Impurity-induced modulation of terahertz waves in optically excited GaAs

The effect of the photoinduced absorption of terahertz (THz) radiation in a semi-insulating GaAs crystal is studied by the pulsed THz transmission spectroscopy. We found that a broad-band modulation of THz radiation may be induced by a low-power optical excitation in the spectral range of the impurity absorption band in GaAs. The measured modulation achieves 80\%. The amplitude and frequency characteristics of the resulting THz modulator are critically dependent on the carrier density and relaxation dynamics in the conduction band of GaAs. In semi-insulating GaAs crystals, the carrier density created by the impurity excitation is controlled by the rate of their relaxation to the impurity centers. The relaxation rate and, consequently, the frequency characteristics of the modulator can be optimized by an appropriate choice of the impurities and their concentrations. The modulation parameters can be also controlled by the crystal temperature and by the power and photon energy of the optical excitation. These experiments pave the way to the low-power fast optically-controlled THz modulation, imaging, and beam steering.Comment: 5 pages, 3 figure

Electron-positron annihilation into Dirac magnetic monopole and antimonopole: the string ambiguity and the discrete symmetries

We address the problem of string arbitrariness in the quantum field theory of Dirac magnetic monopoles. Different prescriptions are shown to yield different physical results. The constraints due to the discrete symmetries (C and P) are derived for the process of electron- positron annihilation into the monopole-antimonopole pair. In the case of the annihilation through the one-photon channel, the production of spin 0 monopoles is absolutely forbidden; spin 1/2 monopole and antimonopole should have the same helicities (or, equivalently, the monopole-antimonopole state should be p-wave $^1P_1$).Comment: 14 pages, revtex, 3 figure

ЦВЕТНАЯ МЕТАЛЛУРГИЯ УКРАИНЫ:

The monograph contains information on modern technological schemes and hardware design of processes during enrichment, pyrometallurgical production, including during refining, as well as during isolation from solutions during hydrometallurgical processes. Not only are the fulfillment of requirements for obtaining the appropriate quality and quantity of these metals and the complexity of ore processing, but also the fulfillment of increasing environmental requirements, which emphasizes the importance of involving recycled materials containing precious metals in the modern technological process. The monograph is intended for a wide range of scientific and engineering workers, industrialists – metallurgists, technologists, chemists, concentrator, miners and second specialists whose activities are associated with the processing of mineral ore and precious metal waste, as well as for graduate students and students mining and metallurgical specialties. Indexing:         Монография содержит сведения о современных технологических схемах и аппаратурном оформлении процессов при обогащении, пирометаллургическом производстве, в т.ч. при аффинаже, а также при выделении из растворов при гидрометаллургических процессах. Приведено не только выполнение требований по получению соответствующего качества и количества этих металлов и комплексность переработки руды, но и выполнение все возрастающих экологических требований, что подчеркивает значимость вовлечения в современный технологический процесс вторичного сырья, содержащего благородные металлы. Монография предназначена для широкого круга научных и инженерно-технических работников, производственников - металлургов, технологов, химиков, обогатителей, горняков и других специалистов, деятельность которых связана с переработкой рудного минерального сырья и отходов благородных металлов, а также для аспирантов и студентов горных и металлургических специальностей. Індексація:          &nbsp