This thesis aims to gain a better understanding of dusty galaxies by studying the thermal emission of interstellar dust at submillimeter (submm) wavelengths. We conduct 350μm observations of two galaxy samples selected based on entirely different criteria, using the Submillimeter High Angular Resolution Camera II (SHARC-II, Dowell et al. 2003) at the 10.4m Caltech Submillimeter Observatory (CSO).
The first galaxy sample consists of 18 luminous infrared galaxies (LIGs) in the local Universe (0.003 ≤ z ≤ 0.042). We estimate the global properties in this galaxy sample; dust temperature (Td = 38.6 ± 7.7 K), emissivity index (β = 1.6 ± 0.3), far-infrared (FIR) luminosity (Lfir = 1011.2±0.6 L☉}) and dust mass (Md = 107.4±0.6 M☉). Guided by theoretical considerations and laboratory measurements, we derive a Td-β inverse correlation for the local LIG sample; Td = [5.03 x 109]1/4.46 + β.
The second galaxy sample consists of 36 ultraluminous infrared galaxies (ULIGs) at moderate redshifts (0.1 < z < 1.0), out of which 28 galaxies are detected. The newly acquired 350μm data, in combination with the pre-existing IRAS 60μm and 100μm data, leads to meaningful estimates of the global properties in these galaxies for the first time; Td = 40.9 ± 6.9K, Lfir = 1012.2±0.5 L☉, Md = 108.3±0.3 M☉ and intense starburst activity (SFR = 102.5±0.5 L☉ yr-1). There is strong circumstantial evidence that the Td-β inverse correlation observed in the local LIG sample remains valid in ULIGs at moderate redshifts. We find that the FIR-radio correlation observed in local star-forming galaxies continues to hold for ULIGs over the redshift range of 0.1 < z < 1.0 and dust heating originates predominantly from star formation. Lfir and Td derived for dusty galaxy samples over a wide range of redshifts show significant scatter, due to differing selection biases and variations in dust mass and grain properties. We argue that the observed Td, as a large-scale SED parameter, is linked to the global star formation efficiency (SFE) and the spatial extent characteristic of dominant star formation in a galaxy.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/8ZM9-6671, author = {Kovács, Attila}, title = {SHARC-2 350 Micron Observations of Distant Submillimeter Selected Galaxies and Techniques for the Optimal Analysis and Observing of Weak Signals}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/8ZM9-6671}, url = {https://resolver.caltech.edu/CaltechETD:etd-06022006-123747}, abstract = {New 350 micron data constrain accurately the thermal far-infrared spectral energy distributions (SEDs) for 12 distant submillimeter selected galaxies (SMGs). The results confirm that the linear radio to far-infrared correlation, established for local infrared galaxies, holds out to high redshifts z ~ 1–3. The low correlation constant q ~ 2.14 is more indicative of star formation than AGN-fueled dust heating. The sample exhibits an apparent luminosity–temperature relation (L_FIR ~ T_d^2.89), possibly owing to selection effects. As a result, photometric redshifts in the radio or far-infrared may not be viable, but expressions may relate the observed quantities for current flux and volume limited SMG samples. These suggest that SED estimation may be possible, for objects similarly selected, based on a single radio or far-infrared flux measurement.
