Low-noise THz Niobium SIS Mixers

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/k_b 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.

Measurements of mantle velocities of P waves with a large array

A large array has been used to investigate the P-wave velocity structure of the lower mantle. Linear array processing methods are reviewed and a method of nonlinear processing is presented. Phase velocities, travel times, and relative amplitudes of P waves have been measured with the large array at the Tonto Forest Seismological Observatory in Arizona for 125 earthquakes in the distance range of 30 to 100 degrees. Various models are assumed for the upper 771 km of the mantle and the Wiechert-Herglotz method applied to the phase velocity data to obtain a velocity depth structure for the lower mantle. The phase velocity data indicates the presence of a second-order discontinuity at a depth of 840 km, another at 1150 km, and less pronounced discontinuities at 1320, 1700 and 1950 km. Phase velocities beyond 85 degrees are interpreted in terms of a triplication of the phase velocity curve, and this results in a zone of almost constant velocity between depths of 2670 and 2800 km. Because of the uncertainty in the upper mantle assumptions, a final model cannot be proposed, but it appears that the lower mantle is more complicated than the standard models and there is good evidence for second-order discontinuities below a depth of 1000 km. A tentative lower bound of 2881 km can be placed on the depth to the core. The importance of checking the calculated velocity structure against independently measured travel times is pointed out. Comparisons are also made with observed PcP times and the agreement is good. The method of using measured values of the rate of change of amplitude with distances shows promising results.