Several modes of operation of point contacts as heterodyne detectors in the frequency range 0 to 1010 Hz were investigated experimentally. A point contact in a superconducting loop, under the application of two externally generated ac fields, behaved as expected on the basis of a model given previously. Likewise, a point contact in a partly resistive loop, in which an externally generated signal is heterodyned with the Josephson oscillation of the contact, gave the expected results. In both cases the heterodyne signal amplitude exhibits a decaying oscillatory (Bessel-function-like) dependence on applied signal level; the heterodyne power at the first peak being approximately 0.1φ02f0/L, where f0 is the heterodyne frequency, L the loop inductance, and φ0 the flux quantum. On the other hand, an anomalously large heterodyne signal was obtained from a pair of closely coupled Josephson-oscillating contacts. No explanation of this result is offered, but it appears that a sort of amplification of the heterodyne signal is effected when the Josephson frequencies are larger than the heterodyne frequency.
Several modes of operation of point contacts as heterodyne detectors in the frequency range 0 to 1010 Hz were investigated experimentally. A point contact in a superconducting loop, under the application of two externally generated ac fields, behaved as expected on the basis of a model given previously. Likewise, a point contact in a partly resistive loop, in which an externally generated signal is heterodyned with the Josephson oscillation of the contact, gave the expected results. In both cases the heterodyne signal amplitude exhibits a decaying oscillatory (Bessel-function-like) dependence on applied signal level; the heterodyne power at the first peak being approximately 0.1φ02f0/L, where f0 is the heterodyne frequency, L the loop inductance, and φ0 the flux quantum. On the other hand, an anomalously large heterodyne signal was obtained from a pair of closely coupled Josephson-oscillating contacts. No explanation of this result is offered, but it appears that a sort of amplification of the heterodyne signal is effected when the Josephson frequencies are larger than the heterodyne frequency.
Weakly connected superconductors can function as very low power sensors by utilizing the coherence properties of superconductors. One configuration which has produced significant success incorporates a superconducting point contact and a small resistance element in a low-inductance superconducting circuit. When the critical supercurrent (ic) of the point contact is of the order of φ0/L, (φ0=h/2e, L=inductance of the ring), that circuit exhibits coherent quantum behavior. Used as a parametric amplifier, it has been used to measure ultralow voltages, limited by the thermal noise fluctuations in the resistance element R. The voltage sensitivity is approximated by 8kTR/φ0, where k is Boltzmann's constant and T the absolute temperature of the resistance R. We have measured 4×10−16 V across 1.7×10−10 Ω at 4.2°K with a signal-to-noise ratio approaching 10. The operation of this sensor follows from the Josephson oscillation of voltage-biased superconducting point contacts. A dc voltage V0 across the resistance of the circuit produces an oscillating current at the angular frequency ωJ=2πV0/φ0 and an oscillating voltage across the point contact at ωJ. Upon supplying an rf or microwave current at frequency ωD, the point contact mixes ωJ and ωD. Using a homodyne detector at ωD of bandwidth larger than ωJ, we can retrieve the oscillation at ωJ. Direct measurement of this frequency ωJ yields V0. Such superconducting circuits may be useful as cryogenic thermometers, as thermal radiation sensors, and in low-level spectroscopy.
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.
We have performed mixing experiments with CO2 lasers in the 9 to 11 μm wavelength band using superconducting point contacts. The experiments involve difference frequencies ranging from 19 MHz to 106 GHz. The mixing mechanism definitely appears to require the superconducting state since the beat signal amplitude decreases by as much as 60 dB as the bath temperature is raised through the superconducting transition temperature. At least three conceptually distinguishable mechanisms may be involved in the observed response. These include : (1) a conventional Josephson response, (2) a thermally modulated Josephson response, and (3) a single particle tunneling response. The usual signature of a Josephson interaction is one or more current steps in the I-V curve. Steps, the first of which should be at 67 mV, are not observed in these experiments. However, we do not feel that is a definitive test for the mixing mechanism between two external sources at such high frequencies. In fact, Werthamer's theory of tunnel junctions predicts a large response for these experiments, a response as large as that from two X-band sources, for example. It should be noted, however, that the theory assumes the junction is maintained at a fixed low temperature and whether or not that is the case depends directly on the current density at the contact. Very little is known about the current density at the present time. These experiments emphasize a fact that is usually overlooked in Josephson junction mixing experiments with an external local oscillator. If the contact or tunneling region is sufficiently small, it may be thermally modulated at difference frequencies extending into the microwave range. This effect becomes more likely with increasing applied frequencies, particularly in the infrared. Theoretical modeling for the purpose of calculating ultimate sensitivities of these devices requires a detailed consideration of thermal effects. We fully expect large mixing signals from Josephson junctions even at optical frequencies if thermal effects do not prove to be a limitation.
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.
For a current-driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high-frequency currents is calculated with a second-order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic-frequency regions depending on whether the internal self-generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0< or >ω the response could be expressed by easily observable parameters of the dc voltage-current characteristics. This permits comparison of the predictions with experimental results obtained on point-contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the inverse battery current in terms of the critical current, and (iv) the voltage amplitude of the inherent Josephson oscillations. For ω0≫ω ``classical'' detection proportional to curvature is obtained. For ω0∼ω resonance detection occurs, which depends to a considerable degree on fluctuations of the average contact voltage. These predictions are in reasonable agreement with experiments in which point-contact junctions were illuminated with klystron radiation at 10 and 90 GHz. Evidence is presented that the voltage fluctuations limiting the resonance response are the result of noise generated in the junction. Comparison of the noise-equivalent power expected from the simple model with measured results suggests that parasitic elements, such as shunt capacity, are not negligible at 90 GHz.
A heterodyne mixing experiment of microwaves at about 20 GHz was performed using a microwave detector made from granular-type Y1Ba2Cu3O7-x (YBCO) superconductors. Although the bridge included many boundaries, voltage steps corresponding to the Josephson frequency and step height dependence for microwave power similar to those of a single weak-link bridge were clearly observed. The IF signal showed peak amplitudes corresponding to the peak dynamic resistantance due to nonlinearity resulting from the Josephson effect. It was confirmed for the first time that this detector operates stably for the heterodyne mixer at 60 K or below and has a potential for high sensitivity due to its large dynamic resistance.