Two laboratory emission spectrometers have been designed and described previously. Here, we present a follow-up study with special focus on absolute intensity calibration of the new SURFER-spectrometer (SUbmillimeter Receiver For Emission spectroscopy of Rotational transitions), operational between 300 and 400~GHz and mostly coincident with ALMA (Atacama Large Millimeter/submillimeter Array) Band 7. Furthermore, we present a feasibility study to extend the detection frequencies up to 2~THz. First results have been obtained using the SOFIA (Stratospheric Observatory for IR Astronomy) upGREAT laboratory setup at the University of Cologne. Pure rotational spectra of the complex molecule vinyl cyanide have been obtained and are used to give an estimate on the sensitivity to record ro-vibrational transitions of molecules with astrophysical importance at 2~THz.

C. Risacher1,6∗, R. Güsten, J. Stutzki, H.-W. Hübers, R. Aladro, A. Bell, C. Buchbender, D. Büchel, T. Csengeri, C. Duran, U. U. Graf, R. D. Higgins, C. E. Honingh, K. Jacobs, M. Justen, B. Klein , M. Mertens, Y. Okada, A. Parikka, P. Pütz, N. Reyes, H. Richter, O. Ricken, D. Riquelme, N. Rothbart, N. Schneider, R. Simon, M. Wienold, H. Wiesemeyer, M. Ziebart, P. Fusco, S. Rosner and B. Wohler Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany, crisache@mpifr.de I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany Departamento de Ingenieŕıa Eléctrica, Universidad de Chile, Santiago, Chile University of Applied Sciences Bonn-Rhein-Sieg, Sankt Augustin, 53757 Germany IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Heres, France NASA Ames Research Center, Moffett Field, CA 94035, USA SETI Institute, Mountain View, CA 94043, USA

The German REceiver for Astronomy at Terahertz frequencies (GREAT) has been in successful service onboard SOFIA since 2011. GREAT, with its modular approach, is composed of a group of cryostats containing detectors for different frequency bands (until now, between 1.25 and 4.7 THz). At any time, GREAT can carry two cryostats. 4GREAT (4G), a new member of the GREAT constellation, is a 4-color single-pixel module. Two channels, 4G-1 and 4G-2, are implemented using spare flight mixers developed for Herschel's Heterodyne Instrument for the Far-Infrared, namely HIFI band 1 and band 4. The third channel, 4G-3, makes use of the current GREAT L1 detector (1.2–1.5 THz), while 4G-4 covers the frequency range of GREAT Ma,b (2.5-2.7 THz), using a newly developed mixer, (similar in design to the upGREAT HFA mixers). The four channels, co-aligned on sky, are operated in a single closed-cycle cooled cryostat. 4GREAT, scheduled for commissioning in July 2017, will be used simultaneously with the upGREAT-HFA (an array of 7 pixels working at 4.745 THz), allowing multiple frequency observations of astrophysically important species including among many others, the ground-state transitions of many hydrides (HDO, HCl, CH, ammonia NH3, isotopic water H2O, hydroxyl OH), as well as mid-J transitions of carbon monoxide.

The NASA-DLR airborne observatory SOFIA is now performing routine observations, having the German PI instrument (GREAT) as one of its four main instruments. The instrument currently comprises a set of single pixel heterodyne receivers observing in selected frequency windows between 1.25 and 4.7 THz. We are developing new instruments, the upGREAT receivers, which consist of mid-size heterodyne arrays based on superconducting waveguide HEB mixers. The Low Frequency Array (LFA) will cover the 1.9-2.5 THz range using dual polarization 7-pixel HEB arrays. The second receiver, the High Frequency Array (HFA), will observe the [OI] line at ~4.7 THz using a 7-pixel HEB array. We present the status of the LFA receiver, which is in the final stages of integration, testing and characterization. The installation and commissioning aboard SOFIA is planned for May 2015.

The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will survey a section of the Galactic plane in the dominant interstellar cooling line (C II) (1.9 THz) and the important star formation tracer (N II) (1.46 THz) at ~1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic and molecular clouds at 10 kpc. The science flight instrument package hosts four (CII) and four (NII) HEB receivers. There is also one 492 GHz Schottky receiver for observing the (CI) line. In this paper we discuss preparations for the scheduled Antarctic science flight in December 2011.

We present the first successful waveguide HEB mixer in the 2.5 THz frequency band. KOSMA has designed and fabricated the mixer and the RF characterization was performed at 2.523 THz using the FIR gas laser local oscillator and the vacuum hot-cold load with thin Mylar beam splitter at SRON-Groningen. The best uncorrected receiver noise temperature Trec(IF, Vbias) measured is 800 K at 1.25 GHz intermediate frequency (IF). This value is comparable with the results of quasi-optical mixers at this frequency. KOSMA develops waveguide HEB mixers for use in focal plane array receivers for the Stratospheric Terahertz Observatory (STO) and the Stratospheric Observatory for Infrared Astronomy (SOFIA). The mixer consists of a small NbTiN microbridge of 4 nm in thickness, 0.4 μm in length and 1.55 μm in width on a 2 μm SIN membrane substrate that is contacted and mounted to a waveguide mixer block by beam leads. The device shows a noticeable direct-detection response to the loads, which affects evaluation of mixer sensitivity. During each Y factor measurement we therefore adjusted the LO power by means of an attenuator wire-grid in the LO path, keeping the pump level constant as indicated by the HEB bias current. This manual pump level compensation in either direction (for hot or cold load radiation, respectively) was confirmed by identical noise temperatures obtained from successive continuous LO power scans responding to the hot and cold load.

