Optics and Spectroscopy (v.109, #5)

Theoretical studies have been conducted on sub-Doppler resonances of fluorescence of an ultrathin gas layer in a cell with an internal weakness less than or on the order of the wavelength of a monochromatic light beam incident on the given cell in the normal direction. The features of these resonances are analyzed both under steady-state action of the incident radiation and immediately following abrupt termination of its action. The essential dependence of these resonances (at the centers of optical transitions with sufficiently small radiation spectral linewidths) on the transversal dimensions of the gas cell (using the example of a rectangular parallelepiped shape). The main focus is on analyzing the linear optics case when the examined resonances are narrowest. The possibility is noted of using such sub-Doppler fluorescence resonances in nanometer gas cells as references for optical frequency standards.

The narrow-band radiation observed in the range of the resonance line of xenon at 147 nm in the VUV emission spectra of the gas-discharge plasma of a krypton-xenon mixture is proposed to interpret as a manifestation of bound-bound transitions between the vibrational levels of the excited electronic states 0+(3 P 1) and 1(3 P 1) and the ground electronic state 0+(1 S 0) in the KrXe* molecule. A correction of the potential curves of the electronic states under consideration is proposed from a comparison of the calculated and experimental spectra.

Spectral determination of local values of electron concentrations and temperatures in a strongly ionized nitrogen plasma using a CCD sensor by T. Sh. Belyaletdinov; S. V. Goryachev; A. V. Efimov; E. Kh. Isakaev; V. F. Chinnov (662-668).
Using an earlier described automated measuring system and the procedure of processing “matrix” spectra, we detect and process the set of NI, NII, and NIII spectral lines, which allows independent determination of local electron concentration and temperature values in a strongly ionized nitrogen plasma in the temperature range 15–35 kK. From measurements of local contours of lines NI and NII, we obtain the radial electron concentration distributions in the space of an inhomogeneous plasma and compare them with the results of diagnostics on continuous radiation of the plasma. We study local distributions of excited nitrogen ions according to excitation states in order to elucidate the question on the possible quasi-Boltzmann description of these distributions with local electron temperature values.

The UV radiation of glow- and capacitive-discharge lamps based on mixtures of inert gases with iodine vapors are optimized in the spectral range of 175–360 nm, in which working helium-iodine mixtures of different compositions are used. The most intense spectral lines in the bactericidal region of the spectrum were the atomic lines of iodine (183.0, 206.2 nm), and in the region of 320–360 nm, emission of the spectral band of an iodine molecule prevailed with a maximum at λ = 342 nm. For a capacitive lamp with a casing opaque in the spectral range λ < 250 nm, the main part of the plasma emission power is concentrated in the A′-D′ band of an iodine molecule with a maximum at 342 nm. The emission brightness of this lamp is optimized in iodine molecule transitions depending on the partial helium pressure. We present the results of simulating the kinetics of processes in a glow-discharge plasma in mixtures of He, Xe, and iodine vapors. We establish the dependence of the main part of the emission intensity of the 206.2 nm spectral line of an iodine atom and the 342 nm band of an iodine molecule on the helium pressure in a glow-discharge lamp operating on a He-I2 mixture.

Complex cross sections of the spin exchange are calculated for the first time for interacting Na and K atoms based on the data on the singlet (X 1Σ+) and triplet (a 3Σ+) potentials that describe the interaction of these alkali-metal atoms in the ground state. The obtained cross sections allow one to theoretically consider the polarization transfer processes and calculate the relaxation times and the magnetic resonance frequency shifts caused by Na-K spin-exchange collisions.

Vibrational, rotational, and centrifugal spectroscopic constants and radiative parameters, i.e., the Einstein coefficients, oscillator strengths, and wave numbers for vibrational transitions in electronic systems of bands A 1Σ u + -X 1Σ g + (0 ≤ v′ ≤ 25; 0 ≤ v″ ≤ 44), B 1Π u -X 1Σ g + (0 ≤ v′ ≤ 29; 0 ≤ v″ ≤ 47), and the radiative lifetimes for the vibrational levels of excited states of the sodium dimer, are calculated. The calculations are carried out based on semiempirical potential curves constructed in this study. The calculated spectroscopic constants and radiative lifetimes are compared to the experimental values.

