A technique for manufacturing diverging lensed fibers with a concave surface via chemical etching has been modified. The flow patterns arising in the etchant volume near the end face of the optical fiber during the technological process have been characterized. The propagation of radiation in a lensed optical fiber with the resulting geometry has been mathematically modeled. Using scanning electron microscopy, the topology of the etched end-face surface has been examined, and the chemical composition of the core of the silica fiber and adjacent regions has been determined to differentiate areas exhibiting different etching rates. It has been established that the desired end-face topology forms within 15 minutes after the start of the experiment, requiring significantly less time than that needed for fabricating converging lenses.
New optical materials are one of the drivers of the development of integrated and nonlinear photonics. In this paper, we study the prospects for using mixed crystals of X and Z-cut lithium niobate-tantalate (LNT) solid solution for the manufacturing of optical waveguides by the annealed proton exchange (APE) method. We have evaluated the defectiveness of the LNT samples by means of scanning electron microscopy visualization and optical profilometry after wet chemical etching and by X-ray diffraction analysis. The studies showed that the LNT structure has pores with a depth up to 60[Formula: see text]nm and surface density of 2 pcs/mm[Formula: see text]. Also, it was established that LNT crystals possess a higher density of etch pits or dislocations regardless of the crystallographic orientation as compared to conventional lithium niobate. We determined the geometric parameters of single-mode channel APE waveguides by OptiBPM simulation. We experimentally measured the optical losses of channel APE waveguides by the fiber-to-fiber method. The optical losses were 6.2[Formula: see text]dB (Z-cut) and 19.0[Formula: see text]dB (X-cut), which is two times higher than those for APE LN under the same conditions. This effect was related to surface and bulk defects of LNT crystals. The study highlights the relationship between structural and optical characteristics of APE LNT waveguides and contributes to the further study and development of mixed LNT crystals as a material platform for photonics and optics.
Density-driven convective instabilities in porous media control the migration of contaminant plumes in aquifers. We present a comprehensive theoretical and numerical study of miscible Rayleigh-Taylor instability in unbounded porous media, where diffusion acts as a strong stabilizing mechanism that delays instability onset. Using a modified quasi-steady state approximation, we derive analytical expressions for the critical time and the wavelength of maximum growth. The analytical solution is obtained using a piecewise-linear approximation of the diffusive concentration profile. Despite this simplification, the theory accurately predicts the critical time for instability onset. Direct numerical simulations validate the theory in critical time prediction and agreement in growth rates during the linear regime. The results provide quantitative criteria for assessing when density-driven instabilities become significant in contaminated aquifers, informing monitoring network design, remediation strategy selection, and long-term risk assessment for groundwater protection.
Unlike the warm season, winter is the least studied aspect of spider life in temperate regions. The composition and spatiotemporal dynamics of spider groups in the urban protected area of the Botanical Garden with Perm State University during the periods of stable snow cover (in 2021–2025) are studied. The material was collected using pitfall traps with a subnivean exposure at seven plots with different vegetation. Spiders represent the main component in the subnivean complex of epigeic invertebrates (71
3-Aroylpyrrolo[2,1-c][1,4]benzoxazine-1,2,4-triones reacted with α- and β-naphthols to give 3′-aroyl-4′-hydroxy-1′-(2-hydroxyaryl)-2H-spiro[naphthofuran-3(or 1),2′-pyrrole]-2,5′(1′H)-diones. The product structure was confirmed by X-ray analysis. The growth regulatory activity of the synthesized compounds was studied on the microalgae Chlorella vulgaris.
