We describe the design and construction of GREAT (German REceiver for Astronomy at Terahertz frequencies) operated on the Stratospheric Observatory For Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne instrument for highresolution far-infrared (FIR) spectroscopy. Selected for SOFIA's Early Science demonstration, the instrument has successfully performed three Short and more than a dozen Basic Science flights since first light was recorded on its April 1, 2011 commissioning flight. We report on the in-flight performance and operation of the receiver that -in various flight configurations, with three different detector channels -observed in several science-defined frequency windows between 1.25 and 2.5 THz. The receiver optics was verified to be diffraction-limited as designed, with nominal efficiencies; receiver sensitivities are state-of-the-art, with excellent system stability. The modular design allows for the continuous integration of latest technologies; we briefly discuss additional channels under development and ongoing improvements for Cycle 1 observations. GREAT is a principal investigator instrument, developed by a consortium of four German research institutes, available to the SOFIA users on a collaborative basis.

Optical and transport properties of a series of ultrathin NbN films with different thickness grown on sapphire have been evaluated by means of spectral ellipsometry and dc measurements of superconducting critical parameters. The growth process and thus the nitrogen content have been optimized for each film in the series to achieve the highest superconducting transition temperature, which however increases with the film thickness. Optical and transport measurements agree in slowly increasing disorder while the electron density of states at the Fermi level shows a twofold decrease when the film thickness drops from 14 to 3 nm. Nearinfrared extinction spectra of nanowire gratings from our films are well described by the scattering matrix method that uses optical parameters of nonstructured films and the grating geometry. The technique provides an attractive tool for analyzing various devices for nanophotonics.

We have studied supercurrent-assisted formation of the resistive state in nano-structured Nb and NbN superconducting films after absorption of a single photon. In amorphous narrow NbN strips the probability of the resistive state formation has a pronounced spectral cut-off. The corresponding threshold photon energy decreases with the bias current. Analysis of the experimental data in the framework of the generalized hot-spot model suggests that the quantum yield for near-infrared photons increases faster than the photon energy. Relaxation of the resistive state depends on the photon energy making the phenomenon feasible for the development of energy resolving singlephoton detectors.

We present results from an extensive study of fluctuation phenomena in superconducting nanowires made from sputtered NbN. Nanoscale wires were fabricated in form of a meander and operated at a constant temperature ) 0 ( 4 . 0 c T T ≈ . The superconducting state is driven close to the electronic phase transition by a high bias current near the critical one.Fluctuations of sufficient strength temporarily drive a section of the meander structure into the normal conducting state, which can be registered as a voltage pulse of nanosecond duration.We considered three different models (vortex-antivortex pairs, vortex edge barriers and phase slip centers) to explain the experimental data. Only thermally excited vortices, either via unbinding of vortex-antivortex pairs or vortices overcoming the edge barrier, lead to a satisfactory and consistent description for all measurements. c T . At lower temperatures the probability of thermodynamic fluctuations drops exponentially so that they are experimentally no longer observable far below the transition temperature. However, the freezing-out of thermal fluctuations opens up the possibility to observe quantum fluctuations that prevail in the limit 0 = T , for example quantum phase-slips [16].Although well-defined one-and two-dimensional systems have been studied in great detail, the cross-over region between these limiting cases is less understood. This situation is just beginning to change as the size of superconducting conduction paths of devices such as SQUIDs or quantum detectors is continually decreasing, and therefore a better understanding of superconducting structures that are in between the limiting dimensions is required.I and are sensitive in the visible and near-infrared spectral range (3.1 -0.4 eV). It is generally believed that fluctuations are the major source of dark-count events in these detectors [22][23][24]. Measuring the dark-count rate thus gives us direct information about the fluctuation rates in a part of the superconducting phase diagram that is otherwise not easily accessible. The commonly used approach [23] to measure the DC resistance that is then used to infer the fluctuation rate is not appropriate at large bias currents close to the experimental critical current c,e I , since the Joule heating cannot be eliminated. By contrast, Joule heating may influence the amplitude and duration of individual voltage transients in our time-resolved measurements, but it does not superconducting systems [5]