For future ground, airborne and space based single aperture telescopes, multipixel heterodyne imaging arrays are necessary to take full advantage of platform lifetime, and facilitate science requiring wide field spectral line imaging. A first generation of heterodyne arrays with ~10 pixels has already been constructed, i.e. CHAMP, SMART, HERA, DesertStar, PoleStar and HARP. Our group is now constructing SuperCam, a 64 pixel heterodyne array for operation in the 350 GHz atmospheric window. This instrument will realize another order of magnitude increase in array pixel count. Several new techniques were used for SuperCam to maximize integration and modularity. Unlike other SIS array receivers, SuperCam is built around 8 pixel linear mixer modules, rather than independent mixer blocks. These modules house 8 single ended waveguide mixers with SOI substrate SIS devices. Each device is tab bonded to a MMIC based LNA. These modules dissipate only 8 mW of heat, while still maintaining 5 K IF noise temperature and 32 dB gain. Blind mate IF and DC connectors allow each module to be inserted in or removed from the focal plane as a unit. The modules are machined using a state-of-the-art CNC micromilling machine acquired specifically for this project. IF signals are processed by 8 channel IF downconverter boards, which provide gain, baseband downconversion and IF total power monitoring. A real-time FFT spectrometer implemented with high speed ADCs and Xilinx 4 FPGAs produce spectra of the central 250 MHz of each channel at 0.25 km/s spectral resolution. For arrays with an additional order of magnitude increase in pixel count, several additional technical problems must be overcome. Kilopixel arrays will require advances in device fabrication, cryogenics, micromachining, IF processing and spectrometers. In addition, seemingly straightforward receiver systems will require new approaches to realize a kilopixel heterodyne array with manageable complexity and cost. Wire count and 4K heat load must all be reduced significantly compared to SuperCam. IF and DC cabling and interconnects may be replaced with multiconductor microstrip or stripline ribbon. Parallel biasing of LNAs, magnets and even SIS devices is feasible if device uniformity is good enough. IF processing will require further integration, possibly with integrated MMIC chips containing all parts of a IF downconversion chain. Continued advances in FFT spectrometers could allow processing many hundreds of gigahertz of IF bandwidth for a realizable cost. We present results from final SuperCam receiver integration and testing, and concepts for expanding heterodyne arrays to kilopixel scales in the future.

For the 1.4 THz and 1.9 THz channels of the GREAT instrument for SOFIA we have developed waveguide mixers with NbTiN superconducting Hot Electron Bolometer (SHEB) devices on low stress silicon nitride membranes. Comparable mixers will also be used in the balloon-borne Stratospheric Terahertz Observatory (STO). In the current baseline approach for these receivers, the mixer is connected to the low noise IF amplifier by a narrow-band (1.2–1.8 GHz) cryogenic isolator to prevent interactions between the 1–2 GHz amplifier and the mixer. Previous tests have indicated that an isolator is necessary for a stable receiver performance with minimal variations of noise and gain vs. IF frequency. Unfortunately, the isolator has the disadvantage that a significant fraction of the potential IF bandwidth of the mixer and low noise IF amplifier is wasted.

—We present measurements on a balanced mixer for 490 GHz. The system consists of a central -3 dB waveguide branchline coupler, which has been fabricated in split block technique in the KOSMA workshop. It is connected to two SIS mixers and two feed horn antennas. A corrugated horn is used for the signal and a diagonal horn with the same beam parameters is used to feed the coupler with the LO signal. The modular design allows to characterize every component separately. In particular, various waveguide couplers have been investigated with a vector network analyzer at their respective operating frequencies at the university of Bern [1]. The SIS mixers have been tested in standard double sideband mode and show 60-80K DSB noise temperature over the RF band. These results can be compared with simulations and with measurements of the entire balanced mixer in order to gain insight into function and interaction of the different components as well as into critical fabrication tolerances.