In a series of published experimental works, there has been observed nonresonance biphoton excitation, by femtosecond IR pulses (1250–1500 nm) of molecules of bacteriochlorophyll-a and the pigment in the composition of light-absorbing natural “antenna” complexes of photosynthesizing purple bacteria. The authors of these works believe that IR quanta excite hypothetical forbidden levels of pigments of these bacteria in the dual frequency range of 625–750 nm. In this study, an alternative mechanism of intramolecular electron transport apparently responsible for this phenomenon is suggested and substantiated. The mechanism should manifest itself in powerful electric fields, which are achieved in the pulses of picofemtosecond lasers.

The broadening of hyperfine components of rotational transitions of the methyl cyanide molecule is considered in terms of the collision approximation based on the theory previously developed by the author. The role played by possible collisional interference processes is examined. Relaxation parameters are calculated, and broadening coefficients are compared to the experimental data.

The development of simple methods used to analyze the available data and separate the contributions from the main physicochemical processes responsible for the kinetics of instantaneous emission spectra is of considerable interest due to the intensive investigation of the time-resolved characteristics of fluorescence of various quantum objects that are used to examine the most important photophysical and photochemical reactions in excited states. The relation between the functions of the shift in the instantaneous spectra of spontaneous emission of molecules (obtained using kinetic spectroscopy methods) and the time variations in the configurational structure of solvates, as well as the charge transfer in the excited singlet state of molecules of the luminescent probe, has been shown using simple analytical expressions of solvatofluorochromism. The fluorescence of solutions of one of the most important molecular probes, namely, 1-(phenylamino)naphthalene in glycerol, has been investigated with a picosecond time resolution. This molecular probe is known because its electric dipole moment is formed in the excited singlet state, which almost instantaneously initiates processes of intermolecular rearrangement of solvent molecules. It has been demonstrated that it is possible to determine the time dependence of the function of the dielectric response of the polar solvent to the dipole of this probe and, hence, the time evolution of the microcharacteristics of the solvent that determine the above function.

IR spectroscopic study of surface properties of amorphous water ice by A. V. Rudakova; M. S. Poretskiy; I. L. Marinov; A. A. Tsyganenko (708-718).
The state of the surface of amorphous ice with a specific surface area of about 160 m2/g obtained by the condensation of water vapor at 77 K is studied by IR spectroscopy. As the temperature increases to 130–160 K, absorption bands of surface hydroxyl groups vanish, whereas changes in bands characteristic of hydroxyl groups in the bulk of ice are indicative of a phase transition of ice from amorphous to the polycrystalline structure. The surface sites of amorphous ice are characterized with low-temperature adsorption of carbon monoxide. It is shown that there are two types of CO adsorption sites, free hydroxyl groups and oxygen atoms of surface coordinately unsaturated water molecules. Upon adsorption of nitrogen, methane, and carbon monoxide, in addition to the perturbation of surface OH groups, reversible changes in the spectrum are observed in the region of vibrations of bulk hydroxyls, which indicate that the strength of hydrogen bonds between water molecules in the surface layer of icy particles increases approaching the strength of these bonds in the crystal and that the ice surface becomes less amorphous. These results indicate that the properties of the ice surface layer substantially depend on the presence of adsorbed molecules.

Spectrally controlled atom-by-atom photoassembly of silver clusters on the surface of ionic-covalent crystals by A. N. Latyshev; O. V. Ovchinnikov; M. S. Smirnov; D. I. Stasel’ko; P. V. Novikov; D. A. Minakov (719-728).
The fundamental possibility of implementing light-controlled atom-by-atom assembly of silver clusters beginning with dimers and trimers via photostimulated diffusion of individual atoms initially adsorbed on the surface of AgCl and ZnS single crystals by mass-spectrometric ion deposition is shown. Possible mechanisms of this process are discussed, including the charge exchange in adatoms via alternating capture of holes and electrons accompanied by the transition to neighboring, energetically more favorable adsorption sites; hopping displacements of adatoms over the surface by gaining kinetic energy upon nonradiative recombination of photocarriers from their levels; and low-barrier diffusion of short-lived negatively charged adatoms with a captured electron.

