Vol 124, No 2 (2018)

The specific features of absorption in a cell with an antirelaxation coating related to optical pumping and a finite rate of laser-frequency scanning are studied. The internal state of an atom in such cells is likely to remain unaltered after a collision with a wall. This results in optical pumping of an atomic ensemble over all velocities and the entire cell volume. Both the frequency corresponding to the maximum absorption in the D₂ line of ¹³³Cs and the absorption maximum itself depend on the sign of laser-frequency scanning. A theoretical model attributing the power dependence of asymmetry of the absorption contour of the ¹³³Cs D₂ line to the presence of cyclic transition levels in the system is presented.

The energy spectrum of fullerene C₇₀ is calculated within the Hubbard model in the mean-field approximation. Using group-theory methods, irreducible representations are determined that correspond to the energy states, as well as allowed transitions in the energy spectrum of fullerene C₇₀. On the basis of this spectrum, an interpretation of experimentally observed optical absorption bands of fullerene C₇₀ is proposed.

Oxyhalides Ba₂InO₃F, Ba₂InO₃Cl, and Ba₂InO₃Br are synthesized, and their phase homogeneity is confirmed by the method of X-ray diffraction. The influence of the nature of a halide ion on the crystal-lattice parameters and on the In–O bond lengths is established. The ability of the studied phases to undergo hydration and formation of oxygen–hydrogen groups in their structure is proven.

To identify the structure of emissive tunnel recombination sites in the emulsion microcrystals of silver bromide AgBr(I) with iodine contaminations and to determine the role of an emulsion medium in their formation, the temperature dependence of the luminescence spectra in the range from 77 to 120 K, the kinetics of the growth of the maximum luminescence intensity value at λ ≈ 560 nm, and the luminescence flash spectrum stimulated by the infrared light are investigated. Two types of the AgBr_{1 – x}(I_x) (x = 0.03) microcrystals—namely, obtained in an aqueous solution and on a gelatin substrate—are used in the studies. It is established that the emissive tunnel recombination sites with a luminescence maximum at λ ≈ 560 nm in AgBr_{1 – x}(I_x) (x = 0.03) are the {(I_a^-I_a^-)Ag_i⁺} donor–acceptor complexes with the I_a^- iodine ions located in neighbor anionic sites of the AgBr(I) crystal lattice, next to which the Ag_i⁺ interstitial silver ion is positioned. With an increase in the temperature, the {(I_a^-I_a^-)Ag_i⁺} sites undergo structural transformation into the {(I_a^-I_a^-)Ag_in⁺} sites, where n = 2, 3, …. Moreover, the {(I_a^-I_a^-)Ag_in⁺} sites (n = 2) after the capture of an electron and hole also provide the tunnel recombination with a luminescence maximum at λ ≈ 720 nm. The influence of an emulsion medium consists in that gelatin interacts with the surface electron-localization sites, i.e., the interstitial silver ions Ag_in⁺, n = 1, 2, and forms the complexes {Ag_in⁰ G⁺} (n = 1, 2) with them. The latter are deeper electron traps with a small capture cross section as compared to the Ag_in⁺ sites (n = 1, 2) and that manifest themselves in that the kinetics of the luminescence growth in AgBr(I) to a stationary level at λ ≈ 560 nm is characterized by the presence of “flash firing.” At the same time, the luminescence flash stimulated by IR light, for which the Ag_in⁺(n = 1, 2) electron-localization sites are responsible, is absent. It is supposed that the electrons localized on the {Ag_in⁺G⁺} complexes (n = 2) retain the capability for emissive tunnel recombination with holes localized on paired iodine sites with a luminescence maximum at λ ≈ 750 nm.

Luminescence characteristics of an analogue of the mineral component of dental enamel—nanocrystalline B-type carbonate-substituted calcium hydroxyapatite (CHAP)—with defects (nanopores ∼2‒5 nm in size) on the surface of nanocrystals are studied. It is shown that laser-induced luminescence of CHAP samples synthesized by us occurs in the region of ∼515 nm (∼2.4 eV) and is related to the existence of CO₃ groups substituting PO₄ groups in the CHAP lattice. It is determined that the luminescence intensity of the CHAP samples depends on the amount of structurally bound CO₃ groups and decreases with decreasing concentration of these intracenter defects in the apatite structure. The results obtained in this work are of potential importance for developing the fundamentals of precision and early detection of caries in human hard dental tissue.

Zinc oxide (ZnO) nanorods were grown on glass substrates coated with a conducting indium tin oxide film using the hydrothermal method. The nanorods are 2–2.5 μm long and 70–200 nm in diameter. Under UV irradiation the nanorods exhibit photoluminescence with a maximum at 382 nm. It is found that changes in angle between the nanorods growth direction and the emission recording direction give rise to an appearance of a violet emission band centered at ∼400 nm. It is possible dependence of the luminescence spectrum on the ZnO nanorods’ spatial orientation is due to localization of the violet emission centers in the surface layer.

A new method of investigation of photoelectric properties of layered thin-film structures based on broadband Fourier spectroscopy exhibiting a harmonically modulated optical delay is proposed. In contrast to traditional approaches to study photoelectric properties, which are based on application of dispersive spectral devices, the proposed method allows not only simultaneously covering the ultraviolet, visible, and infrared spectral ranges, while demonstrating a wide dynamic range and high spectral resolution, but also easily varying low-frequency modulation of the action of light. The capabilities of the method are demonstrated using a polycrystalline organic heterostructure as an example. Its spectral sensitivity, speed, and specific detectivity are measured. A model and an equivalent electric circuit are proposed for explanation of the results of the measurements.

A method for rapid identification of uranyl compounds based on resonance fiber-optic photoluminescence (PL) excitation by ultraviolet-laser or LED radiation is proposed. This method was used to measure the PL spectra of an extremely small volume (10^–9 cm³) of crystalline uranyl acetate dehydrate UO₂(CH₃COO)₂ ∙ 2H₂O with an exposure of 10^–3 s. Semiconductor LEDs with wavelengths of 369, 385, 410, and 466 nm and a repetitively pulsed nitrogen laser with a lasing wavelength of 337 nm served as sources of excitation radiation. The operating range of a small-sized minispectrometer used in these experiments was 200–1000 nm.

Refractive indices of new nonlinear optical materials based on 4-nitrophenol cocrystals with the aminopyridines 2-aminopyridine, 2,6-aminopyridine, and 4-aminopyridine have been studied. The refractive indices of the compounds under study are presented: n = 1.694 (optical microscopy) and 1.605 (Abbe method) for 2-aminopyridine-4-nitrophenol (2AP4N), n = 1.541 (optical microscopy) and 1.589 (Abbe method) for 2,6-diaminopyridine-4-nitrophenol (2,6DAP4N), and n = 1.561 (optical microscopy) and 1.591 (Abbe method) for 4-aminopyridine-4-nitrophenol (4AP4N).

Particular features of waveguide propagation of modes in a planar structure that consists of alternating layers of a dielectric and graphene are investigated. Within the effective-medium approximation, dispersion relations are obtained for symmetric and antisymmetric waveguide modes. Based on their numerical analysis, the frequency dependences of the propagation and decay constants, of the group and phase velocities, and of the energy flux carried by waveguide modes are constructed. The influence of the fraction of graphene in the structure on the behavior of waveguide modes is analyzed.

The interaction of an electromagnetic E wave with a thin metal film placed between two dielectric media is calculated in the case of different specular reflectances q₁ and q₂ for the reflection of electrons from the surface of the thin metal layer, in the case of variations in the values of dielectric permittivities ε₁ and ε₂ of the media, and in the case of different values of angle of incidence θ of the electromagnetic wave. The behavior of reflection coefficient R, transmission coefficient T, and absorption coefficient A in relation to the frequency of the external field is analyzed.

On the basis of polarized IR-reflection spectra in the range of 5000–350 cm^–1 measured from the natural face of an optically transparent fluorapatite single crystal, components of a complex refractive index (optical constants) for radiation-vector orientations E ‖ c and E ⊥ c have been calculated by the Kramers–Kronig method. The fluorapatite single crystal has been chosen from several samples: it contains a minimum amount of impurities and has a high degree of crystallinity in accordance with the criteria of IR spectroscopy and Raman spectroscopy. Tabular data on optical constants for ordinary and extraordinary rays are given for the IR range of 6–28 μm. The obtained absorption spectra are compared with the results of quantum-chemical ab initio calculations within the B3LYP simulation.

Results of an experimental study of the possibility to use thin holograms with a diffraction efficiency increased due to a triangular profile of the fringe for the formation of light beams with a nonzero orbital moment (optical vortices) are presented. It is shown that the transformation efficiency in this case can be significantly increased (theoretically, up to 100%) without any distortion in the structure of the formed beam.

A generalization of the Cormac algorithm for the inversion of the Radon transformation in an optical medium with an axisymmetric index of refraction has been proposed. The distribution of rays is such that only one ray passes through any two points in the circle. A “parallel scanning scheme” of tomography has been considered: a cylindrical object is illuminated by a parallel beam of light, the rays of which are deflected into the cylinder. There is no refraction on the surface of the cylinder. The algorithm assumes the possibility of attenuation on a ray, which also has an axisymmetric character. Such a type of ray deflection occurs in problematic issues of the tomography of GRADANs, light guides, and plasma.