Optics and Spectroscopy最新文献

In this work, we study orientation properties of nematic liquid crystal systems confined in microfluidic droplets by polarizing optical microscopy. We analyze microfluidic flow conditions that allow for controlling formation of disperse systems in liquid crystal and water flows. The research revealed a different formation behavior of microdroplets depending on the type of disperse systems. In continuous medium of liquid crystals, its molecules tend to align with respect to microchannel walls in tilted position that is close to perpendicular in the presence of added surfactants. Such anchoring was shown to be typical for slow flow below 0.05 mm/s. At higher flow velocities, shear forces realign liquid crystal molecules with respect to the flow axis in a close-to-planar orientation. In small immobilized liquid crystal droplets with a diameter less than half width of the microchannel, a typical orientation of molecules perpendicular to the interfacial boundary is observed at crossed polarizers. At flow velocities up to 10 mm/s, the alignment texture was shown to deform proportionally to the flowrate. In large and long immobile droplets, orientation of the liquid crystal dispersed phase was shown to be similar to that of a single phase system. At average flow velocities above 0.1 mm/s, convection in droplets initiated a transition to irregular dynamics of liquid crystal domains. The behavior of the liquid crystal phase demonstrated in this work allows to perform tailored control of properties of microfluidic liquid crystal droplets and propose them for further potential applications as flow sensors in “lab-on-chip” instruments.

The sorption of vapors from the gas phase of a number of compounds on the surface of a wide-pore adsorbent Chromaton N-AW modified with a mixture of 4-(2-hydroxyethoxy)-4′-formylazobenzene–µ‑oxodimer 2,8,12,18-tetramethyl-3,7,13,17-tetra-n-amylporphine has been studied by the method of inverted gas chromatography. Electrodonor isomers of methylpyridines and dimethylpyridines, isomers of weakly polar xylenes, and a number of enantiomers have been selected as sorbates. The specific retention volumes of sorbates have been calculated, enthalpy and entropy contributions to the sorption process has been shown, and the analytical properties of the resulting stationary phase have been studied. The effect of temperature and the chemical nature of sorbates on the thermodynamic characteristics of sorption has been discussed. The separation factors of both structural and optical isomers and low-boiling hydrocarbons of various structures have been calculated. The influence of both components of the mixture on the sorption process has been considered: the formation of supramolecular ensembles with specific interactions and possible complex formation with iron µ-oxodimer 2,8,12,18-tetramethyl-3,7,13,17-tetra-n-amylporphine. It has been experimentally established that the sorbent based on the proposed mixture of mesogenic 4-(2-hydroxyethoxy)-4′-formylazobenzene and the iron µ-oxodimer 2,8,12,18-tetramethyl-3,7,13,17-tetra-n-amylporphine exhibits high selectivity with respect to low-boiling compounds of various nature, which is unattainable for an unmodified adsorbent. It has been concluded that the use of the presented modifiers in gas chromatography makes it possible to study new stationary phases based on similar mixtures using macrocycles of various structures and nature, promising from the point of view of both structural and chiral gas chromatography.

Temperature dependences of low-frequency IR spectra of glassy polymers are studied in the range of ν = 0.24–4 THz (8–135 cm^–1) at temperatures from 90 to 400 K. Analysis of these dependences revealed absorption corresponding both to individual and correlated torsional vibrations, which initiate the relaxation dynamics. The temperature evolution of the IR spectra in the range of terahertz frequencies exhibits three universal behaviors, namely, the low-temperature regime corresponding to the anharmonicity of torsional vibrational motions, the high-temperature regime caused by the contribution of α relaxation (glass formation), and the intermediate stage related to the activation of the conformational mobility of chains due to the β transition. The results obtained are applicable to polymers with both hydrogen and van der Waals bonds between macromolecules.

A series of compounds with biphenyl as a mesogenic fragment having a structure that models LC dimers and polymers with mesogenic groups of the side chain and a star-shaped structure have been synthesized. An approach to the synthesis of such compounds under soft conditions without a complex procedure of purification with quantitative yield has been developed; it is an implementation of the click chemistry methods for synthesizing LC compounds. It is shown for compounds with biphenyl as a mesogenic fragment that the LC state is observed only when the number of biphenyls in one compound is 3 (i.e., the compounds are similar to polyesters with biphenyl as a mesogenic fragment, which exhibit LC behavior). For compounds with the potentially weak mesogen such as biphenyl, the decisive factor in the formation of the LC state and smectic mesomorphism is the formation of intermolecular hydrogen bonds.

Design of a planar waveguide light-beam combiner that performs summation of an array of external light beams is proposed. Variants of combiner design are analyzed, and a 3D variant with an optical isolator is proposed. Working parameters of combiner elements as a function of refractive indices of the structure layers are calculated. Minimum dimensions of the combiner input/output diffraction elements are estimated.

Optical parameters of a surface layer formed on silica glass in the process of polishing are studied. It is shown that the mechanisms of surface layer formation by mechanical-chemical and ionic polishing have much in common. In both cases, the appearance of a surface layer with a refractive index higher than that of the bulk glass is related to stretching and breaking of Si–O–Si bridging bonds due to hydrolysis during glass polishing. The hydrolysis of Si–O–Si bonds occurs with formation of Si–O and Si–OH groups, which leads to an increase in the refractive index of the surface layer.

A short scientific-methodological review of theoretical studies and approaches created in the field of nonlinear optics of ultimately short pulses in the period from 1970s up to now is presented. Important peculiarities making the nonlinear optics of ultimately short pulses different from the nonlinear optics of quasi-monochromatic signals are underscored.

A tenfold increase in the quantum yield of defect luminescence in the region of 750 nm with a simultaneous increase in its decay time from 4 to 200 ns, due to decoration of surface of quantum dots (QDs) Ag₂S/SiO₂ (5.0 ± 1.5 nm) with Au nanoparticles (NPs) (2.0 ± 0.5 nm). Based on analysis of the luminescence kinetics at temperatures of 77 and 300 K, it was concluded that such a nonspecific manifestation of the plasmon–exciton interaction is caused by the influence of polarization effects from Au NPs on the properties of shallow traps involved in the formation of the defect luminescence kinetics of Ag₂S/SiO₂ QDs.