Coherent Receiver Arrays for Astronomy and Remote Sensing

Monolithic Millimeter-wave Integrated Circuits (MMICs) provide a level of integration that makes possible the construction of large focal plane arrays of radio-frequency detectors—effectively the first “Radio Cameras”—and these will revolutionize radio-frequency observations with single dishes, interferometers, spectrometers, and spacecraft over the next two decades. The key technological advances have been made at the Jet Propulsion Laboratory (JPL) in collaboration with the Northrop Grumman Corporation (NGC). Although dramatic progress has been made in the last decade in several important areas, including (i) packaging that enables large coherent detector arrays, (ii) extending the performance of amplifiers to much higher frequencies, and (iii) reducing room-temperature noise at high frequencies, funding to develop MMIC performance at cryo-temperatures and at frequencies below 150GHz has dropped nearly to zero over the last five years. This has severely hampered the advance of the field. Moreover, because of the high visibility of < 150GHz cryogenic detectors in astrophysics and cosmology, lack of progress in this area has probably had a disproportionate impact on perceptions of the potential of coherent detectors in general. One of the prime objectives of the Keck Institute for Space Studies (KISS) is to select crucial areas of technological development in their embryonic stages, when relatively modest funding can have a highly significant impact by catalyzing collaborations between key institutions world-wide, supporting in-depth studies of the current state and potential of emerging technologies, and prototyping development of key components—all potentially leading to strong agency follow-on funding. The KISS large program “Coherent Instrumentation for Cosmic Microwave Background Observations” was initiated in order to investigate the scientific potential and technical feasibility of these “Radio Cameras.” This opens up the possibility of bringing support to this embryonic area of detector development at a critical phase during which KISS can catalyze and launch a coherent, coordinated, worldwide effort on the development of MMIC Arrays. A number of key questions, regarding (i) the importance and breadth of the scientific drivers, (ii) realistic limits on sensitivity, (iii) the potential of miniaturization into receiver “modules,” and (iv) digital signal processing, needed to be studied carefully before embarking on a major MMIC Array development effort led by Caltech/JPL/NGC and supported by KISS, in the hope of attracting adequate subsequent government funding. For this purpose a large study was undertaken under the sponsorship and aegis of KISS. The study began with a workshop in Pasadena on “MMIC Array Receivers and Spectrographs” (July 21–25, 2008)1, immediately after an international conference “CMB Component Separation and the Physics of Foregrounds” (July 14–18, 2008)2 that was organized in conjunction with the MMIC workshop. There was then an eight-month study period, culminating in a final “MMIC 2Workshop” (March 23–27, 2009).3 These workshops were very well attended, and brought together the major international groups and scientists in the field of coherent radio-frequency detector arrays. A notable aspect of the workshops is that they were well attended by young scientists—there are many graduate students and post-doctoral fellows coming into this area. The two workshops focused both on detailed discussions of key areas of interest and on the writing of this report. They were conducted in a spirit of full and impartial scrutiny of the pros and cons of MMICs, in order to make an objective assessment of their potential. It serves no useful purpose to pursue lines of technology development based on unrealistic and over-optimistic projections. This is crucially important for KISS, Caltech, and JPL which can only have real impact if they deliver on the promise of the technologies they develop. A broad range of opinions was evident at the start of the first workshop, but in the end a strong consensus was achieved on the most important questions that had emerged. This report reflects the workshop deliberations and that consensus. The key scientific drivers for the development of the MMIC technology are: (i) large angular-scale Bmode polarization observations of the cosmic microwave background—here MMICs are one of two key technologies under development at JPL, both of which are primary detectors on the recently-launched Planck mission; (ii) large-field spectroscopic surveys of the Galaxy and nearby galaxies at high spectral resolution, and of galaxy clusters at low resolution; (iii) wide-field imaging via deployment as focal plane arrays on interferometers; (iv) remote sensing of the atmosphere and Earth; and (v) wide-field imaging in planetary missions. These science drivers are discussed in the report. The most important single outcome of the workshops, and a sine qua non of this whole program, is that consensus was reached that it should be possible to reduce the noise of individual HEMTs or MMICs operating at cryogenic temperatures to less than three times the quantum limit at frequencies up to 150 GHz, by working closely with a foundry (in this case NGC) and providing rapid feedback on the performance of the devices they are fabricating, thus enabling tests of the effects of small changes in the design of these transistors. This kind of partnership has been very successful in the past, but can now be focused more intensively on cryogenic performance by carrying out tests of MMIC wafers, including tests on a cryogenic probe station. It was felt that a properly outfitted university laboratory dedicated to this testing and optimization would be an important element in this program, which would include MMIC designs, wafer runs, and a wide variety of tests of MMIC performance at cryogenic temperatures. This Study identified eight primary areas of technology development, including the one singled out above, which must be actively pursued in order to exploit the full potential of MMIC Arrays in a timely fashion: 1. Reduce the noise levels of individual transistors and MMICs to three times the quantum limit or lower at cryogenic temperatures at frequencies up to 150 GHz. 2. Integrate high-performing MMICs into the building blocks of large arrays without loss of performance. Currently factors of two in both noise and bandwidth are lost at this step. 3. Develop high performance, low mass, inexpensive feed arrays. 4. Develop robust interconnects and wiring that allow easy fabrication and integration of large arrays. 5. Develop mass production techniques suitable for arrays of differing sizes. 6. Reduce mass and power. (Requirements will differ widely with application. In the realm of planetary instruments, this is often the most important single requirement.) 7. Develop planar orthomode transducers with low crosstalk and broad bandwidth. 8. Develop high power and high efficiency MMIC amplifiers for LO chains, etc. Another important outcome of the two workshops was that a number of new collaborations were forged between leading groups worldwide with the object of focusing on the development of MMIC arrays

