The bandwidth and noise temperature of superconducting-insulating-superconducting (SIS) mixers are strongly determined by the matching between antenna and tunnel-junction. Optimal scientific benefit from Band 9 (600 to 720 GHz) of the Atacama large millimeter array requires a proper design of the matching circuit by taking into account the energy gap of the niobium (680 GHz). In preliminary experiments we have found that tapered structures lead to significant improvements, which suggests that a systematic analysis of optimized geometrical shapes might be beneficial. The tuning circuits are analysed using a model, which includes the Mattis-Bardeen theory for the surface impedance of Nb and which allows arbitrary shapes. We will present simulations and experimental results focusing on the transmission efficiency, which can conveniently be evaluated experimentally by using a Fourier transform spectrometer. We will show how the shape influences the transmission efficiency and relate it to observed noise temperatures of the mixers.
The bandwidth and noise temperature of superconducting-insulating-superconducting (SIS) mixers are strongly determined by the matching between antenna and tunnel-junction. Optimal scientific benefit from Band 9 (600 to 720 GHz) of the Atacama large millimeter array requires a proper design of the matching circuit by taking into account the energy gap of the niobium (680 GHz). In preliminary experiments we have found that tapered structures lead to significant improvements, which suggests that a systematic analysis of optimized geometrical shapes might be beneficial. The tuning circuits are analysed using a model, which includes the Mattis-Bardeen theory for the surface impedance of Nb and which allows arbitrary shapes. We will present simulations and experimental results focusing on the transmission efficiency, which can conveniently be evaluated experimentally by using a Fourier transform spectrometer. We will show how the shape influences the transmission efficiency and relate it to observed noise temperatures of the mixers.
This thesis describes the development, modeling, and optimization of Nb superconducting microstrip tuning circuits for Superconductor-Insulator-Superconductor mixers. The ultimate goal is to extend the quantum limited noise performance of Nb/AlOx/Nb SIS mixers to above the Nb gap frequency (680 GHz). The mixers are intended to be used in the Atacama Large Millimeter Array (ALMA) band 9 frequency range (600-720 GHz). The behavior of Nb microstrip tuning circuits especially above the gap frequency is modeled using the complex conductivity given by the Mattis-Bardeen theory of the anomalous skin effect [72]. A computer code utilizing two-port network theory is utilized to predict and optimize the RF coupling efficiency and bandwidth of the tuning circuits and embedding impedances. The effect of material and geometrical variations on the coupling has been evaluated. The installment of the devices in the waveguide has been found to be a crucial factor explaining the observed scattering in measurement results for nominally identical devices. Despite parameter variations, modeling of the RF coupling efficiency proved to be successful within the limitations set by scattering of the measured results. Using this model, the tuning circuit geometry has been optimized for bandwidth and maximum coupling efficiency. During the work several SIS devices have been fabricated and Fourier transform spectroscopy measurements have been made for RF coupling evaluation. Finally, using simulations, it has been found that application of Nb/AlN/Nb junctions would increase the bandwidth to an extent which should dramatically decrease the sensitivity to parameter variations.
A 600-700GHz SIS mixer with the twin-junction tuning circuit has been designed and fabricated [1] The surface impedance of thin-film superconducting microstrip lines, based on Mattis-Bardeen theory [2], is included in the optimization of RF impedance matching. The receiver noise temperature measured in the frequency range of 630-660GHz is below 200K and the lowest receiver noise temperature of 181K is achieved at 656GHz. The FTS response of the SIS mixer shows a good RF coupling from 600GHz to 700GHz. Both the noise performance and the FTS response can be quantitatively described by numerical results using the quantum theory of mixing. Some detail considerations on the mixer model calculation, such as spreading inductance around the junction tuning structure and mixer's embedding impedance, are discussed. Introduction Nb-based superconductor-insulator-superconductor (SIS) mixers have been used very successfully in sub-mm wavelength detection. Their receiver noise temperature has reached to three times the quantum limit, —3 hfrkB, below the junction's gap frequency. However, at high frequency bands, Nb SIS mixers need to be designed carefully because the photon energy is close to or higher than the gap energy of Nb, A-1.4meV. When the frequency band covers the gap frequency of Nb, —670GHz, the RF loss of Nb superconducting transmission lines increases dramatically and the surface impedance has strong frequency dependence. Usually, the RF properties of a superconducting transmission line can be described by the Mattis-Bardeen theory [2] in good accuracy. From results reported so far [4-9], the best receiver noise temperature is close to 5 hi7kB in the band of 600-700GHz. Despite of the input loss contributed by the experimental setup, some other properties of SIS mixers should be taken into account. In this paper, we characterize the performance of a 600-700GHz mixer by comparing the experimental and simulated results.
