Optics and Spectroscopy最新文献

This paper examines the thermal resistance of the mid-infrared “flip-chip” LEDs based on the p‑InAsSbP/n-InAsSb heterostructure. It was shown that the measurement of thermal resistance via a thermal control unit (forward p–n junction voltage) for LEDs based on narrow-gap semiconductors must be carried out at low temperature when thermal control unit is constant.

Photodetectors based on diode heterostracture with InAsSb_x photosensitive region (x = 0.38) with long-wave cut-off λ_0.1 about 10 μm (296 K) are investigated. The dependences of dark current density and detectivity in the temperature range of 200–425 K have been investigated. It is shown that the experimental samples are characterized by values of dark current density about 500 A/cm² at room temperature, detectivity 1.2 × 10⁹ and 5 × 10⁹ cm Hz^1/2 W^–1 at room temperature and 250 K, respectively, and diffusion mechanism of current flow in the temperature range 200–350 K.

The paper considers collisions involving spin polarized alkali Na and Cs atoms in the ground state. The complex cross sections for spin exchange and the elastic scattering cross sections of the considered atoms are calculated in the energy range from 10^–4 to 10^–2 a.u.

A comparison of the generation efficiency of short pulses with coherent and standard passive (incoherent) mode locking with a saturable absorber is made on the basis of numerical simulations. The advantage of the coherent mode locking in comparison to the standard mode locking in lasers with a saturable absorber is shown. Coherent mode locking in two-section lasers is based on the coherent interaction of the radiation with the absorbing and amplifying media. It allows the generation of ultrashort laser pulses with a duration shorter than the polarisation relaxation time T₂ of the absorbing and amplifying media. In conventional lasers with standard (incoherent) passive mode locking with a saturable absorber, the interaction of the laser pulses with the absorbing and amplifying media is incoherent and the duration of the generated pulses is always limited by the polarization relaxation time T₂ of the absorber and amplifier.

Bismuth tantalum pyrochlores doped with iron and magnesium atoms were obtained thermally stable up to a temperature of 1140°C: Bi₂Mg_xFe_{1 – x}Ta₂O_{9.5 – Δ} (x = 0–1). The effect of Fe and Mg codoping on their crystal structure has been studied. The unit cell parameter of solid solutions increases uniformly from 10.4871 (2) Å (x = 0) to 10.54607 Å (x = 1). According to the XPS and NEXAFS spectroscopy data, it was found that the ions in these solid solutions are in the charge states Bi(+3), Mg(+2), Fe(+3), Ta(+5 – δ). The dopant atoms iron and magnesium are introduced into the positions of the tantalum atom without distorting the symmetry of the crystal lattice.

A circuit design of a fiber endoscope is described that allows monitoring of hard-to-reach cavities and other partially transparent and/or reflective objects into which it is possible to pass a thin fiber bundle.

Spectral properties of composite materials based on matrices of high-silica porous glasses activated by silver ions and lanthanum ions have been investigated. The optical density spectra (270–900 nm) and infrared transmittance spectra (11 000–9000 and 9000–4000 cm^–1) of composite materials of different compositions subjected to heat treatment according to one of three modes (120, 500 and 800°C) have been considered. The synthesized composite materials were investigated by X-ray powder diffraction method and energy-dispersive X-ray spectroscopy. The analysis of optical spectra permitted to identify the formation of molecular clusters, clusters, dimers and silver nanoparticles, as well as absorption bands related to charge transfer O²⁺ → La³⁺ (282, 285, 300 nm) and to lanthanum nanoparticles (282, 285 nm) under different conditions of synthesis of composite materials. It was found that the change in the mode of thermal treatment of composites leads to changes in the IR spectra of composites, and the change in their composition leads to the appearance of additional bands associated with the oxygen atom of the OH-group, which can coordinate with several neighboring lanthanum atoms.

Abstract

The mathematical modeling of operating regimes of parametric THz devices based on graphene metamaterials (graphene‒dielectric periodic structures (GDPSs)) has been carried out by solving a nonlinear 3D diffraction problem for Maxwell’s equations in the projection integral form. Using an algorithm for computing bifurcation points of the nonlinear Maxwell operator, the threshold pump wave amplitudes have been calculated at which an unstable mode of the parametric excitation of electromagnetic waves occurs in GDPS-based THz device.

Using the CST Microwave Studio 2023, models of plasmonic graphene antennas (PGAs) with excitation by using discrete and waveguide ports by incident TEM-waves of S- and P-polarization have been constructed. The characteristics of rectangular and square PGAs with dielectric and metallized substrate: scattering matrix elements, radiation patterns for various graphene parameters and chemical potential values have been calculated.

Computational algorithm for determining the descriptor of autonomous blocks with virtual Floquet channels (MFABs) containing magnetically functionalized carbon nanotubes (MFCNTs) with encapsulated magnetic nanoparticles is developed. The algorithm is based on solving the 3D problem of diffraction for the Maxwell’s equations together with the Landau–Lifshitz equation by applying the Galerkin projection method. Electrodynamic calculation of the complex wavenumbers of the longitudinal and transverse waves propagating in the 3D MFCNT arrays as functions of magnitude of the bias field in the millimeter and terahertz frequency ranges is carried out based on the characteristic equation by means of the developed computational algorithm for calculation of the MFAB conductivity matrix.