The detection of these faint objects (~10 mJy at 350 micron) from the ground is complicated by a bright (~1000 Jy) and highly variable (~10 Jy RMS in 10 minutes of integration) atmosphere with a 1/f^2 noise spectrum and by instrumental 1/f noise. To reach optimum sensitivities, a careful analysis of the data is required, and well-chosen observing strategies are helpful. The principal techniques that aid the extraction of weak signals from colored noise are presented. Close to optimal analysis is implemented effectively by the CRUSH software. Both the computing and storage requirement of the implementation scales linearly with the size of the data set, making this approach superior to the computationally expensive alternatives for handling the very large data volumes expected from future instruments.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/7eha-q373, author = {Benford, Dominic James}, title = {Broadband submillimeter instrumentation for the detection of distant galaxies}, school = {California Institute of Technology}, year = {1999}, doi = {10.7907/7eha-q373}, url = {https://resolver.caltech.edu/CaltechETD:etd-06092008-100026}, abstract = {This research details my efforts to search the universe for the faint far-infrared emission of galaxies as far away as possible. I first describe the design, construction, use, and performance evaluation of the Submillimeter High Angular Resolution Camera (SHARC), a 24-element bolometer camera for imaging at 350 microns and 450 microns from the Caltech Submillimeter Observatory (CSO). This instrument achieves background-limited performance and has been available for use by the international community since 1996. In the following two chapters, I detail the astronomical results attained with this instrument when used to observe the thermal dust emission from a sample of nearby galaxies (chapter 3) and a sample of sources as distant as 90% of the way across the spacetime universe (chapter 4). The nearby sample is well-characterized by a single-component greybody dust emission model with a temperature of 37 +/- 4K and a spectral emissivity index of beta = 1.7 +/- 0.4. Our cosmologically distant sample provides the first systematic study of these objects at wavelengths probing near the emission peak at a rest-frame wavelength of approximately 80 microns. We find an average temperature of 53 +/- 8K and determine a median luminosity of (2 +/- 1)x10^13 L_sun and a median dust mass of (3 +/- 2)x10^8 M_sun. This makes these objects some of the most massive and luminous ever observed, with an inferred star formation rate of 2000 M_sun per year. To balance the continuum emission results described above, I have observed both local and high-redshift galaxies with the CSO facility heterodyne receivers in an effort to detect the emission lines of CO, Cii, and Nii. We have observed a sample of 22 nearby (0.02 < z < 0.13) ultraluminous infrared galaxies in the J = 2 to 1 and J = 3 to 2 transitions of CO. Using published 1 to 0 intensities, we find that the 3 to 2 emission most likely arises from an optically thick region, implying that future observations in the higher-J lines can be used to constrain the temperature of the molecular gas in these galaxies. We find a most likely temperature in the range 20 < T < 60K and a molecular gas density of n(H2) [approx] 1500 cm^-3. At high redshifts (z > 2), however, most of our observations have resulted in nondetections, but not without merit. The emission in the Nii 205 micron line from the Cloverleaf quasar is found to be below the amount predicted for a galaxy similar to M82. For two z > 4 quasars, our upper limits to the Cii 158 micron line emission show that the ratio of the line luminosity to the total luminosity is less than 0.01%, ten times smaller than has been observed locally. Finally, I shall detail our effort to design a novel submillimeter spectrometer using a linear bolometer array as a detector element and relying on Fabry-Perot or immersed grating optics to provide the spectral dispersion. This approach promises to provide bandwidths several times larger than are available with existing heterodyne spectrometers, making the detection of cosmologically distant galaxies in their submillimeter line emission a reality.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/9k2a-vf26, author = {Hunter, Todd Russell}, title = {A submillimeter imaging survey of ultracompact HII regions}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/9k2a-vf26}, url = {https://resolver.caltech.edu/CaltechETD:etd-05212007-111551}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.