Relativistic Jets from Active Galactic Nuclei

The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These active galactic nuclei (AGNs) can form jets that are observed on scales from astronomical units to megaparsecs and from meter wavelengths to TeV energies. High-resolution radio imaging and multiwavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: Relativistic AGN jets are formed when the black hole spins and the the accretion disk is strongly magnetized, perhaps on account of gas accreting at high latitude beyond the black hole sphere of influence. AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind. Higher-power jets can emerge from their galactic nuclei in a relativistic, supersonic, and proton-dominated state, and they terminate in strong, hot spot shocks; lower-power jets are degraded to buoyant plumes and bubbles. Jets may accelerate protons to EeV energies, which contribute to the cosmic ray spectrum and may initiate pair cascades that can efficiently radiate synchrotron γ-rays. Jets were far more common when the Universe was a few billion years old and black holes and massive galaxies were growing rapidly. Jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright

Cosmic Microwave Background Observations in the Era of Precision Cosmology

The cosmic microwave background (CMB) radiation is a major arena for testing cosmological theories. Its discovery confirmed the hot-big- bang origin of the universe and ruled out the steady-state theory. Since that time the impact on cosmology of CMB studies has grown steadily, indicating the prevalence of non-baryonic matter and the existence of a negative pressure component in the 1980’s; the discovery of anisotropy in the 1990’s spurred a new generation of experiments and the entry into the era of precision cosmology in 2000 with the demonstration that the geometry is close to flat. The new “holy grail” of the field is the large-scale B-mode polarization component, which would reveal the energy scale of inflation. The sensitivity needed is ~10^(−8) Kelvin, and at this level foreground polarized emission is likely to dominate over most of the sky. New radio-frequency cameras consisting of ~1,000-element MMIC arrays will be deployed over the next few years on a wide variety of instruments and should bring about a revolution in radio astronomy with enormous consequences, not only for cosmology, but also for a wide variety of astrophysical studies

The First Billion Years

Spectral measurements of atomic and molecular lines embedded in the cosmic microwave background (CMB) have the potential to open entirely new probes of the early Universe. Two avenues are of great interest: 1) Spectral line deviations from the CMB blackbody spectrum will enable the study of hydrogen and helium recombination physics during and before the time of the surface of last scattering, and could provide the potential for game-changing discoveries by testing dark matter annihilation in the redshift range 6000> z > 1000, by allowing a test of the time-dependence of the fine-structure constant at a critical epoch, and by testing inflation models using an independent method. 2) Extension of CMB anisotropy measurements to detect unresolved spectral line emission from starforming galaxies during reionization (6 < z < 10) would directly delineate the large-scale structure of the galaxies responsible for reionizing the Universe and provide the only foreseeable measurements on scales sufficiently large to compare with upcoming observations of reionization by way of the redshifted hydrogen 21 cm line. CO, [C II], and Ly-a lines were investigated as promising targets. CO and [C II] line transitions emerged as particularly compelling. The two science objectives identified in the Program share some common core technological requirements based on the shared need for approximately 1000-element feed arrays followed by broadband, highresolution spectral correlators. The technical requirements lead to a roadmap for development of large feed arrays beginning with applications in a ground-based CO mapping instrument and leading to a spaceborne recombination-line all-sky spectrometer. The key technical issues include compact and light-weight integrated spectral dual-polarization inexpensive receiver modules, large high-resolution spectral correlators (analog and/or digital), and light-weight feeds. In parallel we recommend long-term investigations into high precision calibrators and calibration techniques that will be required for the recombination line instrument. A second roadmap addresses technical developments required for a 2-D spectroscopic instrument for [C II] mapping

125 - 211 GHz low noise MMIC amplifier design for radio astronomy

To achieve the low noise and wide bandwidth required for millimeter wavelength astronomy applications, superconductor-insulator-superconductor (SIS) mixer based receiver systems have typically been used. This paper investigates the performance of high electron mobility transistor (HEMT) based low noise amplifiers (LNAs) as an alternative approach for systems operating in the 125 — 211 GHz frequency range. A four-stage, common-source, unconditionally stable monolithic microwave integrated circuit (MMIC) design is presented using the state-of-the-art 35 nm indium phosphide HEMT process from Northrop Grumman Corporation. The simulated MMIC achieves noise temperature (T_e) lower than 58 K across the operational bandwidth, with average T_e of 38.8 K (corresponding to less than 5 times the quantum limit (hf/k) at 170 GHz) and forward transmission of 20.5 ± 0.85 dB. Input and output reflection coefficients are better than -6 and -12 dB, respectively, across the desired bandwidth. To the authors knowledge, no LNA currently operates across the entirety of this frequency range. Successful fabrication and implementation of this LNA would challenge the dominance SIS mixers have on sub-THz receivers

Polarization Observations with the Cosmic Background Imager

We describe polarization observations of the CMBR with the Cosmic Background Imager, a 13 element interferometer which operates in the 26-36 GHz band from Llano de Chajnantour in northern Chile. The array consists of 90-cm Cassegrain antennas mounted on a steerable platform which can be rotated about the optical axis to facilitate polarization observations. The CBI employs single mode circularly polarized receivers which sample multipoles from ℓ~400 to ℓ~4250. The instrumental polarization of the CBI was calibrated with 3C279, a bright polarized point source which was monitored with the VLA

Optical Spectroscopy of Bright Fermi LAT Blazars

We report on HET and Palomar 5 m spectroscopy of recently identified $\gamma$-ray blazars in the {\it Fermi} LAT Bright Source List. These data provide identifications for 10 newly discovered $\gamma$-ray flat spectrum radio quasars (FSRQ) and six new BL Lacs plus improved spectroscopy for six additional BL Lacs. We substantially improve the identification completeness of the bright LAT blazars and give new redshifts and $z$ constraints, new estimates of the black hole masses and new measurements of the optical SED.Comment: 8 pages, 5 figures, 2 tables. Accepted for publication in Ap

A Physical Model for Drain Noise in High Electron Mobility Transistors: Theory and Experiment

We report the on-wafer characterization of $S$-parameters and microwave noise temperature ($T_{50}$) of discrete metamorphic GaAs high electron mobility transistors (HEMTs) at 40 K and 300 K over a range of drain-source voltages ($V_{DS}$). From these data, we extract a small-signal model and the drain noise temperature ($T_{d}$) at each bias and temperature. We find that $T_d$ follows a superlinear trend with $V_{DS}$ at both temperatures. These trends are interpreted by attributing drain noise to a thermal component associated with the channel resistance and a component due to real-space transfer (RST) of electrons from the channel to the barrier [1]. In the present devices at the minimum $T_{50}$, RST contributes $\sim 10$% of the drain noise at cryogenic temperatures. At 300 K, the contribution increases to over $\sim 60$% of the total drain noise. This finding indicates that improving the confinement of electrons in the quantum well could enable room-temperature receivers with up to $\sim 50$% lower noise temperatures by decreasing the contribution of RST to drain noise.Comment: 6 pages, 6 figure

Relativistic Jets from Active Galactic Nuclei

The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These active galactic nuclei (AGNs) can form jets that are observed on scales from astronomical units to megaparsecs and from meter wavelengths to TeV energies. High-resolution radio imaging and multiwavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: Relativistic AGN jets are formed when the black hole spins and the the accretion disk is strongly magnetized, perhaps on account of gas accreting at high latitude beyond the black hole sphere of influence. AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind. Higher-power jets can emerge from their galactic nuclei in a relativistic, supersonic, and proton-dominated state, and they terminate in strong, hot spot shocks; lower-power jets are degraded to buoyant plumes and bubbles. Jets may accelerate protons to EeV energies, which contribute to the cosmic ray spectrum and may initiate pair cascades that can efficiently radiate synchrotron γ-rays. Jets were far more common when the Universe was a few billion years old and black holes and massive galaxies were growing rapidly. Jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright