Optics and Spectroscopy (v.123, #2)

An analytical expression describing the frequency shift of nonsoliton pulses in a nonlinear Raman-active medium is obtained by the method of moments. The regime of propagation in which the central frequency of the signal falls into the region of zero group-velocity dispersion is analyzed.

Localized optical states in a liquid-crystal structure adjacent to a metal by M. V. Pyatnov; S. Ya. Vetrov; I. V. Timofeev (189-192).
The spectrum of light transmission through a structure consisting of a metallic layer and a cholesteric liquid crystal with an induced planar defect is calculated. The possibility of existence of localized states at the metal–dielectric interface of this system is demonstrated. The transmission spectra at different positions of the defect in the structure are studied.

The necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index optical fiber are analyzed. The cutoff conditions for such an optical waveguide are investigated taking into account Kerr nonlinearity. The minimal power of optical pulses required for fulfilling the necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index fiber is calculated as a function of the normalized frequency. The allowed ranges of variation of the normalized frequency and optical-radiation power are estimated. It is demonstrated that the conditions necessary for nonlinear excitation of a higher-order mode in a step-index single-mode optical fiber can be created for optical pulses shorter than 500 fs.

It is established, that in modern works on digital holography, three fundamentally different mathematical models of a matrix photodetector are used. Comparative analysis of these models, including analysis of the formula of each model and test calculations, has been conducted. The possibility of using these models to account for the influence of geometrical parameters of a matrix photodetector on the properties of recorded digital holograms is considered.

The use of the diffusion approximation for simulating radiation and thermal processes in the skin by A. E. Kovtanyuk; G. V. Grenkin; A. Yu. Chebotarev (205-210).
Radiation and thermal processes in skin exposed to solar radiation are simulated based on the diffusion model of radiative-conductive heat exchange. Using the model proposed for the parameters corresponding to radiation with a wavelength of 800 nm, the contributions of thermal radiation induced by the skin and the reflection and refraction effects are estimated, and the photoprotective properties of titanium dioxide nanoparticles (TiO2) when introduced into the stratum corneum are studied.

Quantum memory in an orthogonal geometry of silenced echo retrieval by K. I. Gerasimov; M. M. Minnegaliev; S. A. Moiseev; R. V. Urmancheev; T. Chanelière; A. Louchet-Chauvet (211-216).
We experimentally realize a quantum-memory protocol based on retrieval of silenced echo (ROSE) in Tm3+:Y3Al5O12 crystal in an orthogonal geometry of the signal and control light fields. The silenced echo signal revival efficiency of ~13% with 36 μs storage time is demonstrated. To achieve that we implemented a high-precision atomic coherence control via amplitude- and phase-modulated laser pulses. We also discuss capabilities of this configuration, ways to increase quantum efficiency and to combine it with a single-mode optical cavity.

Analytic expressions for the probability density of states of a molecule with internal rotation and the probability of finding the state in the potential well are derived for the first time. Two methods are proposed for assigning conformers to potential wells. A quantitative measure of localization and delocalization of a state in the potential well is introduced. The rotational symmetry number is generalized to the case of asymmetric rotation. On the basis of the localization criterion, a model is developed for calculating the internal rotation contribution to thermodynamic properties of individual conformers with low rotational barriers and/or at a high temperature.

We propose an improvement of the basis for the solution of the stationary two-centre Dirac equation in Cassini coordinates using the finite-basis-set method presented in our earlier article [J. Phys. B 43, 235207 (2010)]. For the calculations in the above article, we constructed the basis for approximating the energy eigenfunctions by using smooth piecewise defined polynomials, called B-splines. In the present article, we report that an analysis of the employed representation of the Dirac matrices shows that the above approximation is not efficient using B-splines only. Therefore, we include basis functions which are defined using functions with step-like behavior instead of B-splines. Thereby, we achieve a significant increase of accuracy of results.

Geometrical and electronic structure and optical properties of several substituents and derivatives of 1,8-naphthosultam are studied by quantum-chemical DFT and TDDFT. It is found that the substituents –NO2, –CF3, and –N(CH3)2 affect the geometrical and electronic structure the most. It is shown that the Stokes shift is greatest (≈260 nm) for compounds with the strong donor substituent –N(CH3)2, while strong acceptor substituents provide a quite small Stokes shift. The dependence of the Stokes shift on the difference in energies of the frontier orbitals of the ground and excited states of molecules is found. Of the 1,8-naphthosultam substituents considered, compounds with –N(CH3)2 substituent, which emit in the biological window region, can be advised for use in optical bioimaging. The results can be used as a basis for the development and creation of new functional materials and biologically active compounds.

The vibrational–rotational absorption spectrum of D2O in the range from 10 120 to 10 450 cm–1 is recorded on a Fourier transform spectrometer with a resolution of 0.05 cm–1. The measurements were performed using a multipass White cell with an optical path length of 24 m. A light-emitting diode with brightness higher than that of other devices was used as a radiation source. The signal-to-noise ratio was about 104. The spectrum is interpreted as consisting of lines of more than 400 transitions. The spectral characteristics of lines (centers, intensities, and half widths) are determined by fitting the Voigt profile parameters to experimental data by the least-squares method. The intensities of lines and the experimental rotational energy levels of the (301) vibrational state of the D2 16O molecule with high rotational quantum numbers are determined for the first time.

We have quantitatively analyzed the vibronic parameters of two cross-conjugated δ-dimethylaminoketones. The presence of the –N(CH3)2, C=O, and –NO2 groups in the benzene ring has been shown to affect the manifestation of the vibronic parameters of characteristic bands that describe the state (vibrations, types of deformation upon excitation) of polyene systems with aromatic rings. Data on the influence of the nature of the substituent on the parameters of intra- and intermolecular interactions in the examined compounds have been presented.

Probe Raman spectroscopy of sodium uranyl-acetate microcrystals by V. S. Gorelik; A. A. Anik’ev; V. M. Korshunov; Yu. P. Voinov (255-257).
The technique for recording Raman spectra in uranium compounds using a probe minispectrometer with a multielement radiation detector has been developed. A selective filter, which reflects exciting laser radiation and transmits Raman signals, has been used to suppress the exciting radiation. It is shown that the technique developed makes it possible to record Raman spectra from a small amount (~10–5 g) of sodium uranyl-acetate polycrystals in a wide spectral range for several seconds.

The concentration behavior of the parameters of spectral lines that correspond to stretching vibrations of OH groups in the IR absorption spectra of LiNbO3:Zn (0.04–4.46 mol % ZnO) and LiNbO3:Mg (0.19–5.91 mol % MgO) crystals has been studied. It has been found that, in the range of threshold concentrations of Zn2+ and Mg2+ doping cations, the line parameters experience a jump. In this case, the widths of some lines decrease, which indicates the ordering of OH groups in the crystal. It has been shown that nonstoichiometric crystals are characterized by the occurrence of different positions of OH groups and different values of the quasi-elastic constants of O–H bonds in the crystal structure. In the stoichiometric LiNbO3 crystal, all the positions of OH groups are the same, and the quasi-elastic constants of O–H bonds are identical.

Spectral properties of RuAl2 and RuGa2 compounds: Ellipsometric analysis by Yu. V. Knyazev; Yu. I. Kuz’min (264-268).
The optical properties of RuAl2 and RuGa2 intermetallic compounds have been investigated in the spectral range of 0.22–14 μm. The nature of interband light absorption has been interpreted based on a comparative analysis of calculated and experimental frequency dependences of optical conductivity. The data obtained confirm the existence of pseudogaps with a width of ~0.8 eV localized at the Fermi level in the electron densities of states of these materials, which was predicted in previous energy-band calculations.

IR spectra of co-crystals based on 4-nitrophenol with derivatives of aminopyridines are studied within the range of 450–4500 cm–1. As samples, 2-aminopyridine–4-nitrophenol and 2,6-diaminopyridine–4-nitrophenol are chosen. The samples are prepared by two methods, i.e., by pressing of potassium bromide pellets and by coating of fluorite with a mixture of the sample with vaseline oil. Comparative analysis shows that the spectra of co-crystals differ from the spectra of mixtures of their components. It is found that some vibrations in the spectra correspond to vibrations of aminogroups probably involved in bonding between components in co-crystals, which confirms the formation of co-crystals.

The nonlinear spectroscopy using harmonic generation in the extreme ultraviolet range became a versatile tool for the analysis of the optical, structural and morphological properties of matter. The carbon-contained materials have shown the advanced properties among other studied species, which allowed both the definition of the role of structural properties on the nonlinear optical response and the analysis of the fundamental features of carbon as the attractive material for generation of coherent short-wavelength radiation. We review the studies of the high-order harmonic generation by focusing ultrashort pulses into the plasmas produced during laser ablation of various organic compounds. We discuss the role of ionic transitions of ablated carbon-containing molecules on the harmonic yield. We also show the similarities and distinctions of the harmonic and plasma spectra of organic compounds and graphite. We discuss the studies of the generation of harmonics up to the 27th order (λ = 29.9 nm) of 806 nm radiation in the boron carbide plasma and analyze the advantages and disadvantages of this target compared with the ingredients comprising B4C (solid boron and graphite) by comparing plasma emission and harmonic spectra from three species. We also show that the coincidence of harmonic and plasma emission wavelengths in most cases does not cause the enhancement or decrease of the conversion efficiency of this harmonic.

A new nonlinear physical model of the formation of ordered structures of a surface topography of a semiconductor by ultrashort pulses of linearly polarized laser radiation has been proposed. The model is based on the formation of the dynamic near-surface waveguide due to the optical Kerr effect, interference with the part of waveguide modes, and the formation of microgratings on the surface, which are oriented by polarization of radiation. The model is confirmed by experimental data.

We consider the excitation by a dipole of metallic photonic crystals—systems of infinitely extended parallel-oriented metallic cylinders with a small circular cross section. Based on the rigorous statement of the problem, analytical expressions are obtained that make it possible to give a simple estimate of frequencies that determine the band gap edges in the spectrum of eigenstates of such a crystal and, also, to describe the amplitude characteristics of radiation in the low-frequency range, in which the approximations used in our study are correct. We compare our results with those obtained by strictly numerical simulation.

A method for measuring the phase of the reflection coefficient in the optical wavelength range is proposed. The method is simple in experimental implementation and is based on measuring the energyreflection coefficients of a sample in two media with different refractive indices. Analytical and numerical estimates show that the measurement accuracy of the phase is on the order of 1°. The possibilities of using the results of the phase measurement in practice for a more complete characterization of materials and structures under investigation are considered.

Elimination of bleaching in a material based on aqueous fluid with carbon nanoparticles for optical limiters by A. V. Sokolov; A. L. Nikolaeva; A. Yu. Vlasov; I. M. Kislyakov; A. V. Volkova; S. A. Povarov (305-308).
The possibilities of correcting the functional properties of nonlinear optical limiters based on carbon nanoparticles using polymer dopants have been considered. The results of analyzing the characteristics of nonlinear optical limiting of a suspension of carbon black with addition of sodium dodecylbenzene sulfonate and polyvinyl alcohol are given. The forms in which particles exist in the suspension have been investigated by different physicochemical methods. Optical measurements have confirmed the efficiency of these additives.

A study (based on the magnetic-scanning method) of the absorption of laser radiation with a wavelength of 0.63 μm by neon has revealed a preferred contribution of the central part of extended glow discharge to the absorption signal, which is related to the absorption of its intrinsic radiation.

Two-mode interferometric waveguide sensors based on chalcogenide glasses by V. I. Nalivaiko; M. A. Ponomareva (320-323).
The operation principles of interferometric waveguide sensors have been considered. A comparison of the sensitivity of the two-mode sensors with waveguides of oxide and chalcogenide glass has been performed. The advantages of the two-mode sensors with waveguides made of a material with high refractive index have been shown. It has been shown that the sensors with waveguides made of chalcogenide glass have a sensitivity three orders of magnitude higher.

A series of studies devoted to the theoretical justification and development of methods and tools for angular measurements based on the use of multiple sources of optical beams with a small angular aperture is continued. The source used in this study is a holographic prism: a fluorite single crystal with a system of superimposed holograms recorded in its bulk, which generates a series of diffracted small-angle beams in the form of a flat fan under illumination by a reference laser. This fan has a high spatial stability, including constancy of angles between any pair of fan beams in a wide range of external conditions. Based on the previously introduced notion of an effective beam axis, potential exact methods for measuring angles between fan beams are considered, and a coordinate method using a coordinate measuring machine and a CCD recorder is substantiated and implemented. The accuracy of the proposed method is analyzed. It is shown that its errors can potentially be reduced to a level of 1″ or even less.