This research explores the process of massive star formation in the Galaxy through submillimeter continuum and spectral line observations of ultracompact HII (UCHII) regions. First, I describe the design and operation of the Submillimeter High Angular Resolution Camera (SHARC)-a 24-pixel bolometer array camera for broadband continuum imaging at 350 and 450µm at the Caltech Submillimeter Observatory (CSO). Detailed information is included on the reflective off-axis optical design and the instrument control software interface. Second, I present 10" to 12" resolution SHARC images of 350 and 450µm continuum emission from a sample of 17 UCHII regions with different radio morphologies. Although the dust emission typically peaks at or near the UCHII region, additional sources are often present, sometimes coincident with the position of H2O masers. The combination of submillimeter, millimeter and IRAS far-infrared flux densities forms the basis of greybody models of the spectral energy distributions. The average dust temperature is 40 ± 10 K and the average grain emissivity index ([beta]) is 2.00 ± 0.25. Using a radiative transfer program that solves for the dust temperature versus radius, the distribution of dust around UCHII regions is modeled with a power-law spherical density profile to match the observed radial flux density profiles. By fixing the source boundary at the outer limit of the submillimeter emission, the resulting density profiles n(r) […] can be classified into four categories: 3 regions exhibit p = 2 (isothermal sphere), 4 exhibit p = 1.5 (dynamical collapse), 2 exhibit p = 2 in the outer regions and p = 1.5 in the inner regions, and 6 exhibit p = 1 (logatropic). Although these simplified models may not be unique, a good correlation between the dust luminosity-to-mass ratio and the temperature indicates that the more centrally-condensed sources exhibit higher star formation rates. Third, I present 20" to 30" resolution CO maps which reveal bipolar outflows from 15 out of 17 UCHII regions. The outflow mechanical luminosities and mass ejection rates follow the scaling relations with bolometric luminosity established for less luminous pre-main sequence stars. However, in contrast to lower luminosity sources, the momentum from stellar radiation pressure is comparable to that required to drive the outflows. Many regions show evidence of separate, overlapping outflows. In a final detailed study, 2" resolution images obtained with the Owens Valley Millimeter Array reveal multiple outflows emanating from the molecular core containing the UCHII region G45.12+0.13, while simultaneous outflow and infall motion is seen toward the neighboring, less-evolved core containing G45.07+0.13.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/AZYZ-XP41, author = {Bin, Mei}, title = {Low-noise THz Niobium SIS Mixers}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/AZYZ-XP41}, url = {https://resolver.caltech.edu/CaltechTHESIS:07172014-124951267}, abstract = {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/kb 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.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/xezv-q641, author = {Schoelkopf, Robert J.}, title = {Studies of noise in Josephson-effect mixers and their potential for submillimeter heterodyne detection}, school = {California Institute of Technology}, year = {1995}, doi = {10.7907/xezv-q641}, url = {https://resolver.caltech.edu/CaltechETD:etd-10232007-104241}, abstract = {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.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/rbaq-gb42, author = {Groesbeck, Todd Dickson}, title = {The Contribution of Molecular Line Emission to Broadband Flux Measurements at Millimeter and Submillimeter Wavelengths}, school = {California Institute of Technology}, year = {1995}, doi = {10.7907/rbaq-gb42}, url = {https://resolver.caltech.edu/CaltechETD:etd-03222006-103422}, abstract = {We present measurements of the ratio of flux carried by molecular emission lines to the total flux observed from six astronomical sources in the 330-360 GHz frequency range, demonstrating that the integrated line emission can represent a significant fraction, and in some cases even the majority, of the total flux observed. Given the narrow widths of these emission lines for most sources in the interstellar medium, it has generally been assumed that the contribution of the integrated lines would be negligible when compared to the continuum flux measured by a very broadband instrument. We extend the previous work by Sutton et al. (1984), which demonstrated that line emission was responsible for a large part of the total flux observed from Orion-KL, by presenting observations at higher frequencies of Orion-KL and several additional sources. These observations provide the most comprehensive determination of the ratio of line flux to continuum flux yet made at submillimeter wavelengths.
We have performed spectral line surveys of the star-forming regions Orion-KL, Orion-S, and IRAS 16293-2422, and the evolved stars IRC +10216, VY Canis Majoris, and OH 231.8+4.2. Observations for Orion-KL and IRC +10216 give continuous frequency coverage over approximately the 330-360 GHz band; for the other sources the coverage ranges between 30 and 60% of the same frequency band. Comparisons of the line survey data with total flux measurements demonstrates that the contribution of the integrated line emission to the total flux at these frequencies ranges from as low as ~10% for Orion-S and IRAS 16293-2422 to at least ~60% for Orion-KL and IRC +10216. We also estimate the amount of flux carried in weak lines below our sensitivity limits based on the observed distributions of line intensities.
An improved deconvolution method has been used to obtain a single sideband spectrum from the double sideband observations. We have also developed computer versions of line catalogs which allow immediate line identification and analysis, assuming conditions of local thermodynamic equilibrium (LTE) prevail. The same catalog programs also permit simulation of emission line spectra for arbitrary molecular abundances and excitation temperatures, again assuming LIE. For each of the observed sources, we present simulations of the spectrum from 0 to 1000 GHz, using molecular parameters derived from our data and additional published observations. In all cases, the simulations indicate that the relative importance of the line emission decreases at higher frequencies (~ > 700 GHz). We discuss the significance of these findings regarding the determination of dust parameters from broadband flux measurements, concluding that such measurements must be corrected for possible line flux contributions before they can be reliably used.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/pkm9-jk53, author = {Young, Kenneth Harbour}, title = {Submillimeter and infrared studies of mass lost by asymptotic giant branch stars}, school = {California Institute of Technology}, year = {1994}, doi = {10.7907/pkm9-jk53}, url = {https://resolver.caltech.edu/CaltechETD:etd-09172008-085430}, abstract = {This thesis consists of four separate studies of the material ejected from highly evolved red giant stars, preceded by a brief introductory chapter.
Chapter 2 presents the results of an attempt to identify all the post main sequence stars in the sky with circumstellar shells large enough to have been resolved by the IRAS satellite. Both the 60μm and 100μ survey mode data were used. A total of 512 stars were examined, including all evolved stars with envelopes which had been detected in surveys of mm or submm CO emission. A total of 76 stars were found to have shells with radii exceeding 2’ in the 60μ data. Most of these resolved shells surround semiregular variable stars. Although only 40% of the resolved objects are carbon stars, the fraction of carbon stars with resolved envelopes is higher than that of oxygen—rich stars. The linear sizes of these shells were calculated; the average shell size is 0.76 pc. If a measurement of the expansion velocity was available from CO observations, the age of the shell was estimated using a simple model of the shell’s interaction with the ISM. Some of the shells appear to be detached, and in most cases shells with inner radii greater than ~ 0.3 pc have no detectable CO. The duration of mass loss for Mira variables and carbon stars is estimated to be 10[superscript 5] and 2 x 10[superscript 5] years, respectively. The procedure by which the data were analysed is explained in the appendix to chapter 2, which also contains the results of several tests performed to verify the results statistically.
A survey of CO(3-2) emission from Mira variables comprises chapter 3. The survey examined nearby oxygen—rich Miras, without regard to their infrared properties. Several new detections were made, including some circumstellar shells with expansion velocities lower than any previously reported. Mass loss rates for the detected stars were calculated. While the mass loss rate shows significant correlation with all 4 IRAS fluxes (when normalized by distance), it appears completely uncorrelated with any color derived from these fluxes. A star’s spectral type at maximum light was found to be a good predictor of which stars would be detectable; no star earlier than M5.5 was detected even though some such stars had large IRAS fluxes. A power—law relationship was found between the envelope expansion velocity and the star’s mass loss rate. Extrapolation of this result implies that envelopes with expansion velocities above ~ 18 km s[superscript -1] must be optically thick.
The peculiar submm emission line profiles of CRL 2688, a protoplanetary nebula, are examined in chapter 4. The molecular envelope was found to have three kinematic components, including a previously undetected ~ 100 km s[superscript -1] wind. The mechanical momentum in this high velocity wind appears to be much too large to have been supplied by radiation pressure. A strong self-absorption feature seen in the CO emission lines was examined at very high spectral resolution, and was found to have an extremely sharp blue edge. The sharpness of this feature shows that the velocity field in the extended envelope is very well ordered.
