Optics and Spectroscopy (v.96, #4)

New structures in paraxial Gaussian beams by A. P. Kiselev (479-481).
Within the classical parabolic equation approach, two new families of localized paraxial beams are constructed in the entire space. The solutions are different from the well-known solutions of the Hermite-Gauss or Laguerre-Gauss types. The approach is based on the separation of variables and is quite elementary.

The method of interpolation of relativistic quantum defects is used for determining the energies of Rydberg levels of rubidium-like ions. For this purpose, the values of relativistic quantum defects calculated by the Dirac-Fock method at three points, two of which correspond to discrete levels and the third, to the ionization threshold, are approximated by a second-degree polynomial. By using the continuous function μ(E) thus obtained, one can readily determine the energy value for any discrete level. A formula for calculating the threshold value of the quantum defect μ(0) (the phase shift δ(0)) is given. The approximation coefficients corresponding to the nS 1/2, nP°1/2, and nP°3/2 levels are presented. For better agreement with the experimental results, an empirical correction to the quantum defect is introduced, which weakly depends on energy. The calculations were performed for 17 members of the rubidium isoelectronic sequence (from Rb to Fr50+).

The angular distribution of Auger electrons is considered. The results of numerical calculations of the anisotropy parameter of the angular distribution α2 for the N 3 O 1 O 4, 5 and L 3 M 1 M 4, 5 transitions in the Hg atom are reported. The matrix elements were calculated by the Dirac-Fock method in the relativistic approximation using the intermediate-coupling scheme.

The absorption spectra of Xe + CF4 and Kr + CF4 mixtures in the vacuum ultraviolet region are presented. A considerable broadening of the short-wavelength wings of the atomic absorption lines is explained by a repulsive character of the Rg*-CF4 interaction potential. This repulsive character is also considered to be the most probable reason for the anomalously small cross sections of the quenching of electronically excited atoms of rare gases by CF4 molecules. The influence of CF4 on the emission rate of the trapped resonance radiation of Xe from the cell is discussed.

Application of pattern recognition in molecular spectroscopy: Automatic line search in high-resolution spectra by A. D. Bykov; A. M. Pshenichnikov; L. N. Sinitsa; A. P. Shcherbakov (497-502).
An expert system has been developed for the initial analysis of a recorded spectrum, namely, for the line search and the determination of line positions and intensities. The expert system is based on pattern recognition algorithms. Object recognition learning allows the system to achieve the needed flexibility and automatically detect groups of overlapping lines, whose profiles should be fit together. Gauss, Lorentz, and Voigt profiles are used as model profiles to which spectral lines are fit. The expert system was applied to processing of the Fourier transform spectrum of the D2O molecule in the region 3200–4200 cm−1, and it detected 4670 lines in the spectrum, which consisted of 439000 dots. No one experimentally observed line exceeding the noise level was missed.

The intermolecular photoinduced reactions between triplet ketone molecules and aliphatic amines and pyridine are studied by the quenching of delayed fluorescence of anthraquinone and benzophenone vapors by diethylamine, dibutylamine, cyclohexylamine, triethylamine, and pyridine. In the temperature range 423–573 K, the delayed fluorescence quenching rate constants k q are estimated from changes in the decay rate constant and the intensity of delayed fluorescence upon increasing pressure of bath gases. It is ascertained that, in the gas phase, the mixtures under study exhibit both a negative and a positive dependence of k q on temperature, which indicates that some photoinduced reactions do not have activation barriers. The rate constant k q is shown to increase with decreasing ionization potential of the electron donors. This points to the importance of interactions with charge transfer in the photoreaction of triplet ketone molecules with aliphatic amines and pyridine in the gas phase. The relationship between k q and the change in the free energy ΔG upon the photoinduced intermolecular electron transfer, which is the primary stage of the photochemical reaction, is studied. It is shown that the dependence k q G) for the donor-acceptor pairs under study is described well by the Marcus equation, in which the average vibrational energies of the donor and acceptor are taken into account for the estimate of ΔG.

A modification of the local structure of molecular aggregates in liquid water subjected to the action of an ultrasonic cavitation field is found. This modification is detected by the shift of the center of the Raman band contour of the OH stretching vibrations. It was also found that both the structure and the growth rate of lysozyme crystals grown in redistilled Milli-Q water and in water subjected to cavitation treatment differ considerably from each other.

The bimolecular processes of T-T annihilation and excimer formation of complex organic molecules adsorbed on semiconductor-dielectric structures are studied. The adsorption of molecules on a solid surface is ascertained to be island in character. It is found that the efficiency of bimolecular processes between adsorbed complex organic molecules is affected by the charge of slow traps on the semiconductor-dielectric interface, the oxide layer thickness, and the concentration of adsorbed water molecules.

