Optics and Spectroscopy

Infrared spectra of five samples of sunflower seed oil and five samples of cold-pressed olive oil of different brands are recorded in the range of 650–3800 сm^–1. The structural models of eight fatty acids (oleic, linoleic, palmitic, stearic, α-linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic), as well as of triglycerides of the first four of these acids, are constructed using B3LYP/6-31G(d) methods. The vibrational wavenumbers and intensities in the IR spectra are calculated. The IR spectra of olive and sunflower seed oils are simulated using the supermolecular approach. The dependence of the intensity of vibrational bands at ν_exp = 914 and 3009 сm^–1 on the concentration of triglycerides of oleic and linoleic acids in oils, as well as on the saturation degree of fatty acids, are studied. Experimental and empirical dependences are plotted to estimate the relative concentrations of triglycerides of oleic and linoleic acids in mixtures of olive and sunflower seed oils. The applicability of the density functional theory in combination with IR spectroscopy for characterization of vegetable oils is discussed.

A new line list transition for the water molecule is presented. The line lists is created on the basis of VoTe calculations using cutoff values of 25 000 cm^–1 in transition wavenumber and J max = 50. Calculations use the high accuracy, empirical potential energy surface (PES) of Bubukina et al. LTP2011 and the CVR water dipole moment surface (DMS) of Lodi et al. (2011). Vibration–rotation energy levels up to J = 50 are computed using DVR3D; a novel method of labeling is used which allows more thorough labelling of the energy levels with approximate vibrational and rotational quantum numbers than previous water line lists.

Based on the data of low-frequency (terahertz) IR and Raman spectroscopies, it is established that low-energy vibrational excitations in PMMA and its oligomers with a degree of polymerization n = 2, 7, 9, and 50 are due to correlated small-angle torsional–vibrational dynamics, which prepares the relaxational mobility of macromolecules. The minimum size of the chain segment (“torsional–vibrational segment”), the spectrum of which is identical to the spectrum of the high-molecular polymer, is determined.

Optical characteristics of nitrate and nitrite salts and their aqueous solutions are studied to demonstrate the application of direct spectrophotometric methods for determination of different forms of nitrogen salts in water. Examples of application of the method for determination of nitrate ion in samples of natural and drinking water from wells are given.

On the basis of the data of infrared spectroscopy with synchrotron radiation, the secondary structure of proteins of the dentinal and gingival fluids during the development of cariosity in deep dentin tissues is studied. It is shown that the change in the shape of the profile of the amide I band in the region of 1700‒1605 cm^–1 is associated both with a change in the ratio of the integrated absorption intensities of the α‑helix and β-sheet secondary structures and with the position of the β-coil and β-sheet components in the spectrum. It is established that the α-helix/β-sheet ratio for both dentinal and gingival fluids is below the threshold level, at which significant changes in the secondary structure of proteins of biological fluids are observed, unequivocally indicating the development of pathology in hard dental tissues. The features that we discovered in the profile of the amide I band of biological fluids of the oral cavity, together with the spectral markers of the development of cariosity in dentin, are reliable spectroscopic signatures of the pathology and can be detected using the gingival fluid.

The optical properties of an ultrathin water layer at the interface between two dielectrics (ZnSe–SiO₂) are studied by FTIR spectroscopy. Effective refractive indices and absorption coefficients in the region of stretching vibrations of the OH group are determined. It is found that water molecules in the water–quartz glass interface layer have a planar orientation.

The propagation of monochromatic radiation in coiled spun optical fibers is considered. The local parameters of the elliptical polarization eigenmodes, elliptic birefringence, and the ellipticity angle are the sum of the constant and variable components, with the latter being measured almost according to the harmonic law depending on the length of the fiber. The magnitudes of the constant components are significantly higher than the amplitudes of the variable components. The dependence of the local azimuth of the major axis of the ellipse of the polarization eigenmodes increases almost linearly.