We report on the development of SuperCam, a 64 pixel, superheterodyne camera designed for operation in the astrophysically important 870 μm atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The advent of such a system will provide an order of magnitude increase in mapping speed over what is now available and revolutionize how observational astronomy is performed in this important wavelength regime. Unlike the situation with bolometric detectors, heterodyne receiver systems are coherent, retaining information about both the amplitude and phase of the incident photon stream. From this information a high resolution spectrum of the incident light can be obtained without multiplexing. Indeed, each SuperCam pixel will provide 1,024 simultaneous spectral measurements. In terms of raw power, each observation made with SuperCam will provide 65,536 independent measurements of the properties of the object under study. High resolution spectroscopy can, in principle, be performed in this same wavelength regime using incoherent detectors together with frequency dispersive quasi-optical devices such as gratings and Fabry-Perot interferometers. However, the size requirement of quasi-optical devices and/or the need to scan in order to construct a spectrum make them too cumbersome or insensitive for the scientific objectives of the proposed study. SuperCam will be constructed by stacking eight, 1x8 rows of fixed tuned, SIS mixers. The IF output of each mixer will be connected to a low-noise, broadband MMIC amplifier integrated into the mixer block. The instantaneous IF bandwidth of each pixel will be ~2 GHz, with a center frequency of 5 GHz. A spectrum of the central 500 MHz of each IF band will be provided by the array spectrometer. The spectrometer may be either an array of sixty-four, 500 MHz, 1024 lag correlator chips or sixteen, subdivided, 1 GHz wide A/D converters feeding real-time FFT digital signal processors. Each mixer will have its own electromagnet to suppress unwanted Josephson noise. Mixer, magnet, and MMIC bias of each mixer will be optimized under computer control. Local oscillator power is provided by a frequency multiplier whose output is divided between the pixels by using either a phase grating or a matrix of waveguide power dividers. The mixer array will be cooled to 4K by a closed-cycle refrigeration system. SuperCam will reside at the Cassegrain focus of the 10m Heinrich Hertz telescope (HHT) with a dedicated secondary and re-imaging optics. Each pixel will have a 22” diffraction limited beam on the sky. A prototype single row of the array will be tested on the HHT in early 2006, with the first engineering run of the full array in late 2007. The array is designed and constructed so that it may be readily scaled to higher frequencies. 16th International Symposium on Space Terahertz Technology

We report on heterodyne measurements at submillimeter wavelengths using a receiver with a Superconductor-Insulator-Superconductor (SIS) mixer device and a Microwave Monolithic Integrated Circuit (MMIC) cryogenic low noise amplifier (LNA) module integrated into the same block. The mixer characterization presented in this work demonstrates the feasibility of operating a MMIC LNA in close proximity to the SIS device without penalty in mixer performance due to heating effects. Verification of this functionality is crucial for the ongoing development of SuperCam, a 64-pixel focal plane array receiver consisting of eight, 1 × 8 integrated mixer/LNA modules. The test setup included a mixer block modified to accept a MMIC amplifier. Our tests show that the LNA can be operated over a broad range of Vdrain voltages from 0.40–1.40 V, corresponding to dissipative powers of 2.6–29 mW. We observe no significant effect on the measured uncorrected receiver noise temperatures in the 345 GHz band.

Quantum Cascade Lasers (QCL) are the most promising technology for producing compact, high power (> 1 mW), coherent signal sources above 2 THz. Due to their small size (10 x 25 pm) and rectangular cross-section, the output beam from a QCL laser cavity is highly divergent and non Gaussian A single mode Gaussian beam is desirable for efficient coupling to optical systems. We have designed a vvaveguide spatial filter for this purpose. The 2.7 THz spatial filter consists of two diagonal feed horns connected by one wavelength of square waveguide (92 pm on a side). The mode filtering efficiency and far field beam pattern of the structure have been modeled in CST Microwave Studio. We have fabricated the filter in tellurium copper using a Kern MMP micromilling machine. We present measurements of the QCL's throughput and emergent power pattern with and without the filter. Our preliminary findings suggest that spatial filtering significantly improves the QCL beam pattern, and further measurements are being made to more rigorously explore these results.

Simple waveguide mixers have been fabricated up to 1.9 THz in traditional technology. Advanced mixer concepts such as sideband separation or balancing demand more complicated and precise waveguide technology. Mixer development at KOSMA is based on a dual fabrication technology approach. On the one hand we have extended our traditional, CNC lathe based, metal micro machining capabilities. We will present micro milling results of 490 GHz waveguide couplers with ±5 μm precision. On the other hand deep reactive-ion etching (DRIE) of silicon for fabricating waveguides as a new, very powerful technology is explored. This will be demonstrated with successfully fabricated 1.9 THz waveguide structures. Feature reproducibility, given by the photolithography based processes, is ±1 μm, which is sufficiently precise up to frequencies of 10 THz. Features with two different etch depths, such as a waveguide with substrate channel, have been successfully fabricated by using a dual masking scheme.

Phonon-cooled hot electron bolometer (HEB) mixer elements are promising heterodyne detectors for THz frequencies. The current development status of niobium-titanium nitride (NbTiN) HEB mixers for THz receivers, for example for the GREAT receiver on SOFIA, is presented for both waveguide and quasi-optical mixers. Waveguide mixers are based on 2 /spl mu/md thick silicon nitride membranes which are suspended in a substrate channel and quasi-optical mixers are based on an extended hemispherical silicon lens integrated with logarithmic spiral antennas. The measured DSB receiver noise temperature is 1000 K for waveguide and 2000 K for quasi-optical mixers at 800 GHz RF and 1 GHz IF. The HEB devices are fabricated at KOSMA and consist of a 4-5 nm thin NbTiN film.