The use of Josephson mixers with an external local oscillator for sensitive heterodyne reception at submillimeter wavelengths is discussed. Recent experiments, primarily at 450 GHz (0.66 mm wavelength), together with our present theoretical knowledge, allow realistic estimates of future receiver performance to be made. Although some practical problems remain, the probability of building field‐usable receivers with very competitive sensitivity now appears good. A discussion is also given of the use of the high‐harmonic mixing properties of Josephson devices for frequency measurement in the far infrared.
The use of Josephson mixers with an external local oscillator for sensitive heterodyne reception at submillimeter wavelengths is discussed. Recent experiments, primarily at 450 GHz (0.66 mm wavelength), together with our present theoretical knowledge, allow realistic estimates of future receiver performance to be made. Although some practical problems remain, the probability of building field‐usable receivers with very competitive sensitivity now appears good. A discussion is also given of the use of the high‐harmonic mixing properties of Josephson devices for frequency measurement in the far infrared.
Josephson junctions are highly nonlinear and respond up to infrared frequencies. A wide variety of high frequency applications have been explored. In addition, several high frequency superconducting devices are being developed which do not use the Josephson effect. These include a superconducting version of the Schottky barrier diode for use as a heterodyne mixer and a video detector, and also a composite superconducting bolometer. In this review, the state of development of superconducting mixers, detectors, and parametric amplifiers at microwave, millimeter, and submillimeter wavelengths is discussed. Emphasis will be placed on developments which have taken place since 1974 when a previous review was circulated.
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.
Adjustable niobium Josephson point contacts, mounted in fundamental-mode waveguide cavities, have been used as mixers in an experimental heterodyne receiver with a 450 GHz laser as the local oscillator. It was possible to estimate separately the various factors, such as intrinsic mixer conversion efficiency, mixer noise, and input coupling efficiency that contribute to the overall receiver sensitivity. Mixer performance agreed reasonably well with qualitative predictions of resistively-shunted-junction theoretical models, and there was broad quantitative agreement for some aspects of performance. Spurious, and potentially very misleading non-heterodyne responses to thermal signals were observed under some regimes of applied DC bias and local oscillator power. The lowest overall receiver noise temperature measured was 2100K (double side band), although reduction to 1000K should be possible with quite simple improvements. The intrinsic mixer noise temperatures were usually in the range 500-1000K (single side band), and local oscillator power required by the receiver was usually less than one microwatt. This performance makes these mixers competitive as the basis of very sensitive near-millimetre-wave receivers.
Superconducting tunnel diodes (SISs) are investigated for both the generation and detection of submillimeter wavelengths. For detection, heterodyne receivers using SISs are investigated. Josephson currents are found to have a large effect on SIS mixers in the submillimeter, even though their effect is less at lower frequencies. At 492 GHz, fixer conversion gain increased by 4 dB when Josephson currents are not suppressed by a magnetic field. A much higher noise is seen in this gain mode, but further investigation of this recently discovered phenomenon is warranted. For submillimeter generation, a two-dimensional array of SISs is investigated. The ac Josephson effect in the SIS is expected to deliver submillimeter power into a free-space radiation mode traveling perpendicular to the array. No experimental devices have been developed yet, but the authors show computer simulations and microwave scale model measurements of array structures.
The progress made in relation to the development of sensitive submillimeter heterodyne receiver systems is reviewed. Sensitive superconductor-insulator-superconductor (SIS) tunnel junction mixers were demonstrated at frequencies of greater than 1 THz, with noise temperatures of below 1000 K. An alternative mixer technology is that of the superconducting hot electron bolometer, for which sensitivities of 1.2 THz and 2.5 THz were demonstrated. Local oscillator technology is under development. Gunn oscillators followed by Schottky varactor frequency multipliers have sufficient output power to drive SIS mixers at 1 THz. It is considered that optical photomixer local oscillators have potential for driving hot electron bolometers above 1 THz.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.