We have investigated specific features of reflection of a plane elliptically polarized electromagnetic wave that is incident from an isotropic nonabsorbing medium onto a plane-parallel layer of a hyperbolic metamaterial the optical axis of which is parallel to the interface, while the diagonal values of the dielectric permittivity are smaller than the dielectric permittivity of the isotropic medium. Depending on the angle of incidence and the angle between the plane of incidence and the optical axis of the hyperbolic metamaterial, regimes are possible in which either an ordinary wave or an extraordinary wave, or both waves decay outward from the interface, and, in the latter case, for some values of these angles, the decay may be nonexponential. For these three regimes, the reflection coefficient from a layer of an ITO/Ag nanostructure has been calculated numerically. Upon variation in the polarization of the incident wave, the energy reflection coefficient changes from minimum to maximum values, which depend on the layer thickness. As distinct from conventional anisotropic media, in all cases, the maximum value of the reflection coefficient tends to unity with increasing layer thickness. If the amplitude of an ordinary or extraordinary wave does not decrease outward from the interface boundary, the minimal reflection coefficient periodically turns to zero with increasing layer thickness due to interference. In the case in which both waves decay, with this decay being nonexponential under certain incidence conditions, the minimum reflection coefficient turns to zero at a certain layer thickness, and then tends to unity.

An active area based on InGaAs/InGaAlAs superlattice for laser diodes operating in the spectral range between 1535 and 1565 nm is proposed and realized practically. It is demonstrated that using a superlattice increases the mode gain at the same values of the pump-diode current density relative to a typical active-area design based on an array of InGaAs quantum wells.

The radiation of a two-level resonant medium that is placed into a cavity and is excited by a periodically phase modulated laser pulse is studied theoretically. The radiation is analyzed based on the analytical and numerical solutions of the system of Maxwell–Bloch equations under conditions of a strong coupling between the field and the matter. Under these conditions, this system is similar to a polariton laser. The excitation efficiency of a polariton laser by a phase-modulated radiation pulse is shown to be high compared to a pulse without phase modulation of the carrier frequency. It is shown that the main reason for the efficient excitation of polariton modes in the medium is the appearance of difference combination parametric resonance. The obtained results open up new possibilities for the excitation of radiation of polariton lasers by low-power frequency-modulated pump laser radiation.

A nonstationary regime of squeezed-light generation by a single-atom laser is investigated. Dependences of the quantum-squeezing parameter and radiation intensity on modulation frequency of the atom–field coupling constant are obtained. It is demonstrated that a resonance appears at the modulation frequency equal to twice the average coupling constant, which leads to a more efficient quantum squeezing in a nonstationary harmonic regime than in the case of a stationary regime for the same values of the relaxation and pump parameters.

Samples of thick (about 30 μm) films of undoped zinc oxide on sapphire prepared by magnetron sputtering using an uncooled target have been investigated. The structural and luminescence properties of the initial films and films subjected to additional recrystallization annealing in air have been studied. The time and temperature-dependent characteristics of the samples are considered. It is shown that annealing in air enhances the structural, optical, and luminescence properties of these films.

The problem of plane electromagnetic wave scattering by cylindrical “black holes” with radial profile of dielectric permittivity and magnetic permability in the shell chosen in the form ε(ρ) = μ(ρ) ~ 1/ρ² and with constant permittivity and permeability in the central region is considered. The problem in rigorous formulation is solved both analytically for a basic model of the absorber and numerically using the one-dimensional method of finite elements for a modified model. The results obtained for absorption efficiency, radar cross-section, and field distribution in the cylinder are presented and discussed. It is shown that the absorption efficiency of the cylinder with negative refractive index at realistic values of parameters of the cylinder remains higher than the absorption efficiency of a similar cylinder with positive refractive index. However the indicated advantage is not so great as the advantage compared to the absorption efficiency of the black body predicted in the previous publications for the case of extremely low losses at the surface of the cylinder and extremely high magnitudes of negative permittivity and permeability and losses in the central region of radius approaching zero.

The optical characteristics used in the photovoltaics of nano- and microstructure, evaluated using electromagnetic radiation transfer computation software, mismatch with data obtained at high estimated times. The ability to apply the geometric optics approximations is shown for this purpose. Relevant computations using a 3D ROKS-RG programming tool (the Monte Carlo method) are performed for various experimental results on the optical parameters of nano- and microstructures available in the scientific literature. Given that surface roughness exists as a byproduct of etching during the preparation of structures, the theory is found to satisfactorily agree with experiment. Moreover, most of the radiation is absorbed on account of the surface roughness of substrates, which is expected to induce a nonuniform charge carrier distribution over the structure height and may be among the reasons for the insufficiently high efficiency of solar elements with radial p–n geometry.

