Subject and Purpose. The study concerns spectral transformations of the broad-band radiation generated in polyurethane matrices doped with the Rhodamine 6G organic dye. The purpose is to identify the effects due to thickness of the active layer and dye concentration therein upon the efficiency of luminescent light conversion, and their possible applicability in luminescent spectral converters and solar concentrators. Methods and Methodology. The matrices were fabricated, proceeding from a two-component polyurethane mixture Crystal Clear 204 with a variety of dye concentrations. The study involved measurements of absorption spectra, parameters of luminescence, and optical density of the specimens. The transformation efficiency was evaluated by analyzing the spectral distribution of radiation intensity at the input and output of the matrix, followed by numerical integration within the frequency range of luminescence. Results. As has been found, thinner matrices (e.g., 1 mm thick) demonstrate efficient re-emission of light under the conditions of an optimal dye concentration (about 0.1 mM). Higher concentrations lead to losses in efficiency due to self-absorption of the radiation. Thicker matrices (about 5 mm thick) exhibited significantly lower efficiency due to their increased optical density and higher losses associated with extinction and reabsorption effects. Conclusion. The polyurethane matrices are promising materials for phototransformation-based applications, particularly in thin-film configurations. Further optimization of their optical properties and employment of dyes characterized by higher Stokes shifts would be required for application in bulk systems.
Subject and Purpose. Restoring the world-largest decameter-wavelength radio telescope UTR-2 after the destruction inflictedby the full-scale Russian invasion requires significant efforts for mobilizing financial and organizational resources. In view of this, it is advisable to simultaneously repair the damaged systems and effectuate modernization of the instrument, which should significantly expand its functionality and increase the efficiency of its use. The present work is aimed at familiarizing the scientific community with basic principles underlying modernization of the UTR-2 radio telescope, and with the related expectations as to a significant increase in the information content of the future radio astronomical research projects performed with the facility. Methods and Methodology. Resorting to the expertise gained through half a century work on developing and operating unique radio astronomical equipment (ranging from antennas to data processing complexes) we are able to analyze possibilities for improving parameters of the radio telescope, as well as techniques of its large-scale modernization. The scientific approaches and technical solutions developed at the Institute of Radio Astronomy of the NAS of Ukraine allow expanding the field of view of the radio telescope, increasing the number of pattern beams usable simultaneously, and expanding the operating range towards both lower and higher frequencies. Results. Basic technical solutions have been suggested for modernizing the UTR-2 radio telescope through implementation of a hybrid phasing system for its antenna array. It has been decided to divide the existing array into 68 subsections, with each involving 30 elemental antennas inside. These are phased in an analog way, while phasing between the subsections is performed digitally, which should permit a more efficient use of the entire antenna field. Accordingly, it should be possible to simultaneously use up to 1000 antenna beams of the radio telescope for mapping the celestial sphere. Also, it is planned to increase the operating frequency range of the radio telescope from 8...3 3 MHz to 5...40 MHz. Conclusion. The modernization of the radio telescope UTR-2 opens up new opportunities for radio astronomical observations, ensuring a significant increase in the informative capacity of the radio astronomical research. The novel hybridized phasing system is beneficial for inclusion of the UTR-2 in the network of European low-frequency radio telescopes.
Subject and Purpose. Many of the currently known calculations of the strong absorption of electromagnetic radiation in conductors whose diameter is several hundred times smaller than the radiation wavelength proceed from the physical effects accompanying interaction of an electromagnetic wave with an infinitely long conductor. However, antenna theory shows that resonant phenomena in finite-seized conductors can significantly enhance or suppress the interaction of conductors with electromagnetic radiation. This paper examines the interaction of a thin conductor of finite length with a plane, linearly polarized electromagnetic wave, focusing in particular on the amount of energy absorption and the effects due to the length of the conductor. Methods and Methodology. The solution to the integral equation for the current through a thin conductor has been sought for in an asymptotic averaging technique. Using the Joule-Lenz law and calculating the conductor's resistance, the power absorbed by the conductor is calculated. This allows determining the absorption efficiency factor for the radiation incident on a conductor of finite length. Results. The amount of the electromagnetic radiation energy absorbed within a thin conductor has been studied in dependence on the conductor’s finite length. As has been found, when the length of the conductor is close to half the radiation wavelength, else to a multiple of that value, the absorption increases by tens of times compared to the value demonstrated by an infinitely long conductor. Conclusions. The results permit clarifying the previously existing understanding of the interaction of microwave radiation with thin conducting structures and can find application in the development of compact absorbing loads, field shielding elements and structures for microwave energy transmission.
