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We report on circuit simulation, modeling, and characterization of field-effect transistor based terahertz (THz) detectors (TeraFETs) with integrated patch antennas for discrete frequencies from 1.3 to 5.7 THz. The devices have been fabricated using a standard 90-nm CMOS technology. Here, we focus in particular on a device showing the highest sensitivity to 4.75-THz radiation and its prospect to be employed for power monitoring of a THz quantum cascade laser used in a heterodyne spectrometer GREAT (German REceiver for Astronomy at Terahertz frequencies). We show that a distributed transmission line based detector model can predict the detector's performance better than a device model provided by the manufacturer. The integrated patch antenna of the TeraFET designed for 4.75 THz has an area of $13\times 13$ $\mathrm{\mu }$ m $^2$ and a distance of 2.2 $\mu$ m to the ground plane. The modeled radiation efficiency at the target frequency is 76% with a maximum directivity of 5.5, resulting in an effective area of 1750 $\mu$ m $^2$ . The detector exhibits an area-normalized minimal noise-equivalent power of 404 pW/ $\sqrt{\mathrm{Hz}}$ and a maximum responsivity of 75 V/W. These values represent the state of the art for electronic detectors operating at room-temperature and in this frequency range.

On August 22, 2014, the satellites GSAT-0201 and GSAT-0202 of the European GNSS Galileo were unintentionally launched into eccentric orbits. Unexpectedly, this has become a fortunate scientific opportunity since the onboard hydrogen masers allow for a sensitive test of the redshift predicted by the theory of general relativity. In the present Letter we describe an analysis of approximately three years of data from these satellites including three different clocks. For one of these we determine the test parameter quantifying a potential violation of the combined effects of the gravitational redshift and the relativistic Doppler shift. The uncertainty of our result is reduced by more than a factor 4 as compared to the values of Gravity Probe A obtained in 1976., Comment: 6 pages, 3 figures, published in Phys. Rev. Lett. 121, 231102

We present here the concept for a new experimental test of the Weak Equivalence Principle (WEP) carried out in the gravity field of the Sun. The WEP, stating the independence of the gravitational acceleration a body is subject to from its mass and composition, is at the basis of general relativity theory and more in general of metric theories of gravitation. It is therefore very important to test it to the precision allowable by current technology. The experiment here proposed aims at measuring the relative acceleration of two test masses in free fall, searching for a possible violation of the WEP, which would show up as a non-zero acceleration signal. The core of the experiment is constituted by a differential accelerometer with zero baseline, whose central elements are two test masses of different materials. This differential accelerometer is placed on a pendulum, in such a way as to make the common center of mass coincident with the center of mass of the pendulum itself. Ensuring a very precise centering, such a system should provide a high degree of attenuation of the local seismic noise, which — together with an integration time of the order of tens of days — would allow a test of the WEP with an accuracy improved by at least an order of magnitude with respect to the best measurements achieved so far. One of the strengths of this proposal is the know-how acquired from a previous study and technology development (GReAT: General Relativity Accuracy Test) that involved a test of the WEP in the gravity field of the Earth, in free fall inside a co-moving capsule released from a stratospheric balloon. The concept of the experiment is introduced, with particular attention to the differential accelerometer and its accommodation on the pendulum. A preliminary estimate of the attainable precision is given, along with a critical analysis of the associated challenges.

