Ute Böttger

Low-threshold terahertz Si:As laser

The optical threshold of terahertz intracenter arsenic-doped silicon lasers has been reduced by two orders of magnitude by applying a compressive force to the laser crystal. The Si:As lasers were optically excited with radiation from a CO2 laser operating at a wavelength of 10.59μm. The lowest threshold intensity of 8kW∕cm2 was realized at about 3×108Pa stress applied along the [001] crystal axis. The uniaxial stress breaks the resonant interaction of electrons bound to donors with intervalley f phonons. This changes the upper laser state from 2p± to 2p0, lowers the laser threshold, and increases the output power.

Influence of uniaxial stress on stimulated terahertz emission from phosphor and antimony donors in silicon

The effect of uniaxial stress on terahertz stimulated emission from phosphor and antimony donors in silicon excited by CO2 laser radiation was studied. The laser action originates from 2p0→1s(T2) intracenter transitions. A compressive force applied to the silicon crystal decreases the laser threshold by one order of magnitude. The output power depends nonmonotonically on the stress, while the emission frequency does not change. The results are explained by changes of the donor electronic structure, which do not affect the energy gap between the laser states, and a resonant interaction with acoustic f-TA and g-TA phonons that disappears with increasing stress.

Terahertz Raman laser based on silicon doped with phosphorus

Raman-type stimulated emission at frequencies between 5.0 and 5.2?THz as well as between 6.1 and 6.4?THz has been realized in silicon crystals doped by phosphorus donors. The Raman laser operates at around 5?K under optical excitation by a pulsed, frequency-tunable infrared free electron laser. The frequencies of the observed laser emission are close to the frequencies of the intracenter laser lines which originate from the 2p0 and 2p± phosphorus states. The Stokes shift of 3.16?THz is equal to the difference between the energies of the phosphorus ground state, 1s(A1), and the 1s(E) excited state.

Raman spectroscopic analysis of the calcium oxalate producing extremotolerant lichen Circinaria gyrosa

In the context of astrobiological exposure and simulation experiments in the BIOMEX project, the lichen Circinaria gyrosa was investigated by Raman microspectroscopy. Owing to the symbiotic nature of lichens and their remarkable extremotolerance, C. gyrosa represents a valid model organism in recent and current astrobiological research. Biogenic compounds of C. gyrosa were studied that may serve as biomarkers in Raman assisted remote sensing missions, e.g. ExoMars. The surface as well as different internal layers of C. gyrosa have been characterized and data on the detectability and distribution of β-carotene, chitin and calcium oxalate monohydrate (whewellite) are presented in this study. Raman microspectroscopy was applied on natural samples and thin sections. Although calcium oxalates can also be formed by rare geological processes it may serve as a suitable biomarker for astrobiological investigations. In the model organism C. gyrosa, it forms extracellular crystalline deposits embedded in the intra-medullary space and its function is assumed to balance water uptake and gas exchange during the rare, moist to wet environmental periods that are physiologically favourable. This is a factor that was repeatedly demonstrated to be essential for extremotolerant lichens and other organisms. Depending on the decomposition processes of whewellite under extraterrestrial environmental conditions, it may not only serve as a biomarker of recent life, but also of past and fossilized organisms.

Stimulated terahertz emission due to electronic Raman scattering in silicon

Stimulated Raman emission in the terahertz frequency range (4.8–5.1 THz and 5.9–6.5 THz) has been realized by optical excitation of arsenic donor centers in silicon at low temperatures. The Stokes shift of the observed laser emission is 5.42 THz which is equal to the Raman-active donor electronic transition between the ground 1s(A1) and the excited 1s(E) arsenic states. Optical thresholds of the Raman laser are similar to those observed for other silicon donor lasers. In addition, intracenter donor lasing has been observed when pumping on the dipole-forbidden 1s(A1)→2s transition.

