We present a broadband waveguide power divider for the 400-510 GHz band with low insertion loss and good isolation between the output ports. It is used in the LO distribution system of the CHAI LFA heterodyne receiver for the CCAT observatory to feed 32 balanced SIS mixers with each LO source. A silicon membrane chip with a zebra stripe pattern of resistive titanium nitride (TiN) is placed at the tip of the split block waveguide Y-junction to improve the isolation.

For the CCAT Heterodyne Array Instrument (CHAI) we studied the basic components for the local oscillator (LO) distribution in the 4-pixel block, of which 16 units will constitute the 64 pixels in the 455-495 GHz band. A single LO signal is divided by a cascade of on-chip 3 dB power dividers based on superconducting planar transmission lines, implemented in multipixel waveguide mixer blocks. In this article, we present two different types of power dividers, namely, a microstrip Wilkinson and a coplanar waveguide (CPW) 90(degrees) hybrid, which are designed, simulated, and fabricated. Upon integrating them in a two-pixel block with the intended mixers, we observe an unexpected difference in terms of equal power distribution to both mixers. The 90(degrees) phase difference between the outputs of the hybrid, in the presence of standing waves due to an imperfectly terminated isolated port, causes an imbalance between the LO-power transmitted to both mixers. This inequality is frequency-dependent and alternates considerably across the band. The Wilkinson due to its in-phase power division is immune to this effect and therewith demonstrates a significantly more even power transmission to the mixers.

The CCAT-p Heterodyne Array Instrument (CHAI) is being developed at the I. Physikalisches Institut of the Universität zu Köln. Here we present the proposed architecture of the receiver and the advances made in testing its basic components.

We present the details of a coupling structure to embed a 1.9-THz double-metal quantum cascade lasers (QCLs) into a 120-mu m-wide full-height rectangular waveguide. We describe the split-block manufacturing of the waveguide coupler together with a diagonal feed horn and present power and beam shape measurements of two different devices. The two devices differ in coupling factor, which can be chosen in a wide range by the mounting position of the QCL. Our waveguide embedding allows coupling of a large fraction of the laser power into the waveguide and the subsequent horn antenna emitting with a large Gaussian mode content, enabling efficient integration into a diffraction limited optics setup. This is illustrated by a self-mixing experiment and by using the embedded QCL as a local oscillator in a heterodyne receiver.

In this paper we present the experimental realization of a Nb tunnel junction connected to a high-gap superconducting NbTiN embedding circuit. We investigate relaxation of nonequilibrium quasiparticles in a small volume Au layer between the Nb tunnel junction and the NbTiN circuit. We find a saturation in the effective heat-transfer coefficient consistent with a simple theoretical model. This saturation is determined by the thickness of the Au layer. Our findings are important for the design of the ideal Au energy relaxation layer for practical SIS heterodyne mixers and we suggest two geometries, one, using a circular Au layer and, two, using a half-circular Au layer. Our work is concluded with an outlook of our future experiments.

We present the first superconducting hot electron bolometer (HEB) waveguide mixer operating at 4.7 THz. The 5.5-nm-thick, 300-nm-long, and 3600-nm-wide NbN HEB microbridge is integrated into a normal metal (Au) planar circuit on a 2 μm thick silicon substrate. This circuit is integrated in a 24 μm × 48 μm × 21 μm waveguide cavity and a 14 μm × 7 μm × 200 μm substrate channel, which is directly machined into a CuTe alloy block. The power spectrum of the HEB mixer, measured with a Fourier transform spectrometer, is in good agreement with the results of 3-D EM circuit simulation. Measured mixer performance shows a state-of-the-art double sideband noise temperature of 1100 K, averaged over the IF bandwidth of 0.2-3.5 GHz. The 3-dB noise roll-off is 3.5 GHz. This mixer is used in the German REceiver for Astronomy at Terahertz frequencies (GREAT) at the airborne Stratospheric Observatory for Far Infrared Astronomy (SOFIA).

We report on our hot electron bolometer mixer development for the focal plane array extension upGREAT of the German Receiver for Astronomy at Terahertz frequencies (GREAT) operated on SOFIA. For (up)GREAT we have pushed the waveguide technology to 4.7 THz and present RF performance results. We describe the RF planar circuit design, the micro fabrication employing NbN microbridges on 2 μm thin Si membrane substrates and the machining technology used for the waveguides. One of the 4.7 THz mixers was used in the high frequency channel on GREAT in May 2014 and performed as expected from the laboratory characterization.