— We report on the development of a 4.7-THz local oscillator (LO) for the heterodyne spectrometer GREAT on SOFIA. The design of the LO and its performance in terms of output power, frequency accuracy, frequency stability, and beam profile as well as its implementation in GREAT will be presented. I. INTRODUCTION H ETERODYNE spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and planetary research. It allows for the study of the chemical composition, the evolution, and the dynamical behaviour of astronomical objects such as molecular clouds and star-forming regions. For frequencies beyond 2 THz, SOFIA, the Stratospheric Observatory for Infrared Astronomy, is currently the only platform which allows for heterodyne spectroscopy at these frequencies. One example is the OI fine-structure line at 4.7448 THz, which is a main target to be observed with GREAT, the German Receiver for Astronomy at Terahertz Frequencies, on board of SOFIA. II. RESULTS The local oscillator (LO) combines a quantum-cascade laser (QCL) with a compact, low-input-power Stirling cooler. The 4.7-THz QCL is based on a hybrid design and has been developed for continuous-wave operation, high output powers, and low electrical pump powers [1]. Efficient carrier injection is achieved by resonant longitudinal optical phonon scattering. This design allows for an operating voltage below 6 V. The amount of generated heat complies with the cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power [2]. The QCL has a lateral first-order distributed feedback (DFB) grating, which is optimized for 4.745 THz. This yields single-mode emission over most of the driving current of the laser. Outcoupling is achieved through one of the end facets of the single-plasmon waveguide. The beam of the QCL is shaped with a dedicated lens and a spatial filter into an almost Gaussian profile. The M2 value which can be achieved with this method is approximately 1.2 [3]. The peak output power of the QCL is 0.5 mW. Frequency stabilization is achieved by using a low-noise current source for the QCL and a dedicated temperature stabilization of the heat sink of the QCL. In this way, a frequency stability better than 1.6 MHz (full width at half maximum, FWHM) is achieved. This can be further reduced by locking the emission from the QCL to an absorption line of CH3OH at low pressures (approximately 1 hPa). Using a pyroelectric detector and a proportional-integral-derivative controller, an additional improvement of the frequency stability is achieved [4]. Using this scheme, the FWHM of the laser line is below 0.5 MHz within 30 minutes measurement time. The absolute frequency of the LO has been determined by measuring the absorption spectrum of CH3OH and comparing this with data from the literature. It has been found that the LO emits in a range of ±4 GHz around the OI line. III. SUMMARY We have developed an LO for the heterodyne spectrometer GREAT on board of SOFIA. The LO is based on a QCL with a lateral DFB grating and a single-plasmon waveguide. The LO provides up to 0.5 mW output power in an almost Gaussian beam with an M2 value of 1.2. Its frequency is tunable by current and temperature within approximately ±4 GHz around the OI line. The LO fulfills all requirements and will be operated on SOFIA in 2014 for the first time. REFERENCES [1]. L. Schrottke, M. Wienold, R. Sharma, X. Lu, K. Biermann, R. Hey, A. Tahraoui, H. Richter, H.-W. Hubers, and H. T. Grahn, “Quantum-cascade lasers as local oscillators for heterodyne spectrometers in the spectral range around 4.745 THz,” Semicond. Sci. Technol. vol. 28, 035011 (2013). [2]. H. Richter, M. Greiner-Bar, S. G. Pavlov, A. D. Semenov, M. Wienold, L. Schrottke, M. Giehler, R. Hey, H. T. Grahn, and H.-W. Hubers, “A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler,” Opt. Express vol. 18, pp. 1017710187 (2010). [3]. H. Richter, A. D. Semenov, S. G. Pavlov, L. Mahler, A. Tredicucci, H. E. Beere, D. A. Ritchie, K. S. Il’in, M. Siegel, and H.-W. Hubers, “Terahertz heterodyne receiver with quantum cascade laser and hot electron bolometer mixer in a pulse tube cooler”, Appl. Phys. Lett. vol. 93, 141108 (2010). [4]. H. Richter, S. G. Pavlov, A. D. Semenov, L. Mahler, A. Tredicucci, H. E. Beere, D. A. Ritchie, and H.-W. Hubers, “Submegahertz frequency stabilization of a terahertz quantum cascade laser to a molecular absorption line”, Appl. Phys. Lett. vol. 96, 071112 (2010).

Context. Observing supernova remnants (SNRs) and modelling the shocks they are associated with is the best way to quantify the energy SNRs re-distribute back into the Interstellar Medium (ISM). Aims. We present comparisons of shock models with CO observations in the F knot of the W28 supernova remnant. These comparisons constitute a valuable tool to constrain both the shock characteristics and pre-shock conditions. Methods. New CO observations from the shocked regions with the APEX and SOFIA telescopes are presented and combined. The integrated intensities are compared to the outputs of a grid of models, which were combined from an MHD shock code that calculates the dynamical and chemical structure of these regions, and a radiative transfer module based on the 'large velocity gradient' (LVG) approximation. Results. We base our modelling method on the higher J CO transitions, which unambiguously trace the passage of a shock wave. We provide fits for the blue- and red-lobe components of the observed shocks. We find that only stationary, C-type shock models can reproduce the observed levels of CO emission. Our best models are found for a pre-shock density of 104 cm-3, with the magnetic field strength varying between 45 and 100 {\mu}G, and a higher shock velocity for the so-called blue shock (\sim25 km s-1) than for the red one (\sim20 km s-1). Our models also satisfactorily account for the pure rotational H2 emission that is observed with Spitzer.