The final chapter examines atomic carbon emission in the envelope of the young planetary nebula NGC 7027. Neutral carbon, traced by the 609μm hyperfine line, is extended over a region 30-40" in diameter, and appears to be coextensive with CO throughout most of the neutral envelope. A 158μm spectrum of C II was used to calculate a lower limit to the C II mass. There appears to be roughly equal amounts of CO, C I and C II in the inner neutral envelope. Most of the atomic carbon must have been liberated by photodissociation of CO. Estimates of the mass loss rate for this object derived from CO observations may be too low by a factor of ~ 3.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/1dkj-1503, author = {Büttgenbach, Thomas H.}, title = {Quasi-optical sis receivers and astrophysical observations at submillimeter wavelengths}, school = {California Institute of Technology}, year = {1993}, doi = {10.7907/1dkj-1503}, url = {https://resolver.caltech.edu/CaltechTHESIS:11302012-115924624}, abstract = {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.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/AZ4Q-DF35, author = {Stern, Jeffrey Aaron}, title = {Fabrication and testing of NbN/MgO/NbN tunnel junctions for use as high-frequency heterodyne detectors}, school = {California Institute of Technology}, year = {1991}, doi = {10.7907/AZ4Q-DF35}, url = {https://resolver.caltech.edu/CaltechETD:etd-12202004-091326}, abstract = {This thesis describes the development and testing of NbN/MgO/NbN tunnel junctions for use as superconductor-insulator-superconductor (SIS) mixers. SIS mixers are the most sensitive heterodyne detectors in the millimeter wavelength region. Most SIS mixers use Pb alloy tunnel junction. These tunnel junctions have several problems associated with the soft nature of Pb and its low superconducting transition temperature. NbN-based tunnel junctions are being developed to overcome these difficulties. These devices are intended to be used as mixers at millimeter and submillimeter wavelengths. This thesis describes the fabrication process involved in making NbN junctions, and the results of measurements on these devices. The purpose of these measurements is to determine the future possibilities of NbN tunnel junctions as high-frequency mixers. The first chapter is an introduction to SIS mixers and explains how tunnel junction properties affect mixer performance. The basic theory of tunneling and mixing in SIS mixers is first presented. A description of quantum mixer theory is included in this presentation. This theory makes several interesting predictions that cannot be explained using classical theories. This is followed by a description of how real SIS tunnel junctions differ from ideal junctions. The physical origin of these differences is discussed, along with how they affect SIS mixer performance. Finally, the advantages and disadvantages of the various superconducting materials available are discussed. The decision to develop NbN devices is based on these material properties. The second chapter describes the methods used to fabricate small-area NbN/MgO/NbN tunnel junctions. The chapter begins with a description of the various methods that have been used to deposit NbN films. Reactive magnetron sputtering is chosen as the best method for tunnel junction fabrication. Details on the vacuum systems and the methods used for depositing superconducting NbN are discussed. Next, the process used for depositing junction trilayers (NbN/MgO/NbN) is described. The probable growth mode of MgO on NbN is presented. The importance of this growth mode to the junction quality is explained in some detail. Next, standard junction processing steps are reported. The details and limitations of each step are put forth. The standard process allows for the fabrication of 1 [square micron] tunnel junctions. Finally, this chapter discusses several methods of fabaricating submicron junctions that are being pursued. The status of this work is given. The third chapter describes the characterization