Terahertz lasing from silicon by infrared Raman scattering on bismuth centers

Stimulated emission at terahertz frequencies (4.5–5.8 THz) has been realized by electronic Raman scattering of infrared radiation on bismuth donor centers in silicon at low temperatures. The Stokes shift of the observed laser emission is 40.53 meV which corresponds to the bismuth intracenter transition between the 1s(A1) ground state and the excited 1s(E) state. The laser has a low optical threshold and the largest frequency coverage in comparison with other Raman silicon lasers based on shallow donor centers. Time-resolved pump spectra enable the separation of donor and Raman lasing.

Mineralogical analyses of surface sediments in the Antarctic Dry Valleys: coordinated analyses of Raman spectra, reflectance spectra and elemental abundances

Surface sediments at Lakes Fryxell, Vanda and Brownworth in the Antarctic Dry Valleys (ADV) were investigated as analogues for the cold, dry environment on Mars. Sediments were sampled from regions surrounding the lakes and from the ice cover on top of the lakes. The ADV sediments were studied using Raman spectra of individual grains and reflectance spectra of bulk particulate samples and compared with previous analyses of subsurface and lakebottom sediments. Elemental abundances were coordinated with the spectral data in order to assess trends in sediment alteration. The surface sediments in this study were compared with lakebottom sediments (Bishop JL et al. 2003 Int. J. Astrobiol. 2, 273–287 (doi:10.1017/S1473550403001654)) and samples from soil pits (Englert P et al. 2013 In European Planetary Science Congress, abstract no. 96; Englert P et al. 2014 In 45th Lunar and Planetary Science Conf., abstract no. 1707). Feldspar, quartz and pyroxene are common minerals found in all the sediments. Minor abundances of carbonate, chlorite, actinolite and allophane are also found in the surface sediments, and are similar to minerals found in greater abundance in the lakebottom sediments. Surface sediment formation is dominated by physical processes; a few centimetres below the surface chemical alteration sets in, whereas lakebottom sediments experience biomineralization. Characterizing the mineralogical variations in these samples provides insights into the alteration processes occurring in the ADV and supports understanding alteration in the cold and dry environment on Mars.

Imaging terahertz radar for security applications

Detection of concealed threats is a key issue in public security. In short range applications, passive imagers operating at millimeter wavelengths fulfill this task. However, for larger distances, they will suffer from limited spatial resolution. We will describe the design and performance of 0.8-THz imaging radar that is capable to detect concealed objects at a distance of more than 20 meter. The radar highlights the target with the built-in cw transmitter and analyses the returned signal making use of a heterodyne receiver with a single superconducting hot-electron bolometric mixer. With an integration time of 0.3 sec, the receiver distinguishes a temperature difference of 2 K at the 20 m distance. Both the transmitter and the receiver use the same modified Gregorian telescope consisting from two offset elliptic mirrors. The primary mirror defines limits the lateral resolution of the radar to 2 cm at 20 m distance. At this distance, the field of view of the radar has the diameter 0.5 m. It is sampled with a high-speed conical scanner that allows for a frame time less than 5 sec. The transmitter delivers to the target power with a density less than ten microwatt per squared centimeter, which is harmless for human beings. The radar implements a sensor fusion technique that greatly improves the ability to identify concealed objects.

Influence of an electric field on the operation of terahertz intracenter silicon lasers

We have investigated the influence of a pulsed electric field on the intracenter population inversion between phosphorus donor states in silicon. Terahertz-range electroluminescence resulted from the populated excited donor states. It grows linearly above the impurity breakdown field but saturates at excitation powers exceeding 100 W at 5 μs pulse length. An electric field applied to the optically pumped silicon laser reduces the efficiency of the 2p0→1s(T2) phosphorus transition already at voltages below the impurity breakdown. The appearance of a current through the laser sample results in a fast quenching of the laser emission that indicates a reduction in the inversed population between the laser states.

Optimizing the Operation of Terahertz Silicon Lasers

In this paper, a thorough characterization of terahertz silicon lasers with respect to doping concentration and operation temperature has been carried out. Several factors limiting the laser operation, such as heating of the laser crystal and absorption by photoinduced free carriers, are discussed. The optimal doping concentration has been determined. The influence of the pump geometry on the laser efficiency has been investigated. It was found that an external uniaxial force applied to the laser crystal lowers the pump threshold and increases the output power.