The hydroxyl radical (OH) is found in various environments within the interstellar medium (ISM) of the Milky Way and external galaxies, mostly either in diffuse interstellar clouds or in the warm, dense environments of newly formed low-mass and high-mass stars, i.e, in the dense shells of compact and ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar OH involved the molecule's radio wavelength hyperfine structure (hfs) transitions. These lines are generally not in LTE and either masing or over-cooling complicates their interpretation. In the past, observations of transitions between different rotational levels of OH, which are at far-infrared wavelengths, have suffered from limited spectral and angular resolution. Since these lines have critical densities many orders of magnitude higher than the radio wavelength ground state hfs lines and are emitted from levels with more than 100 K above the ground state, when observed in emission, they probe very dense and warm material. We probe the warm and dense molecular material surrounding the UCHIIR/OH maser sources W3(OH), G10.62-0.39 and NGC 7538 IRS1 by studying the $^2\Pi_{{1/2}}, J = {3/2} - {1/2}$ rotational transition of OH in emission and, toward the last source also the molecule's $^2\Pi_{3/2}, J = 5/2 - 3/2$ ground-state transition in absorption. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe these OH lines, which are near 1.84 THz ($163 \mu$m) and 2.51 THz ($119.3 \mu$m). We clearly detect the OH lines, some of which are blended with each other. Employing non-LTE radiative transfer calculations we predict line intensities using models of a low OH abundance envelope versus a compact, high-abundance source corresponding to the origin of the radio OH lines., Comment: Accepted for publication in A&A (SOFIA/GREAT special issue)

Methods: We used the dual-band receiver GREAT on board the SOFIA airborne telescope to perform observations of the [C II] 158 {\mu}m fine-structure line at the postitions of two giant molecular clouds (GMC) in the center of IC 342 (GMCs C and E) and compared the spectra with corresponding ground-based data for low- and mid-J CO and [C I]. We performed model calculations assuming a clumpy photo-dissociation region (PDR) environment using the KOSMA-tau PDR model code to derive physical parameters of the local medium. Results: The [C II] 158 {\mu}m emission resembles the spectral signature of ground-based atomic and molecular lines, which indicates a common origin. The emission from GMC E can be decomposed into a cool, molecular component with weak far-ultraviolet (FUV) fields and low, mean densities of 103 cm^-3 and a strongly excited starburst/PDR region with higher densities of 104 cm^-3 and FUV intensities of 250-300 Draine fields. The emission from GMC C is consistent with gas densities of 5000 cm^-3, FUV intensities of a few Draine fields and total gas masses of 20\times10^6 M$_\odot$. Conclusions: The high spectral resolution of the GREAT receiver allowed us to decompose the [C II] emission of the GMC E into a strongly excited gas component resembling a PDR/starburst environment and a quieter, less excited gas component and to analyze the different components within a single beam individually., Comment: 4 pages, 2 figures, accepted by A&A

Aims. We investigate the gas dynamics and the physical properties of photodissociation regions (PDRs) in IC 1396A, which is an illuminated bright-rimmed globule with internal structures created by young stellar objects. Methods. Our mapping observations of the [C ii] emission in IC 1396A with GREAT onboard SOFIA revealed the detailed velocity structure of this region. We combined them with observations of the [C i] 3 P1− 3 P0 and CO(4–3) emissions to study the dynamics of the different tracers and physical properties of the PDRs. Results. The [C ii] emission generally matches the IRAC 8 μm, which traces the polycyclic aromatic hydrocarbon (PAH) emissions. The CO(4–3) emission peaks inside the globule, and the [C i] emission is strong in outer regions, following the 8 μm emission to some degree, but its peak is different from that of [C ii]. The [C ii] emitting gas shows a clear velocity gradient within the globule, which is not significant in the [C i] and CO(4–3) emission. Some clumps that are prominent in [C ii] emission appear to be blown away from the rim of the globule. The observed ratios of [C ii]/[C i ]a nd [Cii]/CO(4–3) are compared to the KOSMA-τ PDR model, which indicates a density of 10 4 –10 5 cm −3 .