We present design, performance measurements and simulations of Nb-Al-AlO x -Nb superconductor-insulator-superconductor (SIS) mixers on silicon membranes mounted into waveguide mixerblocks with beam leads. The mixers are designed for use in astronomy for radio frequencies (RF) between 290 and 390 GHz and a 4-12 GHz intermediate frequency (IF) output band. Different mixer designs are discussed. Each design includes a pair of SIS junctions, connected in series or in parallel. Measurements confirm that silicon membrane substrates with beam lead contact technology support excellent cooling of the devices and show noise temperatures that can be limited to values between once and twice the quantum limit in the RF bandwidth. Measurement results are compared to simulations based on the Quantum Theory of Mixing (QTM). For the parallel junctions we use an adaption of the QTM to calculate the single-junction DC IV-characteristics separately. The resulting simulated DC IV-characteristics are in excellent agreement compared to 3D electromagnetic (EM) simulations and measurements.

We report on our waveguide hot electron bolometer (HEB) mixer development for the heterodyne focal plane array receiver upGREAT, which is the multi-pixel extension to the German Receiver for Astronomy at Terahertz frequencies (GREAT) currently in operation on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We will present results for our new generation of HEB mixers for operation at frequencies up to 4.7 THz reviewing the RF circuit design, device fabrication and waveguide technology.

This article presents a heterodyne experiment which uses a 380-520 GHz planar circuit balanced Nb-Al2O3-Nb superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4-8 GHz instantaneous intermediate frequency (IF) bandwidth to quantitatively determine local oscillator (LO) noise. A balanced mixer is a unique tool to separate noise at the mixer's LO port from other noise sources. This is not possible in single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further helps to simplify the measurements. The double-sideband receiver sensitivity of the balanced mixer is 2-4 times the quantum noise limit h nu = k(B) over the measured frequencies with a maximum LO noise rejection of 15 dB. This work presents independent measurements with three different LO sources that produce the reference frequency but also an amount of near-carrier noise power which is quantified in the experiment as a function of the LO and IF frequency in terms of an equivalent noise temperature T-LO. Two types of LO sources are used: a synthesizer driven amplifier/multiplier chain and a Gunn oscillator driven multiplier chain. With the first type of LO we find different near-carrier noise contributions using two different power pre-amplifiers of the LO system. For one of the two power pre-amplifiers we measure T-LO = 30 +/- 4K at the LO frequency 380GHz and T-LO = 38 +/- 10K at 420GHz. At the frequency band center 465 GHz of the Gunn driven LO we measure a comparable value of T-LO 32 +/- 6K. For the second power pre-amplifier, a significant higher T-LO value of the synthesizer driven LO is found which is up to six times larger compared with the best values found for the Gunn driven LO. In a second experiment, we use only one of two SIS mixers of the balanced mixer chip in order to verify the influence of near-carrier LO noise power on a single-ended heterodyne mixer measurement. We find an IF frequency dependence of near-carrier LO noise power. The frequency-resolved IF noise temperature slope is flat or slightly negative for the single-ended mixer. This is in contrast to the IF slope of the balanced mixer itself which is positive due to the expected IF roll-off of the mixer. This indicates a higher noise level closer to the LO's carrier frequency. Our findings imply that near-carrier LO noise has the largest impact on the sensitivity of a receiver system which uses mixers with a low IF band, for example, superconducting hot-electron bolometer HEB mixers. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764324]

Supplementing the publications based on the first-light observations with the German Receiver for Astronomy at Terahertz frequencies (GREAT) on SOFIA, we present background information on the underlying heterodyne detector technology. We describe the superconducting hot electron bolometer (HEB) detectors that are used as frequency mixers in the L1 (1400 GHz), L2 (1900 GHz), and M (2500 GHz) channels of GREAT. Measured performance of the detectors is presented and background information on their operation in GREAT is given. Our mixer units are waveguide-based and couple to free-space radiation via a feedhorn antenna. The HEB mixers are designed, fabricated, characterized, and flight-qualified in-house. We are able to use the full intermediate frequency bandwidth of the mixers using silicon-germanium multi-octave cryogenic low-noise amplifiers with very low input return loss. Superconducting HEB mixers have proven to be practical and sensitive detectors for high-resolution THz frequency spectroscopy on SOFIA. We show that our niobium-titanium-nitride (NbTiN) material HEBs on silicon nitride (SiN) membrane substrates have an intermediate frequency (IF) noise roll-off frequency above 2.8 GHz, which does not limit the current receiver IF bandwidth. Our mixer technology development efforts culminate in the first successful operation of a waveguide-based HEB mixer at 2.5 THz and deployment for radioastronomy. A significant contribution to the success of GREAT is made by technological development, thorough characterization and performance optimization of the mixer and its IF interface for receiver operation on SOFIA. In particular, the development of an optimized mixer IF interface contributes to the low passband ripple and excellent stability, which GREAT demonstrated during its initial successful astronomical observation runs.