of NbN films and NbN/Mg0/NbN tunnel junctions. Film properties are described first. The correlation of these properties to deposition conditions is discussed in some detail. Next, values for the various features of the I-V characteristic are given; typical and exceptional values are noted. How these features limit mixer results is described in detail. Several important device attributes were measured using superconducting-quantum-interference-devices (SQUIDs). These attributes are the junction’s specific capacitance and the film’s magnetic penetration depth. The theory and results of measuring specific capacitance and penetration depth are presented. Following the SQUID results is a large section on RP testing. Mixer tests were made at 205 GHz. The receiver design used, integral inductive tuning circuit used and results are discussed. These results are well understood, with the exception of the temperature dependence of the mixer performance. Finally, measurements of the uniformity of many junctions on a single wafer are presented. The importance of junction uniformity is also described. The final chapter discusses the ultimate limits on NbN mixers, and tells what future work must be done to achieve these limits. The primary high-frequency limit on NbN junctions is the capacitance of NbN/MgO/NbN junctions. The limit imposed by the junction capacitance and circuits used to tune out this capacitance are discussed. Also, the low-frequency limits on NbN junctions are discussed. The status of submicron devices is presented. Junction area is the most immediate limitation on NbN mixer results. The possibility of using other barrier materials to increase the RC speed of NbN junctions is presented. Finally, the possibility of operating NbN junctions at temperatures above 4.2 K is discussed.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/79cv-bm83, author = {Grossman, Erich Nathan}, title = {A Far-Infrared Heterodyne Spectrometer for Airborne Astronomy}, school = {California Institute of Technology}, year = {1988}, doi = {10.7907/79cv-bm83}, url = {https://resolver.caltech.edu/CaltechTHESIS:01222013-150156111}, abstract = {
The design and construction of a novel heterodyne spectrometer for airborne astronomy in the 50 µm - 200 µm wavelength range is described. along with laboratory measurements of its performance. A bulk, extrinsic Ge:Ga photoconductor is used as the mixer. Its low bandwidth, determined by the hole recombination rate, necessitates the use of a continuously tunable local oscillator. This is provided by a far-infrared laser sideband generator, which is based on a GaAs Schottky diode mounted at the feed of a comer-cube antenna, the latter combination acting as a reflective FIR modulator.
The first chapter of this thesis describes the astronomical and technical context of the project - in particular, the constraints which the astronomical goals set on the instrument, and the advantages and drawbacks of each of the various broad instrumental strategies that are available for spectrometer design. The chapter’s last section provides a very brief overview of our most successful laboratory results, which are described at greater length in chapters 2- 4. In chapter two we describe the performance of Ge:Ga mixers as heterodyne mixers. We report on an extensive series of measurements of bandwidth, photoconductive gain, and direct detection responsivity for a series of highly compensated, NTD detectors grown specifically for this purpose. Chapter two also describes a nunber of experiments on FIR heterodyne performance, made using the direct, attenuated laser, rather than the output of the sideband generator, as the local oscillator. These confirm the expectation that germanium photoconductors are capable of quantum-limited noise performance with quantum efficiencies of ~10%, at much lower LO powers than required for Schottky diodes. Our best achieved noise temperature is TN(DSB) = 655K at PLO = 1.6µW, a factor of > 25 lower than the best reported corresponding figure for Schottky diodes.