Three Nb-A10x-Nb SIS mixers tuned to operate in the 400-550, 550-700 and 600-750 GHz frequency bands have been investigated by modeling and experimentally by employing Fourier Transform Spectrometer (FTS) technique and mixer measurements. The mixers were of quasioptical type with spiral antennas and twin junction tuning circuits integrated onto the apex area of the antenna arms. Modeling was employed for optimizing the tuning circuits and evaluating its performance. The comprehensive model for the SIS mixer bases on Tucker-Feldman theory of quasiparticle SIS mixer and Mattis-Bardeen theory of anomalous skin-effect. Very good agreement between modeling, FTS measurements and mixer experiment results was achieved. Accuracy in prediction of the tuning band and the coupling efficiency for the tuning circuits is better or about 15% within 400-1000 GHz band. Introduction The SIS mixer optimum performance can be reached when the intrinsic capacitance of the SIS junction is tuned out. Different integrated on chip tuning circuits have been developed for waveguide and open structure (quasioptical) Nb-A10x-Nb SIS mixers for 100-820 GHz band [16] providing broadband low-noise operation of the receivers [7-11]. These tuning circuits mainly utilize different microstrip lines, made from Nb with SiO or Si02 insulator, to resonate out SIS junction capacitance. The focus of this work is theoretical (by modeling) and experimental investigation of twin junction tuning circuit [4-6] for frequency band about 400-1000 GHz. Mattis-Bardeen theory was employed for the tuning circuit modeling. Fourier Transformer Spectrometer technique as well as mixer measurements of developed SIS mixers were used to determine tuning circuits performance, to compare experimental data and results of modeling. At the first step of the tuning circuit development simplified free-of-loss model has been employed for evaluating of the tuning circuit behavior and its optimization [11,191 The coupling efficiency of the spiral antenna output and the tuning circuit input was used as a quality parameter for loss-free circuitry (Fig. 1). However Fourier Transform Spectrometer (FTS) measurements of the tuning circuits [19] have shown noticeable discrepancy between tuning circuit coupling efficiency at above the gap frequency and model predicted performance. This associated with loss in Nb microstrip tuning circuitry. Near the gap frequency of Nb material (-690 GHz for film) the Nb based microstrip lines become dispersing [12, 13] and above this frequency having increasing losses [14, 15]. Mattis– Bardeen theory [16] of skin anomalous effect describes behavior of a superconductor at these frequencies in teillis of complex conductivity a: cr= G–ja2 , (1) where real a l and imaginary 32 components translate directly into the normal-electron and cooperpare currents in a superconductor. Following fundamental paper of R.L. Kautz [14] these components above can be presented in integral form: zo (7) Page 88 Sixth International Symposium on Space Terahertz Technology 2 E2±2 + E j = de[f (s)— f (e + hco)} O 7: ha) 2 2 1/2 2 2 A h + 1 ' er dep. 2f(hco hw i, 21 — A2 — A2} 112 (s2 _2 ) / (11(0 _ e) ha)
Superconducting-insulating-superconducting mixers incorporating a novel design were fabricated and evaluated for heterodyne detection in the frequency range of 600 to 720 GHz (ALMA Band 9). The improved design consists in tapering the corners of the RF transformer and a careful optimization of the dimensions to obtain a flat response over the full band. We demonstrate that this new design together with state-of- the-art technology lead to an improved noise temperature of T_N=100K. Since the working frequency of our devices crosses the superconducting gap frequency, we had to include the Mattis-Bardeen theory to simulate the response of this particular design. The results of the simulation are in excellent agreement with the experimental data.
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.
In this paper, we report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500 GHz) receivers. Nb-based SIS junctions of a current density of 10 kA/cm/sup 2/ and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA receiver specification, which requires wide frequency coverage as well as low noise temperature. Parallel-connected twin junctions (PCTJ) are designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe's feed point, thereby making it much easier to match the low-impedance PCTJ over a wide frequency band. In addition, a superconducting magnet fixed onto the compact mixer block to provide efficient magnetic field coupling is designed. The SIS mixer demonstrates a minimum double-sideband receiver noise temperature of 108 K at the band center and temperatures of less than 167 K over the whole band (for an intermediate-frequency range of 4-8 GHz).