Vol 126, No 3 (2019)

The decaying plasma that is produced by the dielectric barrier discharge (DBD) in helium across a cylindrical tube at a pressure of helium 1–40 Тorr has been spectroscopically investigated. The radical difference between the nature of the barrier discharge afterglow and longitudinal pulse discharge afterglow at the same plasma electron density has been demonstrated. It has been shown that this effect is due to the abnormally low density of metastable He(2¹S₀, ³S₁) atoms in the plasma that is produced by DBD. In such an experiment, it is possible to observe a purely recombination afterglow that is free of processes involving metastable atoms, which is fundamentally important for solving the problem of helium afterglow.

Spectra of surface-enhanced Raman scattering of a copper phthalocyanine molecule adsorbed on a GaP substrate are studied. It is shown that very strong lines forbidden in typical Raman scattering appear in the spectrum. Analysis of the spectra indicates that these lines are caused by the appearance of a strong quadrupole interaction in the system, as well as by strong enhancement of tangential components of electric field strength. As was shown earlier, the last effect is characteristic of surface enhanced Raman scattering by semiconductor and dielectric substrates, where not only normal but also tangential components of the field strength are enhanced on the surface.

The transmission of intrinsic, antimony-doped, and gallium-doped Ge single crystals in the THz spectral range have been experimentally investigated. It is shown that the attenuation coefficient of intrinsic germanium in the range of 160‒220 μm is at a level of ~0.5 cm^‒1, a value comparable with that for silicon. The free-carrier absorption cross sections of silicon and germanium are significantly different, which may be caused by the difference in the mechanisms of carrier–phonon interaction in these materials.

The results of the performance tests of a three-layer foil separator for a high-current electron gun of an ELA excimer laser are presented. The separator consisted of two layers of thin titanium foil with a polyimide film between them. Such a composition withstood 5630 electron beam pulses with duration of 80 ns and an energy density per pulse of about 9 J/cm². In this case, the polyimide film (Dupont Kapton polyimide film) with a thickness of 38 μm was irradiated with a dose of 1 GGy. The initial and induced absorption of this film was measured in the range from 20 to 0.4 μm. The mechanical tensile strength of the irradiated film decreased by 17 times.

Multivariate methods are applied to the calibration of temperature in the range from 299 to 423 K for the green fluorescence spectra of erbium in lead–fluoride glass doped with 0.5 mol % of erbium and 10 mol % of ytterbium. It is shown that the regression to latent structures using the combination of moving spectral windows is characterized, among the considered methods, by the lowest value (0.2 K) of the root-mean-squared error of prediction of temperature over the test set. Artificial neural networks using two principal components as input variables, the broadband regression to latent structures, the artificial neural network using all the spectral data samples as input variables, and regression to the principal components are inferior in accuracy of the temperature calibration.

A microscope for studying internal inhomogeneities of the refractive index of optical dielectric microcavities by optical tomography is developed. The influence of these inhomogeneities on the Q factor of optical dielectric microcavities formed by thermal treatment is experimentally studied.

The boundary problem of reflection of electromagnetic waves by a hyperbolic metamaterial layer under conditions of total reflection from the boundary with this layer in the absence of birefringence has been solved. In this case, so-called “special inhomogeneous electromagnetic waves” may propagate in the layer. Analytical expressions are obtained for the vector amplitudes of the waves reflected by the layer and transmitted through it and for the special inhomogeneous electromagnetic waves propagating in this layer. These expressions allows calculating the energy characteristics of reflected and transmitted light for an optically uniaxial metamaterial.

The mechanism of excitation of the impurity luminescence in crystalline materials under intense femtosecond laser irradiation has been investigated. It is established that a high concentration of band electrons and holes, which are successively captured by impurity ions, is formed during the femtosecond three-photon ionization of the intrinsic crystalline material. The efficiency of electron–hole excitation of the impurity composition in crystals (as for the electron beam irradiation) is determined by the degree of difference between the electron systems of the s, p, and d subgroups of the outer shell of cations of the intrinsic material and activator.

The propagation of two-dimensional extremely short optical pulses in graphene with metal nanoparticles has been investigated. The effects that are observed when changing the parameters of the effective Hamiltonian, which takes into account the exchange interaction, have been studied. It has been revealed that the introduction of metallic adatoms into graphene leads to an increase in the amplitude of the pulse. This effect can be enhanced by choosing a particular metal particle .

The excitation and focusing of a surface plasmonic wave on the free surface of a metal film in the Kretschmann scheme have been considered based on the theory of reflection of a plane electromagnetic wave from a flat-layered structure. A method of exciting a radially convergent surface plasmonic wave is proposed. The electric-field distribution at the focus is quantitatively investigated, and the conditions for its maximization are determined. The applications of the results obtained are discussed.

The dispersion equations, the group velocity, the slowdown, and the conditions for forward and backward slow and fast plasmons propagating along thin metal films, as well as along graphene sheets, including those located on a dielectric layer and in a dielectric medium, are investigated. Nondissipative and dissipative surface plasmons are considered. Backward nondissipative symmetric plasmons do not occur in a dissipative silver film. It is shown that the anomalous negative dispersion of a dissipative plasmon does not necessarily mean that this plasmon is backward. For a backward dissipative plasmon, the input impedance of the structure from the side of vacuum should be capacitive, while the plasmon itself should be incoming. Symmetric and nonsymmetric structures with these properties are considered.

The results of experimental observation of stimulated low-frequency Raman scattering (SLFRS) in a single-crystal diamond with a buried graphitized layer created as a result of nonlinear interaction of high-power nanosecond laser pulse with eigen oscillations of nanosized layers of diamond-graphite structures are presented. It is demonstrated that the frequency shift of the SLFRS is on the order of 10 GHz and is inversely proportional to geometrical dimensions of the nanolayers composing the structure.

Problems related to using nanoparticles for absorption of solar radiation and photothermal nanotechnologies are now being actively studied. The efficiency of using nanoparticles as photothermal agents for solar energy is determined by their spectral optical properties. We performed computer simulation of optical properties of homogeneous metal (nickel, titanium, and molybdenum) nanoparticles and their oxides, along with nanoparticles consisting of a metal core and an oxide shell, with radii in the range from 50 to 100 nm in the spectral interval between 200 and 2500 nm. The influence of nanoparticle radius, the type of metal and its oxide on spectral coefficients of efficiency absorption (K_abs) and scattering (K_sca) of radiation by nanoparticles is investigated. The type of nanoparticles suitable for absorption of solar radiation was chosen based on a comparative analysis of the wavelength dependences of absorption efficiency coefficients K_abs, intensity of solar radiation I_S, and parameter P₁ = K_abs/K_sca. Spherical double–layer nanoparticles consisting of nickel or titanium core and oxide shells with a radius of 75 or 100 nm can be used in the spectral interval from 200 to 2500 nm for efficient absorption of solar radiation. These results are a substantial contribution to the investigation of optical properties of nanoparticles that can be used in systems of thermal energy.