The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.

Terahertz quantum cascade lasers have been investigated with respect to their performance as a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator.

Recent experimental and theoretical results of impurity doped germanium and silicon terahertz lasers are reviewed. Three different laser mechanisms exist in p-type germanium. Depending on the operating conditions and the properties of the crystal, laser transitions can occur between light-and heavy-hole subbands, between particular light-hole Landau levels or between impurity states. Electric and magnetic fields are required for laser operation. In n-type silicon lasing originates solely from impurity transitions of group-V donors, which are optically excited. The properties of these lasers depend upon the chemical nature of the impurity centre and the properties of the host material. The principles of operation are discussed in terms of their basic physical concepts. The state-of-the-art performance of these lasers is summarized.

Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10 5 T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H 2 analogues, and for investigation of He 2 , a bound molecule predicted under extreme field conditions.

Doping of silicon with magnesium is investigated by a sandwich diffusion technique. Temperature dependence of the diffusion coefficient in the dislocation-free silicon in the range of 1000-1200 8C is determined. It obeys the Arrhenius behavior over the range of 600-1200 8C, when the data obtained earlier for the lower temperatures are taken into consideration. Preliminary results on Mg diffusion in the dislocated crystals are also presented. The dislocation-free Si:Mg samples are investigated with the Hall-effect measurements and the low-temperature Fourier spectroscopy. A decrease in concentration of Mg interstitials (about 15%) has been observed after 31 months of the samples storage at room temperature, when a commercially available FZ silicon was used as a starting material. The effect of the samples degradation is proposed to be due to a formation of Mg-O complexes. When using a special silicon purified from oxygen and carbon with concentrations below or equal to 1.5 Â 10 14 and 5 Â 10 14 cm À3 , respectively, a decrease in the density of interstitial magnesium has not been noticed during this period. The storage of Si:Mg samples prepared from pure silicon gives rise to the formation of an unknown center, whose ionization energy is between the corresponding values for the interstitial Mg 0 centers and (Mg-O) 0 complexes.

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.

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The deep double donor levels of substitutional chalcogen impurities in silicon have unique optical properties which may enable a spin/photonic quantum technology. The interstitial magnesium impurity (Mgi) in silicon is also a deep double donor but has not yet been studied in the same detail as have the chalcogens. In this study we look at the neutral and singly ionized Mgi absorption spectra in natural silicon and isotopically enriched 28-silicon in more detail. The 1s(A1) to 1s(T2) transitions, which are very strong for the chalcogens and are central to the proposed spin/photonic quantum technology, could not be detected. We observe the presence of another double donor (Mgi * ) that may result from Mgi in a reduced symmetry configuration, most likely due to complexing with another impurity. The neutral species of Mgi * reveal unusual low lying ground state levels detected through temperature dependence studies. We also observe a shallow donor which we identify as a magnesium-boron pair.arXiv:1806.01965v3 [cond-mat.mtrl-sci]