Chapter 3 describes the operating principles and construction of our FIR laser, which formed a basic tool in nearly all our laboratory experiments. A brief discussion of the Lorenz instability in FIR lasers is also given, in connection with various observations we have made of spontaneous pulsations and excess low-frequency noise on the laser output, and which have recently been the subject of considerable study by other researchers. Chapter four describes FIR laser sideband generation using small-area Schottky diodes and comer-cube antennas. The construction and performance of our corner-cubes is outlined, including the first direct measurement of the main beam efficiency of a corner-cube antenna in the FIR, and a comparison with theory. The construction and measured performance of the rest of the sideband generator is also described. A detailed, quantitative model has been developed for the conversion efficiency obtainable from Schottky diodes in this application. We find that the low conversion efficiency (-39 db) measured in our experiments, and comparable to that found by other researchers, is inherent in the diode and well predicted by the model. For our particular experiment, the model predicts -28 db loss due to the diode, plus approximately -10 db loss due to the antenna coupling efficiency. The dependence of conversion efficiency on diode parameters is studied and guidelines for future optimization derived. Unfortunately, the severe conversion loss we measure, combined with low FIR laser power and (somewhat less significantly) poor optics transmission, leads to our presently available LO power being inadequate to obtain astronomically useful sensitivity, by a large factor.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/TYQQ-RZ71, author = {Wengler, Michael James}, title = {Heterodyne Detection with Superconducting Tunnel Diodes}, school = {California Institute of Technology}, year = {1988}, doi = {10.7907/TYQQ-RZ71}, url = {https://resolver.caltech.edu/CaltechETD:etd-02012007-084647}, abstract = {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.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/0F73-PJ76, author = {Blake, Geoffrey A.}, title = {On the Chemical Composition of Interstellar Molecular Clouds: A Millimeter and Submillimeter Spectral Line Survey of OMC-1}, school = {California Institute of Technology}, year = {1986}, doi = {10.7907/0F73-PJ76}, url = {https://resolver.caltech.edu/CaltechETD:etd-12092003-145807}, abstract = {The same basic principles govern the chemical and physical evolution of systems throughout the universe. However, the dissimilar conditions on the Earth and in the interstellar medium lead to remarkably different chemical compositions for these two environments. While less familiar than terrestrial chemistry, the study of the chemical composition of the interstellar medium is important because it bears directly on the understanding of phenomena as diverse as star formation, galactic structure and dynamics, and the cosmological origin of the universe, in addition to providing a unique opportunity to investigate a number of fundamental chemical and physical processes.
We present here results from a millimeter and submillimeter spectral line survey of the core of the Orion molecular cloud (OMC-1). The millimeter-wave survey, conducted at the Owens Valley Radio Observatory (OVRO), covers a 55 GHz interval in the 1.3 mm (230 GHz) atmospheric window and contains emission from 29 molecules. Together with the frequency selective submillimeter observations of H2D+ (372.4 GHz), Cl (492.2 GHz), NH3 (572.5 GHz), and HCl (625.9 GHz) performed aboard NASA’s Kuiper Airborne Observatory, over 800 emission lines have been detected from 33 chemically distinct species during the course of this work. The uniformly calibrated results from the unique and extensive OVRO spectral line survey place significant constraints on models of interstellar chemistry, and have allowed the chemical composition of the various regions in OMC-1 to be definitively characterized.
A global analysis of the observed abundances has shown that the markedly different chemical compositions of the kinematically distinct Orion subsources may be simply interpreted in the framework of an evolving, initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the high velocity outflow from the young star(s) embedded deep within the interior of the molecular cloud. Detailed arguments are presented in this thesis which relate the seemingly disparate chemical compositions of the individual regions to each other and to the expected physical manifestations of the circumstellar mass loss, and which suggest that similar mechanisms may operate in other molecular clouds as well.
By performing supporting laboratory spectroscopy to supplement existing millimeter-wave catalogues only 33 of the over 800 lines remain unidentified, in contradiction to earlier expectations which had predicted that the near millimeter-wave spectrum of molecular clouds would contain hundreds of strong, unidentifiable emission features. It is probable that a number of the unidentified lines left in the OVRO survey are due to transitions between states of either isotopically substituted or highly excited abundant and complex molecules such as CH3OH, CH3OCH3, and HCOOCH3 whose rotational spectra are poorly known at present. The very small percentage and weak strength of the unidentified lines implies that the dominant chemical constituents visible at millimeter wavelengths have been identified in the Orion molecular cloud.
