Volume 127, Nº 3 (2019)

Features of Penning ionization in cold gaseous media of Rydberg alkali-metal atoms have been investigated. In contrast to the hydrogen atom, the corresponding autoionization widths exhibit a strong (by orders of magnitude) dependence on the orbital quantum numbers of the atoms involved in the long-range dipole–dipole interaction. An important feature of Penning ionization is the nontrivial dependence of its efficiency on the Rydberg particle size. For all types of alkali-metal atoms, the optimal highly nonsymmetric configurations of Rydberg pairs have been found that lead to the explosive (by several orders of magnitude) intensification of the free electron formation by means of the Penning ionization processes. This property makes Penning ionization an important means of forming primary charged particles during the cold Rydberg plasma formation. The numerical data for pairs of potassium atoms are reported that demonstrate a significant effect of the Förster resonance on the values of the Penning ionization rate constants.

The nonlinear Hanle effect is generally considered in the modern literature as a particular case of coherent population trapping (CPT) at degenerate Zeeman sublevels of the ground atomic state. The shape, amplitude, and sign of observed resonances and their dependence on the excitation parameters and observation geometry were investigated theoretically and experimentally in many studies because of the wide range of application of this effect. The sensitivity of Hanle resonances to the experimental conditions increases significantly in cells with antirelaxation coating, because the atomic ensemble retains its coherence after a large number of collisions with the cell walls. In this paper, the main attention is paid to the joint influence of the excitation radiation intensity and spurious magnetic fields on CPT resonances observed in fluorescence. The theoretical description is based on the numerical solution of an algebraic system of equations for density matrix \(\hat {\rho }\) in the formalism of irreducible tensor operators. The parameters relating different polarization moments in each equation are visualized using the modified sparse matrix of the system. It is established by numerical simulation that the shape, width, amplitude, and sign of nonlinear magnetooptical resonances change nonmonotonically with a change in the parameters of the excitation radiation and atomic system. The presented theoretical results are in good agreement with the experimental data obtained under various conditions.

Decaying neon plasma has been investigated by kinetic spectroscopy. The experimental conditions were neon pressure of 0.2–152 Torr and electron density in the initial stage of decay [e] ≤ 5 × 10¹⁰ cm^–3. The plasma was created by a pulsed barrier discharge with electrodes on the outer surface of a cylindrical glass tube. The discharge frequency was 40–160 Hz. It has been shown based on a comparative analysis of the dependences of the intensities of spectral lines on the time and electron temperature in the afterglow that the 2р⁵4р levels are clearly divided into two groups in terms of population mechanisms. The lower levels (from 3р₁₀ to 3р₃ (Paschen notations)) are related to dissociative recombination of \({\text{Ne}}_{{\text{2}}}^{ + }\) molecular ions with electrons, and the lines emitted by them behaved identically to the lines of the transitions 2p⁵3s ← 2p⁵3p and 2p⁵3p ← 2p⁵3d up to pressures of 0.6 Torr. The kinetics of the upper levels (3р₅, 3р₂, 3р₄, and 3р₁) had a more complicated character, and, at low pressures, the populations of all 2p⁵4p levels were related to a collisional-radiative recombination of Ne⁺ ions. Significant differences in the dependences of the relative populations of 3d- and 4p levels on neon pressure are discussed.

The length-dependent low-frequency terahertz absorption spectrum of the essential amino acid chains has been investigated. Since this special type of amino acids cannot be synthesized inside the human body, the feasibility of noninvasively monitoring their deficiency is of significant importance. Here, the considered chains consist of two to six identical essential amino acids forming homogeneous polypeptides. The terahertz vibrational modes are calculated for different chain sizes of these polypeptides. The low-frequency terahertz spectra show unique and specific absorption peaks. Several fascinating patterns for the chain-size-dependent position of the peaks are reported here that are feasible of being applied for their noninvasive detection. For instance, different shared and identical characteristics peaks are observed for the specific polypeptides which are independent of the chain-size. Furthermore, it is shown that the observed trends for the maximum populated peak in some of the essential polypeptides can be considered as a possible footprint for in the terahertz spectrum.

An atomic structure of crystals of the complex [Eu₂(Quin)₄2Н₂O2Dipy]₂ · 2(NO₃) · Н₂O, (Quin, anion of quinaldic acid; Dipy, 2,2'-dipyridyl), which exhibited intense luminescence and triboluminescence, has been investigated via the X-ray diffraction analysis. The structure of crystals of the compound appeared to be of an islelike type and were represented by two separate crystallographically independent complex dimers, outer-sphere [NO₃]⁻¹-groups, and water molecules. Structural aspects of the formation of the luminescent and triboluminescent properties of the compound have been discussed. The role of cleavage planes at crystal destruction has been established.

Luminescent polymer compositions based on polystyrene (PS), polycarbonate (PC), and polymethyl methacrylate (PMMA) doped with dibenzoylmethanate boron difluoride (DBMBF₂), anthracene–acetonate boron difluoride (AntAcBF₂), and their mixture have been investigated. The maximum quantum yield of luminescence has been demonstrated by the composition based on PS. The role of PS in increasing the efficiency of energy transfer from the donor (DBMBF₂) to the acceptor (AntAcBF₂) has been established and appeared to be caused by the formation of exciplexes of DBMBF₂ with phenyl rings of PS.

The incidence of two coherent waves traveling in opposite directions on opposite surfaces of a planar layered-periodic graphene–dielectric structure has been investigated. It has been shown in the long-wave approximation that it is possible to control the intensity of the outgoing waves and absorption of radiation in the structure by changing the phase difference of the waves that are incident on the structure. There are close to optimal conditions for the intensity modulation effect in the terahertz frequency domain and modulation of the absorption capacity of the structure is possible in the near infrared and visible ranges.