Thermochromic transitions in poly(di-n-hexylsilane) films by N. I. Ostapenko; V. Zaika; S. Suto; A. Watanabe (520-525).
The spectra of absorption (77–398 K) and luminescence (5 K) of poly(di-n-hexylsilane) films are studied in relation to the heating of the films above the phase transition temperature. Two new absorption bands in the region of the band of the gauche conformation are found. The position and intensity of these bands depend on the annealing temperature, annealing regime, and sample thickness. These bands are assumed to be associated with the appearance of a liquid crystalline phase in the polymer, which is determined by intermolecular interactions between neighboring polymer chains. The appearance of the liquid crystalline phase in the heat-treated polymer films is confirmed by the increase in their birefringence in comparison with the birefringence of the initial films and by the observation of a stable orientation of an E 7 nematic liquid crystal applied to the surface of an annealed sample. In addition, the absorption spectra of the films cooled after the annealing exhibit many new bands in comparison with the spectra of the initial samples. These bands are associated with the appearance in the polymer of defect states and longer segments as a result of orientation of polymer chains. The appearance of longer segments is consistent with the observation of a long-wavelength band in the luminescence spectra of the annealed samples.

Effect of higher order nonlinear optical processes on optical absorption in the photorefractive BSO and BGO crystals by R. A. Ganeev; A. I. Ryasnyanskiĭ; R. I. Tugushev; M. K. Kodirov; F. R. Akhmedzhanov; T. Usmanov (526-531).
The processes of nonlinear refraction and nonlinear absorption are studied in the photorefractive Bi12SiO20 (BSO) and Bi12GeO20 (BGO) crystals at a wavelength of a picosecond Nd:YAG laser of 1064 nm. The nonlinear refraction in the crystals is shown to be related to the Kerr effect, and the nonlinear absorption at this wavelength, to three-photon absorption. The three-photon absorption coefficients of the BSO and BGO crystals are equal, respectively, to (2.5±0.8) ×10−20 and (4.4±1.3) ×10−20 cm3W−2.

Luminescence of F + and F centers in YAlO3 by Yu. V. Zorenko; A. S. Voloshinovskiĭ; I. V. Konstankevych (532-537).
In YAlO3 crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ=30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F + centers in the band at 355 nm, with τ=2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F + and F centers in oxides of the Al2O3-Y2O3 system is analyzed on the example of the compounds α-Al2O3, YAlO3, and Y3Al5O12. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed.

The analytic properties of the complex amplitude reflection and transmission coefficients of plane-parallel crystal plates in the exciton spectral region in the presence of additional waves are considered in a wide range of plate thicknesses and angles of incidence of light. It is shown that violations of the classical amplitude-phase Kramers-Kronig relations for the reflection and transmission can be described quantitatively by the contributions of the reflection and transmission zeros appearing due to the interference of ordinary and additional exciton-polariton waves. The decrease in the total absorption coefficient in the vicinity of the exciton resonance is also explained by the presence of transmission zeros in the upper half-plane of the complex frequency.

Magnetic circular polarization of luminescence of dysprosium-yttrium aluminum garnet Dy0.2Y2.8Al5O12 by U. V. Valiev; J. B. Gruber; Sh. A. Rakhimov; V. Yu. Sokolov (549-555).
Magnetic circular polarization of the luminescence of the radiative 4f-4f transitions 6 H 15/26 F 9/2 in dysprosium-yttrium aluminum garnet Dy0.2Y2.8Al5O13 was studied at T=85 K. The revealed features of the spectral dependences of the magnetically polarized luminescence of Dy3+ ions are attributed to the quasi-Ising behavior of rare-earth ions in the garnet structure. The symmetry of the wave functions for a number of Stark sublevels of the multiplets of the ground configuration 4f(n) that combine in observed radiative transitions is determined.

A semiclassical theory of electromagnetically induced transparency in a Bose-Einstein condensate is considered. A nonlinear Schrödinger equation that describes the interaction of the Bose-Einstein condensate with an electromagnetic field is derived. An analysis of the obtained solutions to this equation demonstrates a decrease in the group velocity of the light pulse.

It is proposed to use a result of indirect measurements—the average relative frequency of counts of a detector selective for the states of atoms escaping from a micromaser—for time selection of subensembles of a field mode. Analytical expressions for the reduced density matrix, average number of photons, and Mandel Q parameter for mode subensembles were obtained by the method of periodic paths. The generation of a random sequence of results of indirect quantum measurements was performed by the Monte Carlo method. The results of the analytical and numerical calculations are compared. It is found that the statistics of subensembles is sub-Poisson even when the calculation based on the matrix for the entire ensemble (in the absence of quantum measurements) gives super-Poisson statistics. It is noted that the time selection of subensembles is possible within a limited range of variation in the pumping parameter Θ, when the probability of quantum jumps is relatively low.

Transversely one-dimensional localized field structures in a wide-aperture interferometer with inertialess nonlinearity of a threshold form are analyzed theoretically for the case when modulation instability of the transversely homogeneous field distributions is absent. It is shown that dissipative solitons exist even in the case of monostability (in the absence of bistability of the homogeneous distributions). In this case, the number of types of solitons turns out to be finite. The possibility of existence of pulsating solitons in an interferometer with inertialess nonlinearity is indicated.

