The Martian Moons eXploration (MMX) mission led by JAXA to Mars moons Phobos and Deimos involves a small rover developed by DLR/CNES that will be operating on Phobos’ surface. Aboard it is the Raman Spectrometer for MMX (RAX), whose main scientific objectives address Phobos surface mineralogy, its heterogeneity and relation to the Mars mineralogy. Raman spectrometers require strong suppression of straylight, since this technique operates with few nano-Watt signals that should have significant contrast to all other sources of light inside the instrument. The mission requirements involving RAX call for a compact and sophisticated optical design, precluding space for straylight suppressive elements. To optimize straylight suppression in RAX, Raman scattering, Photoluminescence and reflection were characterized for candidate coatings representing different absorbing materials and fabrication technologies over spectral ranges between 530 nm and 680 nm. This was complimented by mechanical testing to aid selection of the coatings for parts inside the RAX flight model.
Heterodyne spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and
planetary research. One example is the OI fine structure line at 4.7 THz. This is a main target for the observation with
GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be operated on board of SOFIA. We
report on the development of a compact, easy-to-use source, which combines a quantum-cascade laser (QCL) with a compact,
low-input-power Stirling cooler. This work is part of the local-oscillator development for GREAT/SOFIA. The QCL, which is
based on a two-miniband design, has been developed for high output power and low electrical pump power. Efficient carrier
injection is achieved by resonant longitudinal optical phonon scattering. The amount of generated heat complies with the
cooling capacity of the Stirling cooler. The whole system weighs less than 15 kg including cooler, power supplies etc. The
output power is above 1 mW. With an appropriate optical beam shaping, the emission profile of the laser becomes a
fundamental Gaussian one. Sub-MHz frequency accuracy can be achieved by locking the emission of the QCL to a molecular
resonance.
Silicon-based semiconductors offer optically low-loss and high-thermal-conducting lattice for the broad-band terahertz
active media that can be used in the range of 5-7 THz. We report on realization of the terahertz-range stimulated
emission from monocrystalline natural and isotopically enriched silicon crystals doped by group-V donor centers due to
nonlinear frequency conversion. Lasing in the frequency bands of 1.2 - 1.8 THz; 2.5 - 3.4 THz has been achieved from
silicon crystals doped by phosphorus and in the frequency band of 4.6 - 6.4 THz from different donors under optical
pumping by radiation of mid-infrared free electron laser at cryogenic temperatures. Analysis of the data shows that the
emission in high-frequency band corresponds to electronic Stokes-shifted Raman-type lasing. The low-frequency bands
indicate on high-order nonlinear frequency conversion processes similar to four-wave mixing accompanied by highenergy
intervalley g-phonons and f-phonons of host lattice. These lasers supplement terahertz silicon lasers operating on
transitions between donor states.
The terahertz (THz) portion of the electromagnetic spectrum provides specific spectroscopic information for substance
identification. It has been shown that the spectral features of explosive materials might be used for detection and
identification at stand-off distances. We report on the development of a THz spectrometer for explosive detection and
identification. The system is based on THz quantum cascade lasers working at different frequencies. These are used for
illumination of the object under test. The reflected and backscattered radiation from the object under test is detected with
a sensitive heterodyne receiver. As a first step a single frequency, liquid-cryogen free heterodyne receiver operating at
2.5 THz has been developed. In order to realize maximum sensitivity a phonon-cooled NbN hot electron bolometric
mixer with a quantum cascade laser as local oscillator were chosen. The concept of the system and first results will be
presented.
Photoconductivity spectra of unstressed and stressed Ge:Ga detectors were measured. The experiments were performed
with a polarizing step scan Fourier transform spectrometer using the synchrotron facility BESSY, which was operated in
a dedicated mode with a low momentum compaction factor. By this way powerful and coherent synchrotron radiation
below 50 cm-1 was generated. We observed a significant response of unstressed and stressed Ge:Ga detectors below
50 cm-1 and 25 cm-1, respectively. This response can be attributed to transitions between bound excited states or from
bound excited states to the valence band. The results indicate that in germanium detectors a fraction of the recombining
holes is captured into bound excited states.
Quantum cascade lasers (QCLs) operating at 2.5 THz have been used for gas phase spectroscopy and as local oscillator
in a heterodyne receiver. One QCL has a Fabry-Perot resonator while the other has a distributed feedback resonator. The
linewidth and frequency tunability of both QCLs have been investigated by either mixing two modes of the QCL or by
mixing the emission from the QCL with the emission from a 2.5 THz gas laser. The frequency tunability as well as the
linewidth is sufficient for Doppler limited spectroscopy of methanol gas. The QCLs have been used successfully as local
oscillators in a heterodyne receiver. Noise temperature measurements with a hot electron bolometer and a QCL yielded
the same result as with a gas laser as local oscillator.
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