With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial distribution and association of the ionized, neutral and molecular gas components in the M17 SW nebula. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain a 5'.7x3'.7 map of the 12CO J=13-12 transition and the [C II] 158 um fine-structure line in M17 SW and compare the spectroscopically resolved maps with corresponding ground-based data for low- and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to compare velocity-resolved [C II] emission maps with molecular tracers. We see a large part of the [C II] emission, both spatially and in velocity, that is completely non-associated with the other tracers of photon-dominated regions (PDR). Only particular narrow channel maps of the velocity-resolved [C II] spectra show a correlation between the different gas components, which is not seen at all in the integrated intensity maps. These show different morphology in all lines but give hardly any information on the origin of the emission. The [C II] 158 um emission extends for more than 2 pc into the M17 SW molecular cloud and its line profile covers a broader velocity range than the 12CO J=13-12 and [C I] emissions, which we interpret as several clumps and layers of ionized carbon gas within the telescope beam. The high-J CO emission emerges from a dense region between the ionized and neutral carbon emissions, indicating the presence of high-density clumps that allow the fast formation of hot CO in the irradiated complex structure of M17 SW. The [C II] observations in the southern PDR cannot be explained with stratified nor clumpy PDR models., Comment: 4 pages, 4 figures, letter accepted for the SOFIA/GREAT A&A 2012 special issue

Although water is an essential and widespread molecule in star-forming regions, its chemical formation pathways are still not very well constrained. Observing the level of deuterium fractionation of OH, a radical involved in the water chemical network, is a promising way to infer its chemical origin. We aim at understanding the formation mechanisms of water by investigating the origin of its deuterium fractionation. This can be achieved by observing the abundance of OD towards the low-mass protostar IRAS16293-2422, where the HDO distribution is already known. Using the GREAT receiver on board SOFIA, we observed the ground-state OD transition at 1391.5 GHz towards the low-mass protostar IRAS16293-2422. We also present the detection of the HDO 111-000 line using the APEX telescope. We compare the OD/HDO abundance ratio inferred from these observations with the predictions of chemical models. The OD line is detected in absorption towards the source continuum. This is the first detection of OD outside the solar system. The SOFIA observation, coupled to the observation of the HDO 111-000 line, provides an estimate of the abundance ratio OD/HDO ~ 17-90 in the gas where the absorption takes place. This value is fairly high compared with model predictions. This may be reconciled if reprocessing in the gas by means of the dissociative recombination of H2DO+ further fractionates OH with respect to water. The present observation demonstrates the capability of the SOFIA/GREAT instrument to detect the ground transition of OD towards star-forming regions in a frequency range that was not accessible before. Dissociative recombination of H2DO+ may play an important role in setting a high OD abundance. Measuring the branching ratios of this reaction in the laboratory will be of great value for chemical models., Comment: 6 pages, 6 figures, 3 tables, accepted for publication in A&A SOFIA/GREAT special issue

Context. The Stratospheric Observatory for Infrared Astronomy (SOFIA) with its 2.5 m telescope provides new science opportunities for spectroscopic observations of planetary atmospheres in the far-infrared wavelength range.Aims. This paper presents first results from the 14 May, 2014 observing campaign of the Martian atmosphere at 4.7 THz using the German REceiver for Astronomy at Terahertz frequencies (GREAT) instrument.Methods. The atomic oxygen 63 μ m transition, OI, was detected in absorption against the Mars continuum, with a high signal-to-noise ratio (~35). A beam-averaged atomic oxygen from a global circulation model was used as input to the radiative transfer simulations of the observed line area and to obtain a new estimate on the column density using a grid-search method.Results. Minimizing differences between the calculated and observed line intensities in the least-square sense yields an atomic oxygen column density of (1.1 ± 0.2) × 1017 cm-2 . This value is about twice as low as predicted by a modern photochemical model of Mars. The radiative transfer simulations indicate that the line forms in the upper atmospheric region over a rather extended altitude region of 70–120 km.Conclusions. For the first time, a far-infrared transition of the atomic oxygen line was detected in the atmosphere of Mars. The absorption depth provides an estimate on the column density, and this measurement provides additional means to constrain the photochemical models in global circulation models and airglow studies. The lack of other means for monitoring the atomic oxygen in the Martian upper atmosphere makes future observations with the SOFIA observatory highly desirable.

Context: We analyze the NGC 2024 HII region and molecular cloud interface using [12CII] and [13CII] observations. Aims: We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the HII region. Methods. Observations of [12CII] and [13CII] emission at 158 {\mu}m with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the PDR. Results: The [12CII] emission closely follows the distribution of the 8 mum continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [13CII] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [12CII] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2 in the gas emitting the [CII] line. A large fraction of this gas has to be at a temperature of several hundred K. The self-absorption is caused by a cooler (T