Optics and Spectroscopy (v.94, #2)

Spectroscopic characteristics of ions of the palladium isoelectronic sequence with nuclear charge Z=50–63 are theoretically studied. The Pd-like scheme of x-ray lasers is one of the most efficient schemes. Optical transitions promising for the development of lasers operating in the vacuum ultraviolet region are determined. The wavelengths and probabilities of these radiative transitions are presented, along with their cross sections and the rates of electron-impact excitation of the active levels. The results obtained are compared with experimental data and calculations available in the literature.

The effect of a strong magnetic field on the collision-induced change in the polarization characteristics of radiation emitted by excited helium-like ions is considered. Concrete calculations for the C V, Al XII, and Ti XXI ions in the 1 P 1 state showed that magnetic fields of a few hundred tesla lead not only to considerable collisional change in the polarization characteristics of radiation, but also to the induction of new types of ordering, which cannot manifest themselves in the absence of such a field.

An expression for the cross section of light scattering by axisymmetrically aligned atomic systems is derived in a compact form. The cross section under consideration is proportional to the state multipole of the fourth rank. The effect of the second-order alignment determined by the fourth-rank state multipole on the polarization and the angular distribution of scattered light is investigated. The polarization of incident light can be arbitrary and, in the general case, is specified by the Stokes parameters. In particular, it is demonstrated that the second-order alignment cannot induce circular polarization of scattered light and does not influence the dissipation-induced effects of circular dichroism associated with the first-order alignment but leads to a change in the angular distribution and the degree of linear polarization of the scattered light.

The results of nonempirical calculations of the electronic structure, spectroscopic constants, and potential-energy curves for the covalent and triplet ionic-pairing states of the F2 molecule are presented. It is shown that the covalent 13Πu state is weakly bound, with the depth of the potential well of this state being of about 0.05–0.2 eV. The effect of the spin-orbital interaction on the ionic-pairing states, which can result in the perturbation of the Ω=0+ and Ω=1 components of the 3Σ u and 3Πu states, is analyzed. It is demonstrated that the lasing observed at λ ≈ 157 nm is caused by the transitions between the ionic-pairing state 23Πg and the weakly bound covalent state 13Πu.

The frequencies of the normal vibrations and the intensities of the bands in the IR and Raman spectra of pyridine N-oxide and three of its isotopomers are calculated with the density functional theory by using the Becke exchange and Lee-Yang-Parr correlation functionals with the 6−311+G(d, p) basis set. The scaling of the force constants in independent natural coordinates is performed. The force field of pyridine N-oxide in independent symmetry coordinates is obtained. A new assignment of the frequencies of some vibrations in the isotopomers is proposed. On the basis of the calculated frequencies and intensities, the vibrational IR and Raman spectra of pyridine N-oxide are modeled.

Polarizability of two interacting molecules N2 and O2 by M. A. Buldakov; B. V. Korolev; I. I. Matrosov; V. N. Cherepanov (185-190).
In the context of the Silberstein theory, by introducing model atoms, the polarizability of pairs of interacting molecules N2-N2, O2-O2, and N2-O2 was studied in relation to the mutual orientation and the intermolecular distance of the molecules in the pairs. The polarizability tensor for the equilibrium configurations of the (N2)2 and (O2)2 dimers was calculated.

The intensities of vacuum-ultraviolet emission of excimers in a krypton-xenon mixture are calculated in terms of the homogeneous model of a barrier discharge. The mechanisms of the formation of excimer molecules are investigated at different xenon concentrations. The obtained dependences of the efficiency of vacuum-ultraviolet emission on the reduced electric field E/N (where E is the electric field strength and N is the particle concentration of the gas) and the xenon content exhibit a plateau and a subsequent steep decline. The efficiency of vacuum-ultraviolet emission reaches a maximum at the plateau when the xenon content δ is approximately equal to 40%. The concentrations of xenon excimers can be as high as 1014–1015 cm−3. This is comparable to the concentrations in a barrier discharge in pure xenon.

An analytical linear-response theory based on a phenomenological molecular model is proposed. The complex permittivity ε*=ε′+iε′’ of liquid water (H2O) in the submillimeter-wavelength region is described in terms of the harmonic oscillator model. This description is based on the assumption that the specific features of the spectrum are associated with the damped vibrations of hydrogen-bonded water molecules. The calculated frequency dependences of the real (ε′) and imaginary (ε′’) parts of the permittivity at room temperature are in agreement with available experimental data. The contribution of the vibrations to the absorption band located at ∼200 cm−1 is determined. The nonspecific contribution to this band due to reorientations of polar water molecules in a rectangular intermolecular potential well is evaluated. The dielectric response to reorientations is calculated within the hybrid model, which is also applied to the description of the main (Debye) relaxation region and the librational absorption band (centered at ∼650 cm−1) of liquid water. The harmonic oscillator-hybrid model proposed makes it possible, for the first time, to describe two processes in the framework of a unified approach. The first (vibrational) process in the range from 10 to ∼300 cm−1 is specific to systems with hydrogen bonds and includes the dielectric response associated with the translational degrees of freedom of dipoles in an aqueous medium. The second (nonspecific) process involves the dielectric response to orientational motion of a polar water molecule and is responsible primarily for the Debye relaxation region and the librational absorption band. The fundamental differences between D2O and H2O are considered within the molecular model.

The reflectance and transmittance of light by plane-parallel crystal plates were found in the exciton-transition spectral region upon oblique incidence of light with polarization in which two transverse and one longitudinal normal exciton-polariton wave are excited. The reflection spectra calculated for a model crystal with the parameters of the CdSe crystal are analyzed for different thicknesses of the plates, angles of incidence, and damping constants. It is shown that accounting for additional exciton-polariton waves and multiple reflections of light inside the plate results in the possibility of an abrupt change in the phase (by more than 2π) in the vicinity of the exciton transition.

