Vol 64, No 10 (2019)

Two techniques for calculation of the problem of diffraction of a plane wave by a dielectric revolution body located in a homogeneous dielectric half-space are suggested. The case, in which the permittivity of the medium inside the body is the coordinate function independent of the angular coordinate in the cylindrical coordinate system, is considered. When the first technique is used (in the case of a homogeneous medium of the body), the discrepancy of the boundary condition on the contour of the axial section of the body is constructed. Two methods are compared using the example of the problem of the plane wave scattering by a homogeneous sphere and circular cylinder of finite dimensions that are located in a half-space. The results of calculation of the field scattering pattern are presented for the case of diffraction by a finite circular cylinder having variable material characteristics.

A two-dimensional problem of the diffraction of a plane wave of the light range on a cluster consisting of two self-similar, silver nanocylinders of different diameter is considered. The frequency characteristics of the scattering diameter, the spatial distribution of the field components, and the scatterplot are calculated by strict numerical methods. The influence of the angle of incidence, the cylinder distance, the coefficient of self-similarity and silver loss on the spectra of the scattering diameter, and the scatterplot in the light wavelength range is studied. It is shown that the maximum value of the electric field component in the quasistatic region at the output of such a system cannot be more than ten times the values of the incident field.

A principal layout of a Josephson terahertz radiation oscillator integrated with a transmitting antenna-lens system and a harmonic mixer (HM) for phase locking of radiation has been proposed and was successfully implemented. Two antenna-feeder systems designed for the central frequencies of 0.3 and 0.6 THz and located on the same chip with the oscillator are numerically simulated and fabricated. A microstrip transmission line between the oscillator and the antenna is used as the feeder. A study was carried out on matching the oscillator power and HM for two designs; the frequency range of pumping HM was 0.25–0.45 and 0.5–0.68 THz for the designs at 0.3 and 0.6 THz, respectively. Good agreement was obtained between experimental results and numerical simulations. A study of the spectral characteristics of the radiation of the oscillator into the external space for the 0.6 THz design using a superconducting integrated spectrometer was carried out. The linewidth of an emission line in free-running regime was of the order of several megahertz; in the phase locking regime down to tens of kilohertz with a signal-to-noise ratio of more than 20 dB was obtained.

In this paper, we consider signal constructions based on OFDM signals and error-correcting codes that are resistant to the effects of spectrum-concentrated interference. An algorithm for receiving these signal constructions using weight windows and a criterion for optimal weight windows are given. A number of Kravchenko weight windows based on atomic functions are shown to belong to practically optimal windows. The probabilistic characteristics for the reception of the considered signal structures formed using the convolutional code with a code rate of 1/2 are specified.

An analytical approach is presented for an optical IM/DD system with STBC coded OFDM RF subcarrier modulation over a free space optical turbulent channel. The analysis is developed to find the expressions for the output photodetector current and carrier to noise power ratio at the output of the STBC–OFDM demodulator. The average bit error rate is evaluated numerically considering log-normal distribution of the atmospheric turbulence. The numerical results show that STBC-OFDM optical link suffers power penalty due to atmospheric turbulence significantly. However, the power penalty can be reduced by increasing the number of OFDM subcarriers. For example, power penalty for (2X1) STBC with N = 16 OFDM subcarriers is 7.5 dB and the power penalty reduces to 0.55 dB when N is increased to 64. Results are evaluated for different link distance, OFDM subcarriers and various channel condition.

The nonstationary Josephson effect in a hybrid heterostructure based on the YBa₂Cu₃O_7–δ cuprate superconductor and cuprate antiferromagnetic barrier interlayer Ca_0.5Sr_0.5CuO₂ and with the upper superconducting electrode Au/Nb is experimentally investigated. At the frequency of external exposure f = 70 GHz, close to the plasma frequency of the Josephson junction, a giant increase in the noise signal was detected, which was explained by the appearance of stochastic oscillations on the heterostructure. Stochastic oscillations, but with less intensity, occur at the frequency f = 45 GHz. At the same time, on the current–voltage characteristics, there are intervals with negative dynamic resistance.

The results of precision measurements of the rate of breakdown development in air at atmospheric pressure in the tip–plane electrode system are presented. It is shown that breakdown begins when the threshold voltage U_thr is reached. The higher its value is, the larger is the radius of the tip electrode; a breakdown development rate of 6.2 mm/ns is practically independent of the radius of the tip electrode and is the same for pulses with peak voltages of 25–30 kV. Visualization of the discharge structure with an aluminum film 4 nm thick showed the presence of a large number of microchannels with sizes ranging from 5 to 20 μm.

A converging sheet electron beam with a cross section of 0.05 × 2 mm² and a current density of 200 A/cm² formed by an electron gun is modeled using the synthesis and analysis methods with partial and complete magnetic shielding of the cathode and a linear compression of 10 and 15. The cross-sectional deformation of the beam in a focusing magnetic field is analyzed based on the three-dimensional computer model of electron-optical systems with a sheet electron beam. The possibility of generation of a low-perveance flux with small deformation in the transit channel of a comb slow-wave structure up to 30 mm in length is demonstrated.

A method to fabricate quartz optical fibers for probes employed in fluorescence diagnostics of tumors is proposed. It is shown that the sensitivity of fluorimeters in the near-IR spectral range can be increased using quartz–quartz optical fiber probes with the core made of the Heraeus glass and the SiO₂–F cladding with an increased content of fluorine in the glass of up 7 wt %. The numerical aperture of such optical fibers amounts to 0.32. The optical fibers are used to develop a prototype of a multifiber high-aperture probe for a laser oncofluorimeter with a relatively high sensitivity in a spectral interval of 900–1100 nm.

Using the modified Kane theory, a model has been built for describing the formation of the effective mass and g factor of bound electrons in quantum dots several nanometers in size. It is shown that these values ​​depend critically on the dot size and are significantly different from the corresponding values ​​for the bulk semiconductor. The effective mass and g factor affect the binding energy of an electron on a quantum dot in a magnetic field and are determined by this binding energy, which ultimately forms the local band structure in the vicinity of the quantum dot. In the covariant InAs/AlSb heterostructure, the characteristics have been calculated for which a quantum dot localizes no more than one electron without localizing holes and therefore can serve as a prototype of a solid-state qubit with the controlled g factor.

Effect of the ZnSe/ZnS/GaAs distributed Bragg reflector (DBR) on the parameters of the spectral response of a photodiode based on rectifying contacts in the metal–semiconductor–metal (MSM) system is studied. The calculated photoreflection spectra of the ZnSe/ZnS/GaAs heterostructure are in good agreement with the experimental data. It is shown that the MSM diode provides two-color response of the photodetector at wavelengths of 420 and 472 nm, a sharp decrease in the photosensitivity in the long-wavelength part of the response signal, high quantum efficiency (53%), and low dark current (5 × 10^–10 A). It is demonstrated that the narrow-band two-color response of the detector can be tuned to the desired wavelength using appropriate selection of the parameters of the heterostructure that forms that Bragg reflector.