (RF) loss in the tuning element. The junctions are fabricated with standard photolithography and typical junction areas are 1 f4m 2 . The heterodyne measurements performed at a temperature of 4.2 K show that high frequency video response extends up to 1.1 THz and the best DSB noise temperatures are 700 K at 900 GHz and 1000 K at 1 THz.
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.
We have built a waveguide SIS mixer with Nb-based parallel-connected twin junctions at 800 GHz which is above the gap frequency of Nb (690 GHz). Heterodyne mixing was performed at 810 GHz and a receiver noise temperature of 580 K (DSB), which includes noise contributions due to a vacuum window and a beam splitter, was achieved using Nb/AlO/sub x//Nb junctions with Nb striplines. The performance of this mixer is quite good and it is shown that efficient mixing in Nb-based SIS junctions is still possible above the gap frequency, SIS mixers with Al striplines have also been fabricated and good DC I-V characteristics have been observed. Heterodyne measurements of these mixers are now under preparation.
This thesis describes the development of low-noise heterodyne receivers at THz frequencies for submillimeter astronomy using Nb-based superconductor-insulator-superconductor (SIS) tunneling junctions. The mixers utilize a quasi-optical configuration which consists of a planar twin-slot antenna and antisymmetrically-fed two-junctions on an antireflection-coated silicon hyperhemispherical lens. On-chip integrated tuning circuits, in the form of microstrip lines, are used to obtain maximum coupling efficiency in the designed frequency band. To reduce the rf losses in the integrated tuning circuits above the superconducting Nb gap frequency (~ 700 GHz), normal-metal Al is used to replace Nb as the tuning circuits. To account the rf losses in the micros trip lines, we calculated the surface impedance of the AI films using the nonlocal anomalous skin effect for finite thickness films. Nb films were calculated using the Mattis-Bardeen theory in the extreme anomalous limit. Our calculations show that the losses of the Al and Nb microstrip lines are about equal at 830 GHz. For Al-wiring and Nb-wiring mixers both optimized at 1050 GHz, the RF coupling efficiency of Al-wiring mixer is higher than that of Nb-wiring one by almost 50%. We have designed both Nb-wiring and Al-wiring mixers below and above the gap frequency. A Fourier transform spectrometer (FTS) has been constructed especially for the study of the frequency response of SIS receivers. This FTS features large aperture size (10 inch) and high frequency resolution (114 MHz). The FTS spectra, obtained using the SIS receivers as direct detectors on the FTS, agree quite well with our theoretical simulations. We have also, for the first time, measured the FTS heterodyne response of an SIS mixer at sufficiently high resolution to resolve the LO and the sidebands. Heterodyne measurements of our SIS receivers with Nb-wiring or Al-wiring have yielded results which arc among the best reported to date for broadband heterodyne receivers. The Nb-wiring mixers, covering 400 - 850 GHz band with four separate fixed-tuned mixers, have uncorrected DSB receiver noise temperature around 5hv/kb to 700 GHz, and better than 540 K at 808 GHz. An Al-wiring mixer designed for 1050 GHz band has an uncorrected DSB receiver noise temperature 840 K at 1042 GHz and 2.5 K bath temperature. Mixer performance analysis shows that Nb junctions can work well up to twice the gap frequency and the major cause of loss above the gap frequency is the rf losses in the microstrip tuning structures. Further advances in THz SIS mixers may be possible using circuits fabricated with higher-gap superconductors such as NbN. However, this will require high-quality films with low RF surface resistance at THz frequencies.
It is expected that the integration of NbTiN striplines with Nb-based SIS junctions will extend the range of operation of sensitive SIS mixers up to the energy gap of NbTiN (— 1.2-1.25 THz) to meet the requirements of the HIFI instrument for the Far-Infrared Space Telescope (FIRST). We have fabricated prototype RS waveguide mixers in which Nb/A1-A10,INb SIS junctions are integrated with NbTiN and Al wiring layers. DC current-voltage measurements of these devices show that the voltage gap is somewhat lower and the sub-gap current is somewhat higher for devices with two NbTiN wiring layers in comparison with those in which the top wiring layer is Al (Vgap = 2.7 mV vs. 2.8 mV, and R 2.0/RN — 32 vs. 35). Additionally, the gap voltage of the NbTiN stripline devices is further suppressed by the application of 890-1040 GHz local oscillator power during heterodyne measurements. These effects are attributed to the trapping of heat in the Nb junction electrodes due primarily to quasi-particle trapping at the Nb/NbTiN interface. FTS measurements result in what is believed to be the first reported observation of direct detection response in a NbTiN stripline device above 1.1 'THz. From an analysis of the resonance frequencies of several devices, it appears that the frequency response of the NbTiN striplines is predicted reasonably well by the MattisBardeen theory for the conductivity of a superconductor in the extreme anomalous limit. Furthermore, we present what is believed to be the first reported heterodyne measurements of NbTiN stripline devices in the 0.9-1 THz range. These measurements yield a receiver sensitivity of TN = 830 K at 930 GHz at a bath temperature of 3.3 K, after correction for beamsplitter losses.
We report on the continuing development of submillimeter quasi-optical slot antenna SIS mixers which use two-junction tuning circuits [1]. The mixers use 10 kA/cm2 Nb/Al-Oxide/Nb junctions and Nb wiring, and have generated DSB receiver noise temperatures around 5hv/kB to 700 GHz, and 16hv/kB (620 K) at 798 GHz. We present Fourier-transform spectrometer (FTS) measurements and heterodyne measurements on several such devices. In general, the measured response at frequencies below the gap of Nb is in good agreement with the predicted performance. Introduction SIS mixers with Nb-trilayer tunnel junctions offer excellent performance at frequencies below 800 GHz, and should perform competitively at frequencies as high as 1.4 THz [2-5]. Our goal is to develop a suite of ultra-low-noise SIS mixers which cover frequencies from = 400 GHz to more than I THz. At frequencies below the Nb gap (< 700 GHz), existing devices perform quite well [6], but further improvements in noise temperatures remain important. In addition, it is quite useful to be able to predict device performance given the design parameters. One can then optimize the device design for a particular frequency band, and confirm that we indeed understand the physics of the device. Progress in the design and operation of devices in this frequency range is presented below. At frequencies above the Nb gap, RF photons can break Cooper pairs, resulting in greater signal loss and higher receiver noise temperatures. The loss is particularly important in the slender microstrip lines used to resonate the junction capacitance and to transform the antenna impedance down to the RF junction impedance. However, the actual coupling of radiation into Nb junctions above the gap should still be reasonably efficient -more than 30% at 800 GHz [7]. In addition to RF loss, one is also affected by increased dispersion at frequencies near the gap. The Nb microstriplines have therefore been carefully designed to ensure the desired bandpass is achieved. We demonstrate for the first time that all-Nb SIS receivers can have substantially better performance than GaAs Schottky receivers for the astronomically important CI ( P2 — P 1 ) and CO (7-6) transitions near 810 GHz. Receiver Configuration Since waveguide mixers become difficult to construct at short submillimeter wavelengths, we have adopted a quasi-optical approach. Here, lenses take the place of waveguide horns, and the incoming radiation is collected by a planar antenna on the SIS mixer substrate [8]. This offers several other advantages, such as on-chip broad-band silt a SS 114 SSE WA Wa Sta MX Dewar 120 Mylar Window SIS Mixer 295 IQ 77 K Mylar Load Bearnsplitter ICI 1= NZ IC =1 HEN1T Amplifier Polyethylene Lens