Also presented are high resolution laboratory millimeter, submillimeter, and far-infrared absorption spectra of the transient molecular species OH, CN, HOC+, and HCO, The zero-field pure rotational spectrum of the OH radical was observed, for the first time, with a frequency agile far-infrared laser sideband spectrometer which promises to revolutionize high resolution spectroscopy at submillimeter and far-infrared wavelengths, while the HOC+ molecular ion was synthesized in a novel glow discharge cell that increases ion abundances by roughly two orders of magnitude as compared with those produced by previously reported methods. Studies of several ions produced in the new discharge cell have provided a theory of the mechanism responsible for the ion enhancement. Sixty-five transitions of CN in its first four vibrational states have been observed, allowing a detailed examination of vibrational and electronic effects in this astrophysically important free radical. The investigation of HCO is the first extensive zero-field analysis of the formyl radical, and is one of the very few millimeter and submillimeter laboratory studies of a non-linear free radical performed to date.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, } @phdthesis{10.7907/wm40-4k69, author = {Brown, Elliott R.}, title = {Investigation of Bulk Indium Antimonide as Heterodyne Detector for the Submillimeter Wavelength Region}, school = {California Institute of Technology}, year = {1985}, doi = {10.7907/wm40-4k69}, url = {https://resolver.caltech.edu/CaltechTHESIS:07172014-130847340}, abstract = {Bulk n-InSb is investigated at a heterodyne detector for the submillimeter wavelength region. Two modes of operation are investigated: (1) the Rollin or hot electron bolometer mode (zero magnetic field), and (2) the Putley mode (quantizing magnetic field). The highlight of the thesis work is the pioneering demonstration or the Putley mode mixer at several frequencies. For example, a double-sideband system noise temperature of about 510K was obtained using a 812 GHz methanol laser for the local oscillator. This performance is at least a factor or 10 more sensitive than any other performance reported to date at the same frequency. In addition, the Putley mode mixer achieved system noise temperatures of 250K at 492 GHz and 350K at 625 GHz. The 492 GHz performance is about 50% better and the 625 GHz is about 100% better than previous best performances established by the Rollin-mode mixer. To achieve these results, it was necessary to design a totally new ultra-low noise, room-temperature preamp to handle the higher source impedance imposed by the Putley mode operation. This preamp has considerably less input capacitance than comparably noisy, ambient designs.
In addition to advancing receiver technology, this thesis also presents several novel results regarding the physics of n-InSb at low temperatures. A Fourier transform spectrometer was constructed and used to measure the submillimeter wave absorption coefficient of relatively pure material at liquid helium temperatures and in zero magnetic field. Below 4.2K, the absorption coefficient was found to decrease with frequency much faster than predicted by Drudian theory. Much better agreement with experiment was obtained using a quantum theory based on inverse-Bremmstrahlung in a solid. Also the noise of the Rollin-mode detector at 4.2K was accurately measured and compared with theory. The power spectrum is found to be well fit by a recent theory of non-equilibrium noise due to Mather. Surprisingly, when biased for optimum detector performance, high purity InSb cooled to liquid helium temperatures generates less noise than that predicted by simple non-equilibrium Johnson noise theory alone. This explains in part the excellent performance of the Rollin-mode detector in the millimeter wavelength region.
Again using the Fourier transform spectrometer, spectra are obtained of the responsivity and direct detection NEP as a function of magnetic field in the range 20-110 cm-1. The results show a discernable peak in the detector response at the conduction electron cyclotron resonance frequency for magnetic fields as low as 3 KG at bath temperatures of 2.0K. The spectra also display the well-known peak due to the cyclotron resonance of electrons bound to impurity states. The magnitude of responsivity at both peaks is roughly constant with magnet1c field and is comparable to the low frequency Rollin-mode response. The NEP at the peaks is found to be much better than previous values at the same frequency and comparable to the best long wavelength results previously reported. For example, a value NEP = 4.5 x 10-13W/Hz1/2 is measured at 4.2K, 6 KG and 40 cm-1. Study of the responsivity under conditions of impact ionization showed a dramatic disappearance of the impurity electron resonance while the conduction electron resonance remained constant. This observation offers the first concrete evidence that the mobility of an electron in the N = 0 and N = 1 Landau levels is different. Finally, these direct detection experiments indicate that the excellent heterodyne performance achieved at 812 GHz should be attainable up to frequencies of at least 1200 GHz.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Phillips, Thomas G.}, }