Rolf Güsten

Spitzer spectral line mapping of protostellar outflows. i. basic data and outflow energetics

We report the results of spectroscopic mapping observations carried out toward protostellar outflows in the BHR71, L1157, L1448, NGC 2071, and VLA 1623 molecular regions using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations, covering the 5.2-37 μm spectral region, provide detailed maps of the eight lowest pure rotational lines of molecular hydrogen and of the [S I] 25.25 μm and [Fe II] 26.0 μm fine-structure lines. The molecular hydrogen lines, believed to account for a large fraction of the radiative cooling from warm molecular gas that has been heated by a non-dissociative shock, allow the energetics of the outflows to be elucidated. Within the regions mapped toward these five outflow sources, total H2 luminosities ranging from 0.02 to 0.75 L ☉ were inferred for the sum of the eight lowest pure rotational transitions. By contrast, the much weaker [Fe II] 26.0 μm fine-structure transition traces faster, dissociative shocks; here, only a small fraction of the fast shock luminosity emerges as line radiation that can be detected with Spitzer/IRS.

Ultra-wide-band traveling-wave photodetectors for photonic local oscillators

This paper reviews recent advances in the development of high-speed 1.55-/spl mu/m traveling-wave p-i-n photodetectors (TWPD) for photonic millimeter-wave and submillimeter-wave local oscillators. We first discuss the basic physics and performances of high-speed 1.55-/spl mu/m TWPD. Next, we present a frequency-domain optical-heterodyne measurement technique for ultra-wide-band characterization of the TWPD and photonic transmitter modules within the frequency range from almost dc up to more than 1 THz. We further demonstrate ultra-wide-band (0.02-0.7 THz) photonic transmitter modules consisting of a high-speed TWPD coupled to a broad-band bow-tie antenna as well as a narrow-band 0.46-THz photonic transmitter module producing output power levels sufficient to operate a superconductor-insulator-superconductor (SIS) astronomical receiver under optimum conditions. Finally, we will report on ultra-wide-band (0.06-1 THz) photonic transmitter modules consisting of high-speed TWPDs coupled to various rectangular metallic waveguides (WR10, WR8, and WR5).

The first-light APEX submillimeter heterodyne instrument FLASH

Development of a dual-color heterodyne instrument for use with the Atacama Pathfinder EXperiment. Commissioning of the APEX began in mid 2004, and regular science operation has been performed since July 2005. Verification of the telescope required a dualchannel receiver operating at (short) submillimeter wavelengths. It was important for the characterization of the telescope to observe at the highest possible frequency at which routine observations can be performed. For pointing, focus, and tracking verification (simultaneous) operation at lower frequencies was requested. We developed FLASH operating on two channels simultaneously – at orthogonal polarizations – in the 460 GHz and 810 GHz atmospheric windows. The system performs with a wide tuning range (420–500 GHz, 780–880 GHz) and intermediate frequency bandwidths of 2 and 4 GHz, respectively. As backends, we operate two fast-Fourier transform spectrometers (FFTS) with 2 × 1 GHz bandwidth each and a maximum of 16 384 channels. The receiver has been in continuous operation since June 2004. While first used for the telescope commissioning, since the middle of last year the instrument has served as the high-frequency workhorse on APEX. Simultaneous observations of the rotational transitions of warm carbon monoxide (J = 4–3 and J = 7–6) and of the two fine-structure lines of atomic carbon are scientifically attractive. FLASH is a principal investigator instrument, available to the APEX-user community on a collaborative basis with MPIfR. A state-of-the-art dualchannel heterodyne instrument has been developed, which made timely commissioning of the APEX possible. Most of the scientific results presented in this special issue rely on data derived with FLASH.

Fast Fourier transform spectrometer readout for large arrays of microwave kinetic inductance detectors

Microwave kinetic inductance detectors have great potential for large, very sensitive detector arrays for use in, for example, submillimeter imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing ∼1000 s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present here the concept and experimental demonstration of the use of fast Fourier transform spectrometer readout, showing no deterioration of the noise performance compared to the low noise analog mixing while allowing high multiplexing ratios.

4.7-THz Superconducting Hot Electron Bolometer Waveguide Mixer

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).

Traveling-wave photomixer with recessed interdigitated contacts on low-temperature-grown GaAs

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal (MSM) contacts based on low-temperature-grown GaAs. The new recessed MSM geometry led to an improved electric-field distribution inside the photomixer structure and resulted in an up-to-100% increase in the output power of continuously operated devices, compared to conventional MSM devices with standard surface electrodes fabricated on an identical material. The recessed electrode structure also resulted in lower saturation of output power at higher input powers, enabling it to take advantage of higher input powers.

Terahertz photomixing in high energy oxygen- and nitrogen-ion-implanted GaAs

In this letter, the authors elaborate a detailed study of ion-implanted GaAs terahertz photomixers. The authors implanted several GaAs samples with oxygen and nitrogen ions with energies between 2 and 3MeV and doses ranging from 2×1011to3×1013cm−2. The samples were processed by patterning metal-semiconductor-metal structures on the feed point of self-complementary log-periodic spiral broadband antennas. From dc measurements and analysis of frequency roll-off in the 100GHz–1THz range under variable bias conditions, the authors studied systematically the carrier trapping time, terahertz power, and photocurrent dependence on applied voltage and frequency for the different samples.

Gas and Dust in the Inner Few Degrees of the Galaxy

This review discusses the complex morphology and perturbed kinematics, as well as the outstanding physical characteristics of the interstellar medium within a few 100 pc of the galactic center. A total of ~107.9 Mo of dense molecular gas, representing ~10% of the Galaxy’s neutral mass content, has settled in a thin layer [size: 450 × 40 pc], composed of giant molecular cloud complexes. The spatial distribution is highly asymmetric with respect to the center, and motions differ considerably from equilibrium conditions. The dynamical situation is still obscure (explosive event vs. response to distorted gravitational potential), but any disturbance must have occurred quite recently (Tdy ~106 yr). Evidence for large-scale star-forming activity is reviewed, and for a standard IMF, a total star formation rate Ф ~0.5 Mo yr-1 is inferred.

Terahertz ammonia absorption as a probe of infall in high-mass star forming clumps

Cloud contraction and infall are the fundamental processes of star formation. While “blue-skewed” line profiles observed in highmass star forming regions are commonly taken as evidence of infall by an ever increasing number of studies, their interpretation offers many pitfalls. Detecting infall via redshifted absorption in front of continuum sources is a much more direct and reliable method but so far mostly restricted toward absorption in the centimeter toward strong HII regions. Here we present a novel approach by probing absorption of rotational ammonia transitions in front of the strong dust emission of massive star-forming regions. A carefully selected sample of three regions with different stages of evolution is selected to study infall through the evolution of massive star-forming

Terahertz photonic mixers as local oscillators for hot electron bolometer and superconductor-insulator-superconductor astronomical receivers

A pump experiment of two astronomical heterodyne receivers, a superconductor- insulator-superconductor (SIS) receiver at 450GHz and a hot-electron-bolometer (HEB) receiver at 750GHz, is reported. A low-temperature-grown GaAs metal-semiconductor-metal photonic local oscillator (LO) was illuminated by two near infrared semiconductor lasers, generating a beat frequency in the submillimeter range. I-V junction characteristics for different LO pump power levels demonstrate that the power delivered by the photomixer is sufficient to pump a SIS and a HEB mixer. SIS receiver noise temperatures were compared using a conventional solid-state LO and a photonic LO. In both cases, the best receiver noise temperature was identical (Tsys=170K).