Optics and Spectroscopy (v.110, #1)

Excitation of gallium atom in collisions with slow electrons is studied using the method of extended crossing beams with recording of the optical signal of excited atoms. At an electron energy of 30 eV, 66 excitation cross sections are measured. In the electron energy range of 0–200 eV, 24 excitation optical functions are recorded. The dependence of the cross sections on the principal quantum number of an upper level is obtained for eight spectral series of Ga I. The results obtained are compared with the data of the previous experiment.

The electronic structure, the spectra, and the efficiency of photophysical processes of energy deactivation are calculated by a semiempirical method for three positively solvatochromic merocyanine dyes with different polymethine chain lengths. The electronic structure and spectra calculated by different modifications of the INDO method at different molecular geometries are compared, and the optimum geometry of the isolated molecule is chosen. The absorption spectra of dyes are interpreted, including the short-wave-length bands related to transitions to highest excited states. The possibility of a specific electrophilic solvation of these compounds in the ground and fluorescent states, as well as the contribution of specific intermolecular interactions to the total interaction with the medium, is estimated. The role played by the trans-cis isomerization of isolated merocyanine molecules in the deactivation of their excited states is considered.

An analytical model of K α radiation of thin laser targets has been developed. It has been shown that, for such targets, the motion of fast electrons is significant not only in the target itself but also in vacuum. The considered dependences for the free path length of a fast electron and for the absorption coefficient of laser radiation on the laser intensity with allowance for the electron motion in vacuum make it possible to match the results of the proposed model with the experimental data on generation of K α radiation in wide ranges of laser intensities (1018–1021 W/cm2) and thicknesses (1–100 μm) of targets.

A theoretical description of the pump-probe optical method is presented that is adapted to semi-conductor quantum dots and makes it possible to determine the energy relaxation rates of the excited states of electron-hole pairs and excitons in these objects. A scheme of the method is considered in which the carrier frequencies of optical pump and probe pulses are close to the resonance with different interband transitions of the quantum-dot electron subsystem (nondegenerate case). It is assumed that the final states to which electron subsystem passes as a result of the absorption of pump and probe pulses are interrelated by intraband relaxation. The probe-pulse energy absorption induced by the pump pulse is analyzed as a function of the delay time between the pulses. It is shown that this dependence tends to be biexponential under certain conditions. The exponential factors are proportional to the energy relaxation rates of the resonantly excited states of electron-hole pairs and excitons, while the preexponential factors depend on the intraband relaxation rate.

Dynamics of the transformation of bistable centers and lattice in CdF2 crystals under femtosecond pulsed photoexcitation by D. I. Stasel’ko; S. A. Tikhomirov; O. V. Buganov; A. S. Shcheulin; A. E. Angervaks; A. I. Ryskin (33-41).
The kinetics of photoinduced absorption spectra of CdF2 crystals with bistable indium and gallium centers under femtosecond pulsed excitation has been experimentally investigated. Based on the example of indium ions, it is shown that the transmission band in the absorption spectrum of deep centers is formed for 0.8–1 ps, which significantly exceeds the photon absorption time. This process is interpreted as a result of displacement of indium ion from the initial interstitial position to a site of neighboring unit cell; the displacement velocity is estimated to be 200–250 m/s, a value close to the thermal velocity of this ion at room temperature. The times characteristic of the formation of free polarons as a result of the displacement of neighboring lattice ions have been experimentally estimated for the first time at a level of 0.8–1.2 ps. The capture times of free polarons by trivalent gallium and indium ions are estimated (5 and 10 ps, respectively), as well as the corresponding cross sections (2 × 10−16 and 8 × 10−16 cm2).

Spectral properties of neodymium in POCl3-BCl3-Nd3+ by E. A. Seregina; D. V. Kabakov (42-47).
We measured the absorption and luminescence spectra of Nd3+ ions in an inorganic solvent POCl3-BCl3. The spectra were analyzed in terms of the Judd-Ofelt theory. We calculated the Judd-Ofelt parameters, oscillator strengths, spontaneous emission probabilities, luminescence quantum yield, and the stimulated emission cross section for the laser transition 4 F 3/24 I 11/2 of the neodymium ion in a POCl3-BCl3-Nd3+ solution.

Enhancement of Raman scattering by ultramarine using silver films on surface of germanium quantum dots on silicon by E. V. Klyachkovskaya; N. D. Strekal; I. G. Motevich; S. V. Vashchenko; M. Ya. Valakh; A. N. Gorbacheva; M. V. Belkov; S. V. Gaponenko (48-54).
The germanium on silicon (Ge-on-Si) semiconductor nanostructures with a vacuum-deposited silver coating are tested as substrates for detecting microamounts of inorganic crystalline ultramarine pigment by enhanced Raman scattering (SERS). At least tenfold enhancement of Raman lines is obtained. The quantum-chemical calculations are performed and used to assign the bands in the Raman spectra. A significant electrooptic anharmonicity of vibrations of chromophore groups in the presence of an SERS-active compound is found.

Raman scattering spectroscopy of inclusions of carbon in Al2O3 films and its solid solutions with HfO2 by T. P. Smirnova; V. A. Volodin; M. S. Lebedev; V. I. Belyi (55-59).
We apply Raman scattering spectroscopy to study the nature of carbon inclusions in Al2O3 and (HfO2) x (Al2O3)1 − x films deposited using volatile complex compounds. Raman spectra of the films under investigation contain D and G vibrational modes, which indicate that carbon clusters of the sp 2 configuration tend to form in the films. We estimate the size of clusters from the integrated intensity ratio I D /I G and find it to be in the range of 14–20 Å. The content of hydrogen in carbon clusters is calculated from the height of the photoluminescence pedestal and is found to vary from 14 to 30 at % depending on the regime of the film’s synthesis.

The complexation of Eu3+ with doxycycline (DC) antibiotic in the presence of several second ligands and surfactant micelles of different types is studied by the spectrophotometric and luminescence methods. It is found that the efficiency of excitation energy transfer in Eu3+-DC chelate depends on the nature of the second ligand and surfactant micelles. Using thenoyltrifluoroacetone (TTA) as an example, it is shown that the second ligand additionally sensitizes the europium fluorescence, and the possibility of intermediate sensitization of DC and then of europium is shown by the example of 1,10-phenanthroline. In all cases, the excitation energy transfer efficiency was increased due to the so-called antenna effect. The decay kinetics of the sensitized fluorescence of the binary and mixed-ligand chelates in aqueous and micellar solutions of nonionic surfactants is studied and the relative quantum yields and lifetimes of fluorescence are determined.

Long-term luminescence of organic dyes in cells of biological tissues by S. N. Letuta; V. S. Maryakhina; S. N. Pashkevich; R. R. Rakhmatullin (67-70).
We studied the kinetics of delayed fluorescence and phosphoresce of exogenous fluorophores in cells extracted from tumorous and normal tissues of the mammary gland of mice of the BYRB strain. We revealed the specifics and determined regularities of the kinetics of long-term luminescence of fluorophores in different cells. The possibility of developing of a method for the early fluorescence diagnostics of the pathology of biotissues based on the obtained results is discussed.

The interaction of signal and control fields with an extended medium of two-level atoms with degenerate energy levels is considered in the semiclassical approximation. The effects of recording, storing, and release of signal pulses are studied for several schemes under the conditions of electromagnetically induced transparency. The possibility of inverting the succession of the reconstructed pulses is shown.

Quantum fluctuations of the number of photons of the fundamental mode and of the second harmonic in the process of intracavity generation of the second harmonic in the unstable region of the system are studied. The distribution functions of the number of photons of interacting modes are calculated in the positive P-representation. The functions of the joint distribution of the number of photons of the fundamental mode and the number of photons in the second harmonic are also studied.

Collisions of laser solitons by N. N. Rosanov; S. V. Fedorov; A. N. Shatsev (85-96).
A theoretical analysis and numerical simulation of a soliton collider (a wide-aperture laser with saturable absorption and variable length of the cavity) are performed. The presence of the cavity length gradient in a part of the aperture allows one to accelerate the laser soliton up to noticeable values of the transverse velocity. As a result, it becomes possible to implement a high-energy collision of the soliton with another laser soliton located in the other part of the aperture where the length of the cavity is constant. Different scenarios are presented for the strong interaction of colliding laser solitons with different topological charges.

Quantum properties of parametric amplification and frequency conversion of an optical image in coupled parametric interactions are analyzed for the case of a remote object. The coupled-wave interaction consists of a traditional process of parametric amplification in the high-frequency pump field accompanied by the frequency mixing of generated and pump waves. The photon number density and signal-to-noise ratio at the interacting frequencies are studied in the image plane at the output of the nonlinear frequency converter in the fixed pump field approximation. The degree of entanglement of optical images at frequencies above and below the pump frequency is investigated.

Nonlinear diffraction in inhomogeneous superlattice by M. B. Belonenko; E. G. Fedorov (105-109).
We considered the propagation of laser monochromatic radiation in a superlattice that contains regions with an elevated concentration of carriers. The model of the energy spectrum of electrons is chosen in the strong coupling approximation. The electromagnetic field is described quasiclassically with Maxwell equations, which, as applied to the problem under study, are reduced to a non-one-dimensional sine-Gordon wave equation for the vector-potential. We analyzed the wave equation in the approximation of slowly varying amplitudes and phases and obtained and numerically solved an effective equation that describes the electromagnetic field in the superlattice. We studied different regimes of propagation of laser radiation, analyzed diffraction by regions with an elevated electron concentration.

Angular structure of radiation scattered by monolayer of polydisperse droplets of nematic liquid crystal by V. A. Loiko; U. Maschke; V. Ya. Zyryanov; A. V. Konkolovich; A. A. Miskevich (110-118).
We considered light scattering by a polydisperse ensemble of droplets of a nematic liquid crystal. To model light scattering by a monolayer of polymer-dispersed spherical droplets of a nematic liquid crystal with cylindrical symmetry of its internal structure, we proposed a semianalytical modeling method. The method is based on interference approximation of the theory of multiple wave scattering, anomalous diffraction approximation, and effective-medium approximation. The method takes into account cooperative optical effects in concentrated, partially ordered layers and can be used to analyze the small-angle structure of the intensity of scattered radiation in relation to the concentration, size, polydispersity of liquid crystal droplets, orientation of their optical axes, and refractive indices of the liquid crystal and polymer. The obtained relations can be applied to solving direct and inverse problems of light scattering in composite liquid crystal materials using data of polarization measurements. We present graphical results of solving the direct problem for components of the polarization vector of scattered wave. These results illustrate the formation of an angular structure for monolayers with a high concentration of polydisperse droplets of the liquid crystal in the range of small scattering angles (0 < θ s ≤ 8°).

We study the optical properties of periodic structures, each of which consists of arrays of subwavelength slits in thick films that are placed directly against a continuous thin film. We find that the transmittance of the entire system including the thin film is higher than the transmittance of the isolated thin film alone. Our investigations are performed through numerical simulations with the Fourier modal method.

Low temperature hysteresis of light reflection from bismuth germanate single crystals by B. P. Tsapenko; A. B. Tsapenko; V. G. Bondar’ (124-128).
We measured the temperature dependences of light reflection and transmission of bismuth germanate single crystals upon cyclic variation of their temperature in the range of 94–340 K. We revealed an asymmetric temperature hysteresis of reflection, which is accompanied by a change in the Rayleigh scattering. Heating of crystals, in contrast to their cooling, is accompanied by a decrease in the reflection in the range of 100–287 K and near 310 K. We found that reflection minima in the first and second temperature ranges arise as a result of the action of different mechanisms. We assume that temperature changes in the reflection are related to structural phase transitions in the superficial layer.

We studied polymerizable nanocomposites for obtaining polymer-nanoparticle periodic structures by a holographic method. A general approach to choosing components of composites is developed that ensures a maximal contrast and high efficiency of structures for different types of nanoparticles. We found that the optimal monomeric component of a nanocomposite is a combination of single- and multifunctional monomers with substantially different reactivities. In this case, the low-reactivity monomer should posses a low viscosity, be a good solvent for nanoparticles, and have a low thermodynamic affinity to the polymer network formed upon the polymerization of the high-reactivity monomer. We developed a holographic composition based on known commercially produced monomers that ensures the formation of highly efficient periodic structures for nanoparticles of different types. We described the holographic properties of obtained nanocomposites, as well as parameters of bulk gratings recorded in them.

We considered the mechanism by which bulk periodic structures are formed in polymer-nanoparticle nanocomposites. Phase bulk gratings are formed due to the diffusion transfer of nanoparticles between illuminated and nonilluminated regions of a composite upon polymerization in an interference field. We found that, for the majority of studied nanocomposites containing nanoparticles of different natures, the relative modulation of the concentration of nanoparticles exceeds 80%. We showed that the initial composition should be optimized with respect to the mass of nanoparticles, which ensures a maximal recording efficiency at a minimal level of light scattering in the grating. We found that, for each medium, there exist optimal recording conditions (the intensity and period of the field) under which n 1 is determined only by the parameters of the medium and does not depend on the parameters of the field. Examples of the practical use of periodic structures based on developed holographic nanocomposites are given.

Transformation of spatial characteristics of excimer laser radiation by E. V. Gavrilov; A. P. Zhevlakov; S. V. Kascheev; V. Kujanpaa; T. Savinainen (145-149).
An optical attachment of an excimer laser beam that enables the transformation and optimization of spatial characteristics of a coherent UV beam was studied in an experimental setting. Experiments revealed that a rectangular 3 × 20 mm laser beam with the a divergence of 2 × 5 mrad attains a square shape of 20 × 20 mm and a divergence of 5 × 5 mrad in orthogonal directions after passing through the beam attachment. It is demonstrated that the application of a beam attachment in installations for material microprocessing simplifies optical layout of the illumination module allows one to obtain optimal homogeneity of sample illumination (under 2%) upon the projection of the image on the processing plane of the sample with submicron precision and to reduce the radiation load on optical elements via the elimination of hot spots.