Results of studying electro-optic size effects and nonlinear electro-optic effects in ferroelectric PLZT ceramics are presented. It is established that the presence of electro-optic size effects makes microvolume samples of PLZT ceramics promising for designing efficient modulating devices.

Contrast of a diffraction pattern by V. A. Tarlykov (221-224).
The effect of spatial filtering of a diffraction pattern on the diffractometry of microobjects is examined. The ratio of the amplitudes of the harmonics of the absolute value of the Fourier spectrum of a leveled diffraction pattern is suggested for estimating of the contrast of this diffraction pattern.

The interaction between a solitary electromagnetic wave and a narrow layer with an increased electron concentration in a semiconductor superlattice in a preset uniform alternating electric field directed along the superlattice axis is investigated. The model of the electron energy spectrum in the superlattice is chosen in the strong coupling approximation taking into account the second harmonic. It is shown that, for certain relations between the amplitude and frequency of the alternating electric field, a solitary electromagnetic wave approaching the layer with an increased electron concentration can be trapped by this layer.

A thermocapillary depression is induced by a He-Ne laser beam in a plane-parallel layer of a transparent liquid on an absorbing substrate. The thermocapillary response is investigated as a function of the laser beam ellipticity, the laser power, and the layer thickness.

Generation of singular beams in uniaxial crystals by A. V. Volyar; T. A. Fadeeva (235-244).
The process of generation and transformation of the structure of singular beams both coherent and incoherent, in a crystal-polarization compensator-polarizer system is studied. It is shown that this system can transform the topological charge of the initial beam. The value of the transformed charge depends both on the structure of the initial beam and on its polarization. The initial singular beams, transferring topological multipoles, are shown to acquire unique properties after passing through the system. This system, in particular, makes it possible to control the position of bound optical vortices and the magnitudes of their angular momenta, which may find practical application in devices for trapping, transportation, and mutual arrangement of microparticles.

The propagation of an extremely short (one cycle long) pulse of an electromagnetic field in a medium whose resonant transition is characterized by both diagonal and off-diagonal matrix elements of the dipole-moment operator is considered theoretically. The set of Maxwell-Bloch equations without the approximation of the slowly varying envelopes is used. Two types of solutions of this set, which are found describing the steady-state propagation of an electromagnetic pulse in such a medium.

We discuss propagation of an ultimately short (single-cycle) pulse of an electromagnetic field in a medium whose dispersion and nonlinear properties can be described by the cubic-quintic Duffing model, i.e., by an oscillator with third-and fifth-order anharmonicity. A system of equations governing the evolution of a unidirectional electromagnetic wave is analyzed without using the approximation of slowly varying envelopes. Three types of solutions of this system describing stationary propagation of a pulse in such a medium are found. When the signs of the anharmonicity constants are different, then the amplitude of a steady-state pulse is limited, but its energy may grow on account of an increase in its duration. The characteristics of such a pulse, referred to as an electromagnetic domain, are discussed.

The diffraction by plane apertures of linearly polarized electromagnetic ultrashort pulses described by a delta distribution is analyzed within a new approach. Calculations are made explicit for diffraction by slits and circular apertures.

A nonlinear effect of spatiotemporal transformations of a focused linearly polarized Gaussian beam of a He-Ne laser obliquely incident (S and P polarizations) upon an As2S3-Ag film containing a small amount of chlorine and deposited onto a glass substrate is found and interpreted. The effect consists in the appearance, in the process of exposure, of beams diffracted and anisotropically scattered by periodic structures arising due to excitation of the waveguide TE0 and TM0 modes in the film. The diffraction measurements of the effective refractive indices for the TE0 and TM0 modes make it possible to calculate the refractive index and the thickness of the film from dispersion equations. Along with slow evolution of the diffraction and scattering patterns, we have found a persistent optical turbulence in the fringes of scattering from the structures related to excitation of the TE0 modes. It is suggested that the turbulence is an indication of existence of reversible processes in the mechanism of photosensitivity.

On aperture averaging of speckle patterns by V. V. Ragul’skiĭ; V. G. Sidorovich (270-272).
Statistical parameters of the power of a nonuniform (speckled) light field measured by a photodetector with a finite receiving aperture were investigated. Such a field was created by passing coherent laser radiation through a multimode optical fiber and then scanned across the aperture. It was demonstrated that the mean square deviation of the received power is defined by a parameter equal to the ratio of the aperture diameter to the characteristic dimension of the spatial variations of the light field. If this ratio is more than 2, the above deviation does not exceed values permissible for devices using spatially coherent light (e.g., for free-space optical communication systems).

Propagation of polarized light in media with large discrete inhomogeneities by E. E. Gorodnichev; A. I. Kuzovlev; D. B. Rogozkin (273-285).
An approximate analytical method of solution of the vector radiative-transfer equation for an optically isotropic medium with large-scale inhomogeneities is proposed. The method is based on an assumption about the distinct anisotropy of single scattering. The method was used to calculate the polarization characteristics of light multiply scattered in a monodisperse medium with large spherical particles.

The algorithm and results of calculations of the relative energy density in an image point on a square area element are described. The energy distribution in the image point is calculated for aberration-free optical systems with circular and annular pupils in the Fraunhofer diffraction approximation. Longitudinal and transverse displacements of the center of the square area element relative to the paraxial image of the point are considered.

Coherent soft X-ray (λ=18.9 nm) generation in a transient gain scheme in molybdenum plasma by R. A. Ganeev; T. Kanai; A. Ishizawa; T. Ozaki; H. Kuroda (291-294).
We investigate the threshold conditions for coherent soft x-ray generation using a compact hybrid (neodymium-doped glass-sapