This thesis describes the development of a new generation of submillimeter wave receivers aimedat future integrated array receivers and presents astronomical observations taken with a singleelement of such a new receiver in the submillimeter wavelength band.
The technological development presented in this thesis was driven by the need to developheterodyne receivers based on superconductor- insulator-superconductor (SIS) detectors, whichhave proven to be the most sensitive detectors in the millimeter and low submillimeter band, andthat are suitable for the construction of array receivers. A quasi--optical approach was chosen inorder to take advantage of the planar photolithography used in SIS diode manufacturing. Thisallows straightforward integration with planar antennas. The introductory chapter describes aninitial receiver design, based on a planar logarithmic spiral antenna, that was an improvementover existing quasi-optical designs using planar antennas.
This receiver was the first submillimeter SIS receiver ever to be used for astronomicalobservations. However, it also showed that two major problems still had to be overcome todevelop this technology to the mature state where array receivers can be designed. The beam ofan individual element, launched by the planar antenna, has to be of high quality - comparableto that of a waveguide hom antenna - to provide good coupling to a telescope. Second, theimpedance of the SIS detector, dominated by its capacitance, has to be matched to the antenna'simpedance over the operating range of the receiver.
The optics problem was solved by introducing a novel antenna design called the hybridantenna. The hybrid antenna is defined as a dielectric lens-antenna as a special case of anextended hemispherical dielectric lens that is operated in the diffraction limited regime. It is amodified version of the planar antenna on a lens scheme developed by Rutledge. The dielectriclens- antenna is fed by a planar-structure feed antenna and the combination of the two is termeda hybrid antenna. Beam pattern and aperture efficiency measurements were made at millimeterand submillimeter wavelengths as a function of extension of the hemispherical lens and differentlens sizes. An optimum extension distance is found experimentally and theoretically for whichexcellent beam patterns and simultaneously high aperture efficiencies can be achieved. At 115GHz the aperture efficiency was measured to be (76 ± 5 )% for a diffraction limited beam withsidelobes below -17 dB. The hybrid antenna is diffraction limited, space efficient in an arraydue to its high aperture efficiency, and is easily mass produced, thus being well suited for focalplane heterodyne receiver arrays. A single element hybrid antenna receiver yielded couplingefficiencies between the receiver and the Caltech Submillimeter Telescope of values approachingthose achieved by the best waveguide horn based receiver systems.
The problem of tuning the SIS junction's capacitance was solved by using a novel superconductingtransmission line circuit, called the end-loaded-stub, together with a real impedancetransformer. This Nb based circuit is integrated with the planar feed antenna of the hybrid antennaand a Nb/AlO_x/Nb SIS detector. A goal for the matching circuit design was to achieve an octaveof bandwidth and computer modeling of the device correctly predicted the measured bandwidthand characteristic frequencies to within 8%. The bandwidth measurements were carried out byusing the SIS diode in direct detection mode with a fuurier Transform Spectrometer. A goodmatch was obtained from 200 GHz to 475 GHz between the antenna and a relatively large area (1µm^2) tunnel junction with wR_nC ≈ 2-4. Noise measurements were made in heterodyne modeat 318 GHz, 395 GHz, 426 GHz and 492 GHz, yielding uncorrected double sideband receivernoise temperatures of 200 K, 230 K, 220 K and 500 K, respectively. These results are comparableto state of the art waveguide receivers.
The combination of a hybrid antenna with the integrated tuning circuit thus lays the foundationfor 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 492GHz with a single element quasi-optical SIS receiver. The goal was to investigate the large- andsmall-scale distribution of C I in the interstellar medium (ISM). Observations of neutral atomiccarbon in the galaxy IC 342, made at the Caltech Submillimeter Observatory, are presented, whichrepresent the first extragalactic submillimeter detection of C I. The C I emission from IC 342 wasfound to have a main beam temperature of about 1 K, which is a factor of two greater than thatfor ^(13)CO → 1) in the same size beam. The integrated line intensity for the central 15" ofIC 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 thesouth showed that the intensity ratio of C I to CO(2 → 1) is constant at about 1.4. The C I toCO 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 theintensity ratio of C I to CO(2 → 1) is approximately the same for IC 342 as it is for the MilkyWay, despite the fact that the IC 342 measurements are from the center a moderate star burstgalaxy, while the COBE data are an average over the entire Milky Way. From the comparisonbetween the Milky Way and IC 342 and the cut through IC 342 it can thus be concluded thatthere 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 mapcovers the embedded infrared source IRc2, the southern source (OMC-1 south, FIR 3,4) and theionizing stars of the Trapezium. C I is found to be widespread with a typical intensity of about6 x 10^(-6) ergs^(-1)cm^(-2) sr^(-1)and some enhancement at the interface between the ionized and neutralmolecular medium with a peak intensity of 9.6 x 10^(-6) erg s^(-1) cm^(-2) sr^(-1). A typical gas phaseabundance of neutral carbon relative to hydrogen of ≈ 3 x 10^(-6) is found, decreasing two ordersof magnitude in the cores of the condensations.
Theoretical model calculations of ion chemistry of molecular clouds with external UV illuminationwere performed for this thesis, based on a code by Le Bourlot, which allow for anexplanation of the observed relation of neutral carbon to CO and hydrogen on both, large andsmall, scales. It is suggested, based on the observational data presented and the good agreementof the observations with the ion chemistry model, that the C I (1 → 0) emission emanates fromthe bulk of gas in molecular clouds, i.e., from regions similar to those emitting in the low J ^(13)COlines. This requires the bulk of the observed medium to be in the chemically low-density regimewhere charge transfer reactions of atomic species dominate over protonation reactions (started byH^+_3) of oxygen bearing molecules. The fine structure emission lines from neutral carbon are thusa more important coolant for the ISM than the rotational transitions of CO for low to moderatehydrogen densities. This is in contrast to previous PDR models that only had significant neutralcarbon abundances in a thin PDR transition layer on the surface of molecular clouds irradiatedby FUV.