Optical spectra of one-dimensional defect photonic crystals by S. V. Eliseeva; D. I. Sementsov (729-737).
The effect exerted by defects that represent a local disturbance of the structural periodicity on the band spectrum of a one-dimensional photonic crystal has been investigated. A classification of single defects has been proposed. It has been demonstrated that the position of defect minibands in the band gap of the spectrum of the crystal can be controlled by varying the type of a defect and its location in the crystal structure. The presence of two defects in the structure leads to the formation of two minibands, so that the spacing between the minibands and their intensity depend on the type of defects and on the distance between them.

Fluorescence of anthracene in a perfluorosulfone membrane by A. A. Kurova; A. N. Borisov; S. M. Shilov; V. N. Pak (738-741).
The fluorescence spectrum of anthracene introduced into the pore space of a perfluorosulfone membrane reflects the localization of a guest material in carrier nanochannels with the formation of closely spaced molecules able to form either excimers or stable dimers depending on the irradiation conditions.

Fine band structure of the vibrational spectra of fullerite C60 and enhancement of intermolecular interaction in high-temperature phase by N. E. Kornienko; N. P. Kulish; S. A. Alekseev; O. P. Dmitrenko; E. L. Pavlenko (742-752).
The fine structure of the fundamental vibrational bands and some combination tones of fullerite C60 in its IR absorption and reflection spectra, as well as in Raman spectra, has been studied. This structure is due to the overlapping components of Davydov and isotopic splittings and the removal of vibrational degeneracy with symmetry lowering. It is shown that for IR F u (i) bands (i = 1–4) and low-frequency H g (1) and A g (1) bands in the Raman spectrum the splittings at room temperature exceed those for the low-temperature phase. The enhancement of intermolecular interaction at elevated temperatures is explained by the nonequilibrium vibrational excitation of the medium as a result of nonlinear interaction of vibrational modes and by the change in the electronic states.

Motion of dissipative soliton complexes in a nonlinear interferometer with driving radiation by N. A. Veretenov; N. N. Rosanov; S. V. Fedorov; A. N. Shatsev (753-759).
A numerical analysis has been made of the motion of spatial dissipative soliton complexes in a wide-aperture interferometer with the Kerr nonlinearity, which is excited by continuous coherent driving radiation. It has been demonstrated that, depending on the symmetry of the radiation intensity distribution, the complex can exhibit four variants of dynamics, including the curvilinear motion of its center. The results obtained have been compared with the case of laser soliton complexes (without coherent driving radiation) and with the predictions made from the phenomenological model of the motion of soliton complexes.

SPASER laser and Purcell factor by V. S. Zuev (760-762).
This work presents an interpretation of experiments where laser action was observed in the so-called SPASER, a laser with a nanoscale surface plasmon. The experimental implementation of SPASER was reported by M. A. Noginov et al. in the paper entitled “Demonstration of SPASER-Based Nanolaser” and by Xiang Zhang et al. in the paper entitled “Plasmon Lasers at Deep Subwavelength Scale”, both of which were published online in the advance online publication of Nature in August 2009. Nanoscale plasmonic modes experience huge losses; however, simultaneously, the probabilities of radiative processes in atoms (molecules) leading to the emission of radiation into these modes increase 1000-fold compared to the probabilities of radiation in free space. The similarity of the physical nature of the effects in SPASER with SERS is pointed out.

The results of a theoretical analysis of the generation of broadband radiation in the infrared and terahertz spectral ranges upon the excitation of plasma in air by two femtosecond pulses at the fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. It is found that the appearance of long-wavelength radiation in a strong field of pulses of different frequencies can be described in terms of strongly anharmonic oscillations of optical electrons, whereby electrons are pulled far away from their atoms; these oscillations are accompanied by cascade transitions of electrons from their ground state to a bound excited state, followed by a transition to the continuum. It is shown that the generated infrared and terahertz radiation appears in the form of pulses containing a few oscillations of the light field. The efficiency of terahertz generation varies periodically with an increase in the interaction length of the femtosecond pulses of different frequencies.