Subject and Purpose. The scattering of an E-polarized electromagnetic plane wave by pre-fractal conducting impedance strip gratings is addressed. To order strip systems mathematically, the simplest class of self-similar perfect sets with variable fractional dimension is introduced. The considered asymptotic model of sparse-filling strip gratings considerably simplifies the scattering problem and enables effective investigation of basic characteristics of E-polarized wave scattering by conducting strip gratings, including radiation patterns. Methods and Methodology. Mathematical models of the E-polarized plane wave scattering from conducting strip gratings are derived using classical approaches (the integral equation technique and Rayleigh’s method) and their modifications. The basic full-wave mathematical model of E-polarized electromagnetic plane-wave scattering by conducting strip gratings reduces to a first-kind integral equation system that is solved numerically. The scattered far field is explored and yields essential integral characteristics of strip-grating scattering. Results. Mathematical models have been developed to study the scattering of an E-polarized electromagnetic plane wave from pre-fractal sparse strip gratings. These asymptotic models have proven effective for both determining output variables and examining key scattering characteristics. Important integral characteristics, such as radiation patterns, have been obtained and permit examining the fractal attributes of prefractal conducting impedance strip gratings. Conclusions. The presented mathematical models of the E-polarized electromagnetic plane wave scattering from pre-fractal strip gratings are efficient for determining output variables and examining fundamental scattering characteristics, such as radiation patterns, thereby laying the foundation for extensive investigation into the fractal attributes of key scattering characteristics of pre-fractal impedance strip gratings interrogated by a plane E-polarized wave.
Subject and Purpose. Considered is the terahertz-range electrodynamics of a one-dimensional magnetophotonic crystal containing graphene inclusions. The crystal’s periodic structure involves a gyrotropic magnetoactive layer, a graphene sheet lying on a dielectric substrate, and a dielectric spacer. The work is aimed at analyzing the nonreciprocal propagation of electromagnetic waves through the crystal, investigating the effects, due to the graphene chemical potential, upon electrodynamic characteristics of the structure, and studying the electromagnetic field’s localization inside defected structural layers. Methods and Methodology. The analysis is performed within the theory of electromagnetic wave propagation through layered periodic media. The gyrotropic material is described in terms of a dielectric permittivity tensor with nonzero off-diagonal elements. An operator method, based on successive application of transmission and reflection coefficients at the interfaces between the layers, is used for the analysis. Both finite-seized structures with a defected layer and semi-infinite periodic systems are considered. Numerical analysis has been conducted of the electrodynamic characteristics as functions of the incidence angle, graphene’s chemical potential, and defect-layer parameters. Results. The presence of off-diagonal elements in the dielectric permittivity tensor provides for non-reciprocity of the transmission coefficients for the waves incident from opposite directions. As has been found, by varying the chemical potential of graphene it is possible to effectively control the positions of passbands and stopbands on the frequency axis. For the structure containing a defected layer, formation of an ultra-narrow resonance has been revealed inside the forbidden band. At the resonant frequency, the effects of electromagnetic field localization and a significant field strength enhancement occur inside the defected layer. The non-reciprocity manifests itself through difference in field amplitudes of the waves that propagate in mutually opposite directions. Conclusions. By combining gyrotropic materials and graphene inclusions it proves possible to implement a dual mechanism for controlling the spectral characteristics of magnetophotonic crystals through application of an external d.c. magnetic field and variation of chemical potential of the graphene. The defected modes are responsible for field localization and formation of ultra-narrow resonances. The latter are seen as promising tools for development of terahertz filters, sensors, and nonreciprocal devices.
