Superconductor-insulator-superconductor (SIS) mixers are currently the lowest-noise devices available for mixing in the 26–110 GHz frequency range. However, a major source of noise is anticipated to be the increased effects of Josephson oscillations within the mixer as higher operating frequencies are achieved. Simulations have been done to demonstrate these effects. Results indicate that the commonly used three-port analysis is valid over a wide range of conditions, but that even minimal Josephson oscillations have a deleterious effect on mixer performance. The junction shunt capacitance, which serves to short out the Josephson oscillations, is thus an extremely important parameter. However, the characterization of the mixer in terms of an ‘‘ωRC’’ product appears to be an oversimplification. A more elaborate approach to mixer characterization is suggested.
Superconductor-insulator-superconductor (SIS) mixers are currently the lowest-noise devices available for mixing in the 26–110 GHz frequency range. However, a major source of noise is anticipated to be the increased effects of Josephson oscillations within the mixer as higher operating frequencies are achieved. Simulations have been done to demonstrate these effects. Results indicate that the commonly used three-port analysis is valid over a wide range of conditions, but that even minimal Josephson oscillations have a deleterious effect on mixer performance. The junction shunt capacitance, which serves to short out the Josephson oscillations, is thus an extremely important parameter. However, the characterization of the mixer in terms of an ‘‘ωRC’’ product appears to be an oversimplification. A more elaborate approach to mixer characterization is suggested.
We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis describes both theoretical and experimental investigations into the dynamics and noise processes of Josephson junctions, with the intent of evaluating their potential as mixers in heterodyne instruments for submillimeter-wave (i.e., the frequency range from 300 GHz to 3THz) detection. Superconducting tunnel junctions utilizing the nonlinearity due to photon-assisted tunneling of quasiparticles (SIS mixers) have become the state-of-the-art technology for sensitive heterodyne detection up to frequencies of about 700 [...]. Recent progress in the fabrication of high [...], superconductors has led to Josephson-effect devices with [...] products of up to ten millivolts, which might be suitable for mixing at frequencies of many terahertz. The key question for weighing the prospects for high-frequency Josephson mixers is that of the sensitivity which can be attained. Previous experimental work on Josephson mixing suggests the existence of an "excess" noise, which degrades the sensitivity. Theoretical modeling of mixer performance, based on the resistivelyshunted junction (RSJ) model for the dynamics of the device, also indicates the presence of larger noise than expected. The origin of this noise was not clearly understood, however, nor was its exact magnitude or expected scaling with frequency or junction characteristics known. In addition, previous experiments utilized crude devices of the point-contact type, which were unstable and thus undesirable for real applications. In the first part of this thesis research, extensive numerical simulations were performed with the RSJ model, including calculations of mixer noise and conversion efficiency. These calculations have revealed that the source of excess noise is the AC Josephson oscillations of the device, which can be completely incoherent, with a linewidth comparable to their frequency. Thus they appear as a broadband noise source. While this noise is intrinsic and unavoidable, an optimized mixer is still shown to be capable of interesting sensitivity levels, and the excess noise is expected to become relatively less important as the operating frequency is increased. Secondly, a process has been developed for the fabrication of stable, well-characterized, and reproducible Josephson devices based on resistively-shunted Nb and NbN tunnel junctions. The devices utilize submicron-area, high current-density tunnel junctions and a AuGe shunt resistor to yield completely non-hysteretic I-V curves, normal-state resistances of about [...] products of about half a millivolt. These devices should be nearly optimal for mixing at [...]. Heterodyne measurements using these junctions have been performed in a waveguide mixer mount. Receiver temperatures as low as 190 K (DSB), with -6 dB conversion efficiency, have been obtained at 100 [...], but these results are still a factor of about four higher than those predicted by the RSJ simulations. Accurate measurements of the available noise power of the junctions at the intermediate frequency of 1.5 [...] were made, and confirmed that the receivers were limited by elevated junction output noise. The deviations of the noise from theoretical predictions are shown to be caused by the nonlinear interaction of the junction with the embedding circuit. While this work points out some of the complexity introduced by the strong nonlinearity of Josephson devices, it is still expected that Josephson-effect mixers may be useful for heterodyne detection at very high frequencies.
Virtually all analyses of the superconductor-insulator-superconductor (SIS) quasiparticle mixer have employed the quantum theory of mixing in its three-frequency approximate form. This approximation is valid only in the limit of very large junction capacitance. For finite capacitance, these analyses may be seriously in error. To remedy this, a computer program has been developed which uses the quantum theory of mixing in its most general form, treating the large-signal properties of the mixer in the time domain. The terminating impedances at the harmonics of the local oscillator frequency and their sidebands can be arbitrary. Using this analysis, the effect of finite junction capacitance on one SIS mixer's performance is described. This gives an insight into the range of validity of the three-frequency model.
