Development of a 4.7-THz front end for the GREAT heterodyne spectrometer on SOFIA

Heterodyne spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and planetary research. It allows for the investigation 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 4.7 THz, which is a main target to be observed with GREAT, the German Receiver for Astronomy at Terahertz Frequencies, on board of SOFIA. We report on the progress toward a 4.7-THz front end for the GREAT heterodyne spectrometer on SOFIA. 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 two-miniband design and has been developed for continuous-wave operation, high output powers, and low electrical pump powers. The mixer is a phonon-cooled NbN hot electron bolometer (HEB). It consists of a 2-μm-wide, 0.2-μm-long, and 5.5-nm-thin NbN stripe on a high-resistivity (> 5 kΩ) silicon substrate located in the center of a planar logarithmic spiral antenna and glued onto the flat side of an extended hemispherical 12-mm-diameter silicon lens. In addition, we will present results obtained with a liquid-cryogen-free front end, which is integrated in a pulse-tube cooler (PTC). It is based on a QCL-LO, which is mounted on the first cold stage of the PTC, and a HEB mixer mounted on the second cold stage.