The current-induced optical activity in a tellurium single crystal has been experimentally investigated in the mid-infrared spectral region. The phenomenological theory of the current-induced optical activity has been considered and the microscopic mechanism of this phenomenon has been described. The dependence of the degree of spin polarization of holes in tellurium on the electric current density has been determined. An approximate analytical expression relating the current-induced optical activity to the degree of spin polarization of holes has been obtained.

This article discusses specific quantum transitions in a few-particle hole gas, localized in a strongly oblate lens-shaped quantum dot. Based on the adiabatic method, the possibility of realizing the generalized Kohn theorem in such a system is shown. The criteria for the implementation of this theorem in a lens-shaped quantum dot, fulfilled in the experiment, is presented. An analytical expression is obtained for the frequencies of resonant absorption of far-infrared radiation by a gas of heavy holes, which depends on the geometric parameters of the quantum dot. The results of experiments on far-infrared absorption in the arrays of p-doped Ge/Si quantum dots grown by molecular beam epitaxy (MBE) with gradually increasing average number of holes in dot are presented. Experimental results show that the Coulomb interaction between the holes does not affect the resonant frequency of the transitions. A good agreement between the theoretical and experimental results is shown.

We report on the observation and experimental studies of impurity breakdown and terahertz luminescence in $n\text{-GaN}$ epilayers under external electric field. The terahertz electroluminescence is observed in a wide range of doping levels (at noncompensated donor density from $4.5\times {10}^{16}$ to $3.4\times {10}^{18}{\text{cm}}^{-3}$ ). Spectra of terahertz luminescence and photoconductivity are studied by means of Fourier transform spectrometry. Distinctive features of the spectra can be assigned to intracenter electron transitions between excited and ground states of silicon and oxygen donors and to hot electron transitions to the donor states.

The authors report on the observation and study of terahertz radiation emission from modulation-doped AlGaN/GaN heterostructure under conditions of heating of a two-dimensional electron gas in the lateral electric field. The experimental results are compared with the theoretical model of blackbody-like emission from hot two-dimensional electrons. Complementary transport measurements and a theoretical simulation were carried out to determine the dependence of effective electron temperature on electric field. The role of nonequilibrium optical phonon accumulation is discussed.

The authors report on the experimental studies of terahertz emission from strained $\mathrm{Ga}\mathrm{As}\mathrm{N}∕\mathrm{Ga}\mathrm{As}$ microstructures doped with beryllium at the conditions of electric breakdown of the shallow impurity. The terahertz emission spectrum demonstrates several distinctive signatures that are related to spontaneous optical transitions between resonant and localized states of the acceptor. The energy spectrum calculated for the Be acceptor in the $\mathrm{Ga}\mathrm{As}\mathrm{N}∕\mathrm{Ga}\mathrm{As}$ heterostructure fits reasonably well with the experimentally observed peaks.