Frequency conversion of subnanojoule femtosecond pulses in microstructure fibers by S. O. Konorov; A. B. Fedotov; W. Boutu; E. E. Serebryannikov; D. A. Sidorov-Biryukov; Yu. N. Kondrat’ev; V. S. Shevandin; K. V. Dukel’skiĭ; A. V. Khokhlov; A. M. Zheltikov (575-579).
We experimentally demonstrate highly efficient multiplex frequency conversion of unamplified subnanojoule femtosecond pulses of Ti:sapphire laser radiation in fused silica microstructure fibers. Nonlinear optical spectral transformation of femtosecond pulses in an array of fused silica threadlike channels in these microstructure fibers results in the generation of isolated anti-Stokes spectral components within the wavelength range of 400–500 nm. An efficiency of frequency conversion of about 20% is achieved for 800-nm pump pulses with an energy of 0.7 nJ and a pulse duration of 70 fs.

The wavelength dependence of the specific rotation of the plane of polarization of light by quartz crystals is experimentally studied. The chirality coefficients determined from the plots δ(1/λ2) are found to be different for different crystals. The differences between these plots are associated with the perfection of the crystal structure of quartz. The rotation of the plane of polarization is more sensitive to imperfection of the medium and makes it possible to reveal this imperfection without additional irradiation of crystalline quartz by γ quanta.

Polarization-independent modulation of light in gyrotropic cubic crystals by G. V. Kulak; S. N. Kovchur; P. I. Ropot (582-585).
Polarization-independent modulation of light in gyrotropic cubic crystals is investigated in the Bragg and intermediate diffraction regimes. It is found that, for crystals with considerable specific rotation, the photoelastic anisotropy is suppressed by the circular anisotropy and the observed diffraction efficiency depends weakly on the polarization azimuth of the incident light. Good agreement is obtained between modulation parameters calculated theoretically on the basis of the coupled wave theory and experimental results for a bismuth silicate crystal in the intermediate diffraction regime.

It is shown that, by means of measuring the frequency dependence of the ellipticity of light reflected at the Brewster angle from the plane surface of a nonabsorbing dielectric, one can reveal on it nanostructural objects consisting of foreign atoms or molecules. They manifest themselves by the presence of size resonances, arising in these structures in a field of optical radiation because of the dipole-dipole interaction of the atoms (molecules). A theoretical justification of the experimental technique that takes into account the presence of a transition layer on the surface of the dielectric is proposed.

A method of determining the spatial distribution of focused laser radiation from the shape of the resonance maximum in the ion yield, which is due to the ionization of atoms via a resonance induced by the dynamic Stark effect, is proposed. This method gives the same results as the technique based on photometric measurements. The results of the measurements of the spatial distribution are compared with the results of calculation for focused Gaussian laser beams.

The results of optical study and quantum-chemical simulation of a conjugated organic system, 2-cyclooctylamino-5-nitropyridine (COANP)-fullerene, performed to determine its potential for application as a limiter of visible and near-IR laser radiation and as a material for diffraction elements in systems for reversible recording of optical information, are presented. Complexation between a COANP molecule and fullerene is considered as one of the main mechanisms responsible for the corresponding properties of this system. For the first time, nonlinear optical characteristics of COANP-C60 and COANP-C70 systems are comparatively studied and the intermolecular interaction between a COANP molecule and fullerene is analyzed on the quantum-chemical level.

A model for the prediction of the optical characteristics of blood (refractive index and absorption coefficient) and for the determination of the rate of blood flow in the capillary bed under irradiation of a laser beam is proposed. The dependences of the intensity of the laser radiation on the refractive index and absorption coefficient of the system of blood vessels in the upper layer of the dermis and on the rate of blood flow in the capillary bed are obtained.

It is shown that the possibility of negative delay time of a pulse at the output of an optical system (compared to the pulse at its input) is a necessary consequence of the possibility of positive delay time of the pulse and the superposition principle. In particular, two optical systems with different positive delay times are sufficient for creating an optical system with a negative delay time. The negative delay time of a pulse can be considered as a spline extrapolation of its time dependence resulting from the interference of several copies of the pulse with different delays. It is shown that a negative delay time of a pulse arises necessarily in any interferometer in an area of destructive interference of two copies of the initial pulse that are similar in amplitude and undergo different delay times.

The problem of diffraction of a plane electromagnetic wave by a metal fiber with a circular cross section is considered for the case in which the diameter of the fiber differs only slightly from the radiation wavelength. Relations between the positions of the first diffraction minima and the fiber diameter are derived. The formulas obtained are used to determine the diameter of an ultrathin fiber.

Investigation of diffraction gratings with the use of a differential heterodyne microscope by I. M. Akhmedzhanov; D. V. Baranov; E. M. Zolotov (635-640).
A theory of image formation in a scanning differential heterodyne microscope is developed using the results of observations of a metallized periodic structure with a rectangular profile. The phase and amplitude response functions are calculated for TE and TM polarizations. It is demonstrated that the width of grating elements can be measured with a superresolution. The experimental phase and amplitude responses from a metallized grating with a trapezoidal profile are obtained and compared with the results of theoretical calculations.