Materials with perovskite-type crystal structure attract much attention due to their unique optical properties, such as high quantum yield of photoluminescence and high carrier mobility. However, their further utilization as an active media in photonic and optoelectronic devices is limited by the lack of stability of their optical responses together with the presence of lead cations in the chemical composition. Here, a detailed study of the optical properties and their stability under ambient conditions of perovskite films with the chemical composition (CH₃NH₃)_xA_yI_z, where A = Pb, Bi, Sb, was performed. The changes in optical responses of the samples were monitored for 3 months. It was shown that the stability of perovskite films was improved by surface passivation with a thin layer of polymethylmethacrylate onto their surface. The developed procedures of film formation and following surface passivation are of interest in the design of active optical materials based on perovskite films with the increased optical performance.

In this paper, we report Fano resonance in the dielectric disk and rod cuboidal nanoparticles. In these nanoparticles, forward to backward scattering ratio is enhanced as compared to cubic or other symmetrical nanoparticles. Electric and magnetic dipole resonances come closer by tailoring the dimensions of cuboidal nanoparticles so that both of them spectrally overlap and on-resonance scattering of electric and magnetic dipole moments take place in the visible region. Broad electric dipole and narrow magnetic dipole act as bright and dark modes, respectively. Interaction of these modes generate Fano dip in the backward scattering and enhancement in the forward scattering leading to the improvement in directivity and radiation efficiency which can be used for the design of highly directional nanoantennas in the visible region.

The operation of three types of waveguide refractometric sensors—interferometric Mach–Zehnder, interferometric two-mode, and grating waveguide—has been analyzed. The design features of these sensors with division into functional blocks have been considered. The overall dimensions of sensors based on chalcogenide glasses have been calculated to obtain the same detection limit for the gas analyzed medium.

Transition regions at the interfaces of single- and dual-mode regimes have been investigated for some configurations of planar W optical fibers based on numerical solution of the dispersion equation. It is shown that the sizes of the transition regions under consideration at the cutoff frequencies of the fundamental, first odd, and second even modes depend strongly on the geometric and optical parameters of W optical fibers. These circumstances should be taken into account when choosing parameters of W optical fibers in order to ensure in them a stable single-mode or multimode regime, necessary field concentration, and efficient filtering of leaky modes.

A comparative spectral and kinetic study of the reactions of thiol-substituted molecules of diarylethene and spirooxazine with graphene nanoparticles and graphene oxide nanoplates has been performed. Differences in the photochromic transformations of compounds that are in contact with the surface of carbon nanoparticles in relation to the structure of the compounds and the nature of the nanoparticles are revealed, which indicates the chemical and physical interaction of the molecules of photochromic compounds with the surface groups of carbon nanostructures.

The optical properties of an ensemble of metal nanoparticles coated with an oxide layer have been investigated within framework of the classical theory. The influence of the electron-scattering mechanisms on the polarizability of nanoparticles is analyzed. The limiting case of a thin oxide layer is considered, and analytical expressions for the real and imaginary parts of the polarizability are derived. The evolution of the frequency dependences of the polarizability and extinction coefficient upon variation in the particle size and oxide-layer thickness is investigated. It is shown that the consideration of the size dependence of the surface component of the relaxation time changes the character of the size dependence of the frequency of surface plasmons of two-layer nanoparticles.

The dynamics of the consumption of oxygen during photodynamic processes and the subsequent restoration of its concentration in malignant tumors and healthy tissues of mice were studied in vitro by the kinetics of long-term luminescence of xanthene dyes. To assess changes in the oxygen concentration in tissues, delayed fluorescence can be used due to singlet–triplet annihilation of singlet oxygen and a sensitizer in the triplet state. Upon periodic pulse excitation of sensitizers, reversible quenching of delayed fluorescence was detected in tumors, which is associated with a decrease in the amount of oxygen in the tissues during photodynamic processes. A method for visualizing the restoration of the initial oxygen concentration in tissues is proposed.