The properties of optical materials usable in the terahertz (THz) spectral range, which is the boundary between the optical and radio ranges, are examined. The relevance of the research field...

Cleaning and preparation of metal surfaces is one of the main stages of technological process. These operations ensure high resource of final products for many areas of science, technology and industry. Problems due to surface contamination often arise in operation of high-power infrared gas lasers which have 100 percent resonator mirrors made of copper having higher reflectivity in the infrared range. For example, reflectivity of clean copper mirror surface at a wave length l = 10.6 μm can reach R = 0.990.002 [1]. Being operated, the copper mirror degrades under exposure and due to chemical contact with laser medium. As a consequence, the reflectivity can lower down to 0.96, which means the radiation leaks increase from 0.01 to 0.04 (4 times). No-touch surface cleaning methods enable partial recovery of reflectivity, and additional effect of laser cleaning improves mirror quality as compared with the fresh-polished mirrors (R = 0.985). This method can be applied to cleaning of comparatively expensive components made of non-ferrous metals and having polished surface or complex geometry (inner cavities), which can be damaged during cleaning or tolerate no efficient cleaning with any other methods. This work was carried out in Tver State University with the support of Ministry of Education and Science of the Russian Federation in the framework of accomplishment of Federal Targeted program “Investigation and Engineering in High-Priority Lines of Development of Science and Technology in Russia for 2014–2020” agreement 14.577.21.0004.

Laser treatment for samples of copper, its alloys and gold was carried out with a UV pulse of nanosecond duration. After irradiation at subthreshold values of the energy density (E ∼ 0.2 - 0.8 J/cm²) the noticeable changes in the surface layer were revealed. These are traces of thermoplastic deformation resulting from laser exposure. They appear as uneven rise of the irradiated sample surface area up to 1 μm. The effect is cumulative, because the height of the uplifts increases with increasing number of impact pulses. In addition, the characteristic features of high-temperature plastic deformation were observed in the form of crystallographic slip and grain-boundary slippage. At E ∼ 1 J/cm² or more the optical breakdown occurred with the formation of a crater on the metal surface, that precludes the detection of described effects. The mechanical impulse of a laser plasma, when exposed to a metal surface, prevents the thermomechanical expansion of the material, and therefore, similar effects have not been previously observed. On the surface of materials with a significantly larger elastic limit (single crystals of germanium and silicon, a tungsten carbide) this phenomenon was not observed, because the generated thermomechanical stresses were insufficient to create conditions of plastic deformation.

Germanium (Ge) is a system-forming material of IR photonics for the atmospheric transparency window of 8–14 µm. For optics of the 3–5 µm range, more widespread silicon (Si), which has phonon absorption bands in the long-wave region, is predominantly used. A technology for growing Ge single crystals has been developed, allowing the production of precision optical parts up to 500 mm in diameter. Ge is used primarily for the production of transparent optical parts for thermal imaging devices in the 8–14 µm range. In addition, germanium components are widely used in a large number of optical devices where such properties as mechanical strength, good thermal properties, and climatic resistance are required. A very important area of application of germanium is nonlinear optics, primarily acousto-optics. The influence of doping impurities and temperature on the absorption of IR radiation in germanium is considered in detail. The properties of germanium photodetectors are reported, primarily on the effect of photon drag of holes. Optical properties in the THz range are considered. The features of optical properties for all five stable isotopes of germanium are studied. The isotopic shift of absorption bands in the IR region, caused by phonon phenomena, which was discovered by the authors for the first time, is considered.