We report on NbTiN hot electron bolometer (HEB) mixer design and fabrication for the 1.4, 1.9 and 2.5 THz frequency bands. The mixers under discussion are our contribution to the multi-band single-pixel receivers of the German Receiver for Astronomy at Terahertz Frequencies (GREAT), which is a first light instrument for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), and the focal plane array receiver on the balloon-borne Stratospheric Terahertz Observatory (STO). We measure device noise vs. intermediate frequency (IF) and analyse the receiver system output power stability and IF band ripple with newly developed SiGe low-noise amplifiers from the S. Weinreb group (Caltech). The mixers use waveguide technology with the device coupled to the fundamental waveguide mode via an integrated probe antenna. The device is electrically connected through beam leads, which reliably suspend the 2 μm thin Si3N4 membrane with micrometer mounting precision. Electron beam lithography defines the 400 nm long and 4 nm thick NbTiN microbridges and a novel deep reactive-ion etch is used for shaping of the substrates.

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.

We present a 380-520 GHz balanced superconductor-insulator-superconductor (SIS) mixer on a single silicon substrate. All radio-frequency (RF) circuit components are fabricated on a 9 mu m thick membrane. The intermediate frequency (IF) is separately amplified and combined. The balanced mixer chip, using Nb/Al/Al2O3/Nb SIS junctions, is mounted in a tellurium copper waveguide block at 4.2 K using Au beam lead contacts. We find uncorrected minimum receiver double-sideband noise temperatures of 70 K and a noise suppression of up to 18 dB, measured within a 440-495 GHz RF and a 4-8 GHz IF bandwidth, representing state-of-the-art device performance.

We present observations of the rotational ortho-water ground transition, the two lowest para-water transitions, and the ground transition of ionised ortho-water in the archetypal starburst galaxy M 82, performed with the HIFI instrument on the Herschel Space Observatory. These observations are the first detections of the para-H2O(111–000) (1113 GHz) and ortho-H2O(111–000) (1115 GHz) lines in an extragalactic source. All three water lines show different spectral line profiles, underlining the need for high spectral resolution in interpreting line formation processes. Using the line shape of the para-H2O(111–000) and ortho-H2O(111–000) absorption profile in conjunction with high spatial resolution CO observations, we show that the (ionised) water absorption arises from a ∼2000 pc2 region within the HIFI beam located about ∼50 pc east of the dynamical centre of the galaxy. This region does not coincide with any of the known line emission peaks that have been identified in other molecular tracers, with the exception of HCO. Our data suggest that water and ionised water within this region have high (up to 75%) area-covering factors of the underlying continuum. This indicates that water is not associated with small, dense cores within the ISM of M 82 but arises from a more widespread diffuse gas

We present observations of the rotational ortho-water ground transition, the two lowest para-water transitions, and the ground transition of ionised ortho-water in the archetypal starburst galaxy M82, performed with the HIFI instrument on the Herschel Space Observatory. These observations are the first detections of the para-H2O(1(11)-0(00)) (1113 GHz) and ortho-H2O+(1(11)-0(00)) (1115 GHz) lines in an extragalactic source. All three water lines show different spectral line profiles, underlining the need for high spectral resolution in interpreting line formation processes. Using the line shape of the para-H2O(1(11)-0(00)) and ortho-H2O+(1(11)-0(00)) absorption profile in conjunction with high spatial resolution CO observations, we show that the (ionised) water absorption arises from a similar to 2000 pc(2) region within the HIFI beam located about similar to 50 pc east of the dynamical centre of the galaxy. This region does not coincide with any of the known line emission peaks that have been identified in other molecular tracers, with the exception of HCO. Our data suggest that water and ionised water within this region have high (up to 75%) area-covering factors of the underlying continuum. This indicates that water is not associated with small, dense cores within the ISM of M82 but arises from a more widespread diffuse gas component.

We report on our latest waveguide mixer developments for the German Receiver for Astronomy at THz frequencies (GREAT), the German first light instruments of the Stratospheric Observatory for Infrared Astronomy (SOFIA), and for the Stratospheric THz Observatory (STO).