Results of studying stripe quantum cascade lasers emitting at room temperature in the spectral range of 4.8 µm are presented. Power characteristics and turn-on dynamics of the lasers upon pulse pumping are studied. The performed investigations demonstrate the presence of a significant heating of the active region during the pump pulse.

The synthesis of nanostructured materials took much attention due to their advanced optical and nonlinear optical properties, which can be used in various areas of communications, optics, laser physics and medicine. During last two decades, special attention was given to the nonlinear optical properties of the nanoparticles (NPs) of variable morphology. Here we review the recent studies of the nonlinear optical properties of silver and gold NPs. We discuss the Z-scans and pump–probe transient absorption studies allowing determining the nonlinear refractive indices, nonlinear absorption coefficients and electron–phonon interaction times. The analysis of the low- and high-order harmonic generation in the laser-produced plasmas containing Ag NPs and Au NPs is also presented.

The results of complex action of X-ray and nanosecond UV laser radiation on alkali-containing silicate glasses are presented. It is shown that X-ray irradiation of glasses forms luminescent color centers in them, which is related to increasing concentration of structural defects in glass network. Laser radiation destroys these defects, which is accompanied by bleaching of glasses and disappearance of luminescence in irradiated regions. The dynamics of glass bleaching under action of laser radiation and the influence of alkali-metal (Li, Na, K) ions on this process are studied. Mechanisms of destruction of color centers by laser radiation are proposed.

We successfully fabricated polycrystalline zinc oxide catalyst-free nanorods by a successive ion layer adsorption and reaction process. We measured their optical transmittance, reflectance and thickness in addition to performing X-ray diffraction and probing film microscopic topology. We extracted different optical constants such as absorption coefficient, band gap, complex refractive index and complex dielectric function and inferred the behavior of the optical conductivity and show the supremacy of the obtained ZnO nanostructure over a bulk material. It turns out that it is possible to obtain ZnO nanorods ultra-thin film with optical quality almost similar to those obtained by a contaminant metal catalyst approach requiring extra cost.

Phase shifts of light in orders of diffraction grating for the interference linear displacement sensor are considered. Using the phase diffraction grating with given geometric parameters of the surface relief allows one to stabilize phase relations in optical signals and in resulting signals outputted from the displacement sensor. Based on the mathematical simulation data, technically feasible parameters of the grating relief are proposed for creating the required phase shifts between diffracted beams leading to quadrature modulation. It helps reaching the required measurement accuracy with nanometric resolution.

The optical properties of a silica–polymer optical fiber with a light-guide core with a diameter of 430 μm and a reflecting shell with a thickness of 70 μm from a thermoplastic copolymer of tetrafluoroethylene with ethylene of Tefzel brand have been experimentally studied. The polymer coating is applied to a silica fiber directly in the fiber draw apparatus by the die method from a thermoplastic melt. The optical losses of the drawn fiber and the numerical aperture and its dependence on the fiber length are measured. It is established that a light propagating through the fiber is noticeably scattered by the reflecting shell, which is associated with the crystallinity of the polymer. The distribution of the scattered radiation intensity along the fiber axis and the indicatrix of radiation scattering by the shell are measured. The relative contributions of scattering and absorption of light in the shell to the total optical losses of the fiber are estimated. The possible use of optical fibers of this structure in laser medicine is considered.

Indium–zinc oxide (IZO) nanofibers are synthesized on Si–SiO₂ substrates. The size, morphology, crystal structure, and composition of nanofibers are investigated via scanning electron microscopy, atomic force microscopy, energy-dispersion analysis, and X-ray diffraction. The electric properties of nanofibers, as well as the sensitivity to ultraviolet (UV) radiation, are studied at various In and Zn concentrations. The highest sensitivity to UV radiation is shown to be at an indium content of about 50 at %. The increment of the photocurrent relative to the dark current is more than four orders of magnitude. The times of response and recovery are 6 and 50 s, respectively. The results reveal that IZO nanofibers may find application as UV sensors with improved characteristics.

The results of the study of the dynamics of optical and physiological properties of skin upon exposure to and removal of external mechanical compression, based on the analysis of temporal changes in diffuse reflectance spectra of human skin in vivo in the range of 400–2000 nm, are presented. In the spectral region of 500–600 nm (hemoglobin absorption region), the temporal dynamics of skin reflectance indicates that the skin compression causes an exponential decrease in the amount of blood in it, with an average time of blood displacement from the compression area of approximately 4–5 min. After compression removal, the skin blood filling exponentially recovers approximately within 30 s. In the spectral region of 700–2000 nm, skin reflectance after compression monotonically decrease according to two-exponential law with characteristic times of approximately 10 s and several minutes, which may be determined by the displacement of free and bound water from the compression area.

To identify the signs that distinguish natural diamonds from artificial diamonds, a comparative analysis of the luminescence spectra with regards to the Q factor, center of gravity, bandwidth parameter, and energy losses in the diamond crystal lattice under conditions of ohmic and dielectric relaxation of luminescence is performed. The phenomenon of resonant luminescence in the femtosecond time range is detected in diamond. It is established that natural and artificial diamonds noticeably differ in the relaxation frequency and in the energy of resonant radiation.

The structure of nonpolarizing interference systems incorporating the layers manufactured from metallic materials is considered. The method of searching for the structures of metal-dielectric interference systems providing a small deviation between the spectral characteristics of the energy reflection (transmission) coefficient for s- and р-polarizations is presented. The structures developed on the basis of the presented method have a rather simple structure, which makes them easy to implement. The analysis of the spectral characteristics of the obtained structures has demonstrated that the difference between the integral refl-ection coefficients for s- and p-polarizations within the considered spectral region (460–580 nm) is less than 1%.