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.
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.
Heterodyne receivers based on superconductor-insulator-superconductor (SIS) tunnel diodes are the most sensitive available for near-millimeter wavelengths. Since the late seventies, receivers based on SISs have been used for millimeter band observations at radio observatories around the world. The work described here was carried out with the elusive goal in mind of developing ideal SIS receivers for radioastronomy. This thesis describes one researcher's path towards this goal. In the first chapter, a basic description of SIS diodes, their interaction with radiation, and heterodyne detection are given. The important detailed results of J. R. Tucker's tunnel diode heterodyne theory are described, since subsequent chapters rely on Tucker's work quite heavily. Throughout this introductory chapter, extremely simple (by comparison with the algebraic theoretical results) physical models are used to describe superconductivity, SISs, photon-assisted tunneling, and heterodyne detection. It is the author's experience that these physical models can be used to derive correct theoretical results, long before these results are proved rigorously. The second chapter presents a fully quantum mechanical theory of heterodyne detection with diodes. This theory was developed because Tucker's theory for tunnel diodes predicts a greater mixer sensitivity than is possible considering Heisenberg's uncertainty principle for radiation. Tucker does not quantize the radiation incident on the SIS, although his treatment of the isolated tunnel diode is completely quantum mechanical. In chapter 2, the quantization of radiation is carried out for heterodyne diode detectors. The formalism is shown to obey quantum limits on sensitivity. Finally, an ideal SIS mixer is shown to have noise properties identical to those of optical mixers based on ideal photodiodes. In possession of an apparently complete theory for SIS mixers, the third chapter presents a sampling of numerical results from that theory. Four different non-ideal tunnel diodes are used for these calculations so that a quantitative feel for the importance of diode quality can be achieved. The effects of dc and LO bias, signal and image source admittance, frequency of operation, and junction quality are all explored. This information will be useful for the proper engineering of SIS mixers. Finally, the fully optimized performance of the four tunnel diodes is presented as frequency is varied. It is shown that reasonably good quality lead-alloy SISs should behave like photodiodes up to frequencies as high as 1500 GHz. Finally, chapter 4 presents a prototype open-structure SIS mixer. Measurements in the laboratory show this mixer to be quite sensitive for signal frequencies from 115 to 761 GHz. Unlike all other SIS receivers, in which the diode is mounted across a waveguide, this mixer relies on the bowtie-on-quartz antenna structure, which has been investigated by D. B. Rutledge and his students. This difference is essential to the multi-octave spectral coverage of this mixer. It is probable that waveguide designs will never achieve good results above 500 GHz, and as of now, there are no SIS-waveguide mixers which operate well above 300 GHz. Tests at the Owens Valley Radio Observatory verify the suitablility of this mixer for radioastronomy, but these tests have been limited to frequencies of 260 GHz and lower.
Superconducting tunnel diode (SIS) mixers are used for radio astronomy from 100 to 500 GHz. They are being considered for NASA spacebome astronomy at frequencies near 1,000 GHz. We report measurements of gain and noise in SIS mixers at 230 and 492 GHz. We measure high gain and noise associated with Josephson currents which have not been previously reported. These measurements show that Josephson currents are increasingly important as operating frequencies are raised. We discuss the techniques we use to make these measurements. Measurements made with hot and cold blackbodies are shown to be inaccurate at high frequencies. The problem is that SIS 's do not always respond linearly to the signal power incident on them. This is particularly important when 1) very broad band mixers are used and 2) Josephson effect currents are important. Both of these circumstances are present in the quasioptical SIS mixers favored for 500 GHz and higher. We use monochromatic signals to measure gain and noise to get around these problems.
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.
Sensitive heterodyne receivers are being built at ever higher frequencies with superconducting (SIS) junctions as the first mixer. These devices have extremely sharp non-linearities in their current-voltage characteristics as a result of quantum-mechanical tunneling of electrons across thin insulating barriers. The low energy scale set by the magnitude of the superconducting energy gap implies very low local oscillator power requirements for heterodyne operation. Some general system design considerations for astrophysical receivers are reviewed. These principles are illustrated by discussing two specific applications: a 230 GHz SIS receiver recently installed as a facility instrument at the Swedish-ESO submillimeter telescope in Chile, and the broader receiver requirements of the 6-antenna submillimeter array (SMA), an interferometer now being designed at the Harvard-Smithsonian Center for Astrophysics. The SMA will require receivers at frequencies as high as 820 GHz, and will place some unique demands on detector performance.
Quasiparticle and Josephson tunneling currents in superconducting tunnel junctions provide nonlinear low-noise response up to terahertz frequencies. Many types of direct detector heterodyne mixer and parametric amplifier based on these effects have been explored. Only the SIS quasiparticle mixer has been sufficiently competitive to find significant applications. These mixers can have noise approaching the quantum limit TN = ℏω/k and gain larger than 10 dB. They are used at a number of observatories for molecular line radio astronomy at millimeter wavelengths. Similar applications at submillimeter wavelengths are beginning.
Observation and analysis of submillimeter-wave radiation (300GHz-3THz) in astronomy and atmospheric sciences requires increasingly performant receivers. The most sensitive receivers working in this range of electromagnetic spectrum use superconductor-insolator-superconductor (SIS) junctions. In order to increase the bandwidth and the sensitivity, we are developing a quantum-noise limited heterodyne receiver based on several parallel SIS junctions with broad (larger than 30%) fixed tuned bandwidth. These circuits can be viewed as passband filters which have been optimized by varying the spacings between junctions. We have designed such 5-junction arrays for operation in the range 480-640 GHz. Fabrication and heterodyne characterization of these devices has been done. The 1 μm2 junctions current density ranges from 4 to 13 kA/cm2, using optical lithography and Nb/Al2Nb5/Nb trilayer sputtering technology. The fabrication process and yield are presented in this paper, along with measured performances