We present test results of a 100 GHz waveguide receiver using resistively-shunted Nb and NbN tunnel junctions. These junctions were fabricated with a process similar to that used previously with Nb junctions [1], which produces non-hysteretic devices with normal-state resistances of 30-50 Ohms, and IcRiz products of 0.5-0.75 mV. The best receiver temperature obtained to date with NbN is 270 K (DSB) with 6-7 dB of conversion loss. A receiver temperature of 390 K (DSB) was previously obtained with Nb junctions; these devices showed qualitative agreement with the predictions of the simple DC and RF current-biased RSJ model. The improvement in receiver performance is probably due to improved RF coupling of the junctions, and due to the higher IcRn products of NbN. However, these NbN devices display different pumped I-V curves and IF power curves than predicted by the RSJ model. We show that these effects can be caused by the strong interaction of the junction with the embedding circuit, and conclude that an accurate model of the impedance presented to the junction will have to be included in the RSJ simulations to compare with the observed performance. Fourth International Symposium on Space Terahertz Technology Page 443 Introduction We are studying the performance of Josephson-effect mixers using resistivelyshunted tunnel junctions in order to develop a better understanding of their behavior, the sources of noise and its limitation, and to determine how to optimize their performance. Also, we wish to compare the observed behavior and performance with the theoretical predictions of the so-called RSJ (resistively shunted junction) model, so that there will be a guide to the expected performance for different frequency regimes and device parameters. This work may be important as high-Tc SNS devices become available [P.A. Rosenthal and E.N. Grossman, these proceedings] with large Ic h?„ products, for they provide the possibility of using Josephson mixers at frequencies in the terahertz range, above the range where SIS mixers are currently available. In contrast, due to the extremely short coherence lengths of the high-Tc materials, junctions suitable for quasiparticle (SIS) mixers may not be feasible with these materials Device Fabrication We have applied the technique used for creating non-hysteretic Nb Josephson devices[1] to NbN/MgO/NbN tunnel junctions. First the junction tfilayer is fabricated, then the junction areas are patterned by electron-beam lithography and subsequently etched using reactive-ion etching (RJE) in the standard method [3]. After planarization of the etched area with evaporated SiO, a AuGe resistor is fabricated on top of the SiO, and the junction and resistor are then contacted with another layer of NbN. A photomicrograph of a completed Nb junction is shown in Figure la. Since the bottom of the etched trilayer lies under the resistor, it acts as a ground plane, and the resistor structure can be analyzed as a lossy transmission line. The impedance of this short microstrip structure is purely real, even up to frequencies comparable to the gap frequency of the superconductor, thus providing the desired broadband shunt. However, the section of Page 444 Fourth International Symposium on Space Terahertz Technology superconductor which contacts the junction and resistor creates a non-negligible parasitic capacitance when compared to the very small area tunnel junctions used, and therefore the area of this contact pad must be kept to a minimum. From a knowledge of the geometry and thickness of the insulator (Si0), we estimate this parasitic capacitance to be 7-8 fF. Shunting of the junction is necessary to reduce the McCumber beta parameter (Pc = 2eIcRN C Ih) of the unshunted tunnel junctions. Junctions with /3, values greater than one have hysteretic I-V curves [4], and their dynamics can be complicated, including the possibility of chaos. Reduction of 13, is accomplished by resistive shunting, which reduces /3 by the square of the ratio of the initial junction resistance to the final resistance of the parallel combination of the junction and the resistive shunt. However, this shunting has the undesirable effect of reducing the 1,Rn product of the junction. For the NbN junctions used, the critical current density was about 331cA/cm 2 , and the specific capacitance is estimated to be about 150/F 1 itm [5], giving an unshunted of about 12. Thus the optimal shunted junction for these parameters would have a shunt resistance of about one-fourth of R,„ and thus an IcR7, reduced from the unshunted value of 3.0 mV to 0.8 mV. In order to maintain high enough resistances for the final device, very small (0.3pm on a side) area junctions must be used, and the parasitic capacitance will be comparable to the junction capacitance. This further reduces the final IeR, product attainable. The I-V curve of a typical device is shown in Figure lb. This device had a critical current of 11 microamps and a normal state resistance of 39 Ohms, giving an IcR, product of 430 microvolts. The IV curve is nonhysteretic and follows the RSJ model. Mixer Modelling Mixer performance can be predicted using the RSJ model, in a method similar to that used by Taur[6], but extended to the case of finite junction capacitance. The
The results of extensive analog computer simulations of a Josephson junction used as a mixer with an external local oscillator are presented. The Resistively Shunted Junction model was used throughout, and the effects of intrinsic junction noise were included. When the source impedance is much greater than the junction resistance R our simulations permit predictions of conversion efficiency, noise temperature, and saturation level to be made for a wide range of experimental parameters. The possibility of harmonic mixing has also been considered. With a source resistance comparable to R (i.e., conventional matching) the behavior of the system is too complicated to permit such general predictions of performance. From the results of simulations for a particular set of parameters it appears that the best noise temperature will usually be achieved for a source impedance somewhat greater than R. An upper limit for the mixer noise temperature is approximately 40 T over a broad range of junction parameters, where T is the effective temperature of the junction. The Conversion efficiency under these circumstances should be comparable to what is potentially available from Schottky diode mixers. Our simulations show that with stronger microwave coupling it is possible to obtain conversion gain - i.e., a conversion efficiency exceeding unity. This, however, will probably be at the expense of a higher mixer noise temperature.
Josephson currents have been observed to drain instabilities and decrease the sensitivity of a heterodyne superconductor-insulator-superconductor (SIS) receiver operated at submillimeter wavelengths. A static magnetic field is usually applied on the SIS junction to reduce the magnitude of its critical Josephson currents and enable quasiparticle mixing. We measured the SIS mixer output power at the intermediate frequency (IF) versus the dc bias voltage of an array of two 1 μm2 Nb/Al–Al2O3/Nb junctions. We show evidence of additional, magnetic-field-sensitive noise, peaked at the Shapiro steps voltages, which is due to high-frequency Josephson currents converted to the IF by quasiparticle mixing.