Büttgenbach, Thomas H. (1993) Quasi-optical sis receivers and astrophysical observations at submillimeter wavelengths. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11302012-115924624
This thesis describes the development of a new generation of submillimeter wave receivers aimed at future integrated array receivers and presents astronomical observations taken with a single element of such a new receiver in the submillimeter wavelength band.
The technological development presented in this thesis was driven by the need to develop heterodyne receivers based on superconductor- insulator-superconductor (SIS) detectors, which have proven to be the most sensitive detectors in the millimeter and low submillimeter band, and that are suitable for the construction of array receivers. A quasi--optical approach was chosen in order to take advantage of the planar photolithography used in SIS diode manufacturing. This allows straightforward integration with planar antennas. The introductory chapter describes an initial receiver design, based on a planar logarithmic spiral antenna, that was an improvement over existing quasi-optical designs using planar antennas.
This receiver was the first submillimeter SIS receiver ever to be used for astronomical observations. However, it also showed that two major problems still had to be overcome to develop this technology to the mature state where array receivers can be designed. The beam of an individual element, launched by the planar antenna, has to be of high quality - comparable to that of a waveguide hom antenna - to provide good coupling to a telescope. Second, the impedance of the SIS detector, dominated by its capacitance, has to be matched to the antenna's impedance over the operating range of the receiver.
The optics problem was solved by introducing a novel antenna design called the hybrid antenna. The hybrid antenna is defined as a dielectric lens-antenna as a special case of an extended hemispherical dielectric lens that is operated in the diffraction limited regime. It is a modified version of the planar antenna on a lens scheme developed by Rutledge. The dielectric lens- antenna is fed by a planar-structure feed antenna and the combination of the two is termed a hybrid antenna. Beam pattern and aperture efficiency measurements were made at millimeter and submillimeter wavelengths as a function of extension of the hemispherical lens and different lens sizes. An optimum extension distance is found experimentally and theoretically for which excellent beam patterns and simultaneously high aperture efficiencies can be achieved. At 115 GHz the aperture efficiency was measured to be (76 ± 5 )% for a diffraction limited beam with sidelobes below -17 dB. The hybrid antenna is diffraction limited, space efficient in an array due to its high aperture efficiency, and is easily mass produced, thus being well suited for focal plane heterodyne receiver arrays. A single element hybrid antenna receiver yielded coupling efficiencies between the receiver and the Caltech Submillimeter Telescope of values approaching those achieved by the best waveguide horn based receiver systems.
The problem of tuning the SIS junction's capacitance was solved by using a novel superconducting transmission line circuit, called the end-loaded-stub, together with a real impedance transformer. This Nb based circuit is integrated with the planar feed antenna of the hybrid antenna and a Nb/AlO_x/Nb SIS detector. A goal for the matching circuit design was to achieve an octave of bandwidth and computer modeling of the device correctly predicted the measured bandwidth and characteristic frequencies to within 8%. The bandwidth measurements were carried out by using the SIS diode in direct detection mode with a fuurier Transform Spectrometer. A good match was obtained from 200 GHz to 475 GHz between the antenna and a relatively large area (1 µm^2) tunnel junction with wR_nC ≈ 2-4. Noise measurements were made in heterodyne mode at 318 GHz, 395 GHz, 426 GHz and 492 GHz, yielding uncorrected double sideband receiver noise temperatures of 200 K, 230 K, 220 K and 500 K, respectively. These results are comparable to state of the art waveguide receivers.
The combination of a hybrid antenna with the integrated tuning circuit thus lays the foundation for the development of integrated SIS focal plane receiver arrays.
The final chapter describes observations of the neutral carbon C I(^3P_1 → ^3P_0) line at 492 GHz with a single element quasi-optical SIS receiver. The goal was to investigate the large- and small-scale distribution of C I in the interstellar medium (ISM). Observations of neutral atomic carbon in the galaxy IC 342, made at the Caltech Sub millimeter Observatory, are presented, which represent the first extragalactic submillimeter detection of C I. The C I emission from IC 342 was found to have a main beam temperature of about 1 K, which is a factor of two greater than that for ^(13)CO → 1) in the same size beam. The integrated line intensity for the central 15" of IC 342 was found to be (6.2 ± 1.2) x 10^(-6) erg s^(-1) cm^(-2) sr^(-1). A 45" cut from the center to the south showed that the intensity ratio of C I to CO(2 → 1) is constant at about 1.4. The C I to CO abundance ratio is about 15% and the total C I abundance relative to hydrogen is 7 x 10^(-6), yielding about ≥ 10% of all carbon in the gas phase to be in form of neutral atomic carbon.
Comparing the IC 342 data with COBE observations of the Milky Way it is found that the intensity ratio of C I to CO(2 → 1) is approximately the same for IC 342 as it is for the Milky Way, despite the fact that the IC 342 measurements are from the center a moderate star burst galaxy, while the COBE data are an average over the entire Milky Way. From the comparison between the Milky Way and IC 342 and the cut through IC 342 it can thus be concluded that there is a strong correlation of CO with C I emission on large scales.
The small-scale structure of neutral carbon in the ISM is studied with a 3' by 4' (Δα Δδ) map, sampled at 20" intervals with a 15" beam, in the Orion Molecular Cloud (OMC). The map covers the embedded infrared source IRc2, the southern source (OMC-1 south, FIR 3,4) and the ionizing stars of the Trapezium. C I is found to be widespread with a typical intensity of about 6 x 10^(-6) ergs^(-1)cm^(-2) sr^(-1)and some enhancement at the interface between the ionized and neutral molecular medium with a peak intensity of 9.6 x 10^(-6) erg s^(-1) cm^(-2) sr^(-1). A typical gas phase abundance of neutral carbon relative to hydrogen of ≈ 3 x 10^(-6) is found, decreasing two orders of magnitude in the cores of the condensations.
Theoretical model calculations of ion chemistry of molecular clouds with external UV illumination were performed for this thesis, based on a code by Le Bourlot, which allow for an explanation of the observed relation of neutral carbon to CO and hydrogen on both, large and small, scales. It is suggested, based on the observational data presented and the good agreement of the observations with the ion chemistry model, that the C I (1 → 0) emission emanates from the bulk of gas in molecular clouds, i.e., from regions similar to those emitting in the low J ^(13)CO lines. This requires the bulk of the observed medium to be in the chemically low-density regime where charge transfer reactions of atomic species dominate over protonation reactions (started by H^+_3) of oxygen bearing molecules. The fine structure emission lines from neutral carbon are thus a more important coolant for the ISM than the rotational transitions of CO for low to moderate hydrogen densities. This is in contrast to previous PDR models that only had significant neutral carbon abundances in a thin PDR transition layer on the surface of molecular clouds irradiated by FUV.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Physics, Mathematics and Astronomy|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||1 April 1993|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Benjamin Perez|
|Deposited On:||30 Nov 2012 21:40|
|Last Modified:||26 Dec 2012 04:46|
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