Fluorescence from an ensemble of noninteracting molecules excited by continuous laser radiation is considered as a glow of M individual molecules. The expression that governs the autocorrelation function (ACF) of fluorescence from M molecules is derived based on the theoretical expression that governs the ACF of fluorescence from a single-molecule. Fluctuations of blinking fluorescence and its statistical properties are analyzed using a computer simulation. The results of statistical analysis of the computer experiment are in good agreement with the derived formula that governs the ACF of fluorescence from several molecules.

The self-action of elliptically polarized Gaussian laser pulses in an isotropic gyrotropic medium with an anomalous frequency dispersion and cubic Kerr nonlinearity with a finite relaxation time on the order of the pulse duration is numerically studied. It is shown that, at the output of the medium, the pulse polarization nonmonotonically varies with time. The main peak of the pulse is additionally delayed compared to the time of passing the linear medium; the value of this delay significantly depends on the polarization of the incident pulse and achieves a maximum for incident pulses whose degree of ellipticity is equal to the ratio of the material constants characterizing the local and nonlocal nonlinear optical response of the medium. It seems promising to search for possible differences in relaxation times depending on the intensity of additions to the refractive indices of the right and left circularly polarized waves by investigating the time dependence of polarization characteristics at the output of the medium.

The effect of the type of the model describing a non-Gaussian relief, as well as of the problem dimension, on the statistical characteristics of scattered monochromatic light is considered. A difference in the shape of diffuse scattering indicatrices was observed for different types of surface models (with identical height distributions and autocorrelation functions). The effects revealed because more pronounced with an increase in the deviation of surface height distribution from normal and with an increase in the roughness height.

Light propagation in stratified anisotropic media by E. V. Kryukov; V. P. Romanov (792-799).
Light propagation in stratified anisotropic media with arbitrary orientation of optic axes smoothly varying from layer to layer is considered. In the WKB approximation, a general expression for the field is obtained. For the case of a uniaxial medium, the normal waves are found and specific features of the light propagation are analyzed. General conditions are obtained that determine the turning points and forbidden zones. It is shown that the developed approach allows one to find trajectories of rays in anisotropic media with arbitrary layered structure.

Modes of capillary photonic-crystal fibers with hollow core by A. B. Sotsky; O. A. Bel’skaya; L. I. Sotskaya (800-807).
A rigorous method of integral equations has been used to calculate the dispersion characteristics, fields, and radiation patterns of the leaky modes of photonic-crystal fibers with a shell in the form of a bounded 2D photonic crystal composed of parallel quartz capillaries contacting each other and a core in the form of a defect of a photonic crystal with one capillary removed. It is shown that, when thick-walled capillaries (with an air content in the shell of about 28%) are used, these fibers can be single mode and have an overlap integral of the fundamental-mode field with the dielectric medium that is smaller by a factor of more than two (in comparison with similar fibers formed by air channels of circular cross section). The significant dispersion of the group velocity of the fundamental modes of fibers near the edge of the photonic crystal band gap has been found.

Polarization stokes polarimetry of the amplitude and phase characteristics of surface plasmon polariton resonance by L. S. Maksimenko; I. E. Matyash; I. A. Minaĭlova; O. N. Mishchuk; S. P. Rudenko; B. K. Serdega (808-813).
The amplitude and phase characteristics of internal reflection of gold nanofilms have been investigated using polarization modulation of electromagnetic radiation in the Kretschmann geometry. The component Q (the difference between the reflection coefficients R s 2 and R p 2 for the s and p polarizations, respectively) and the component V of the Stokes vector (the difference between the phases of the orthogonal components of linearly polarized radiation) of light reflected from a half-cylinder of the total internal reflection with gold films of different thicknesses on its flat surface have been measured as a function of the angle of incidence of light in the wavelength range 500–1000 nm. It has been demonstrated that, in the range of parameters corresponding to the manifestation of the plasmon polariton resonance (the angle of incidence of light, the wavelength, the metal film thickness), the dispersions of the amplitude and phase characteristics are in quantitative and qualitative agreement with the model concepts of a classical oscillator.

Matrix analysis of a holographic method for the investigation of induced absorption with a specified phase grating is performed. Expressions for calculating the induced absorption by varying the intensity of output beams are obtained. Comparative measurements of the induced absorption dynamics in photochromic glass using the traditional single-beam diffraction method and the new two-beam interference method are performed. Good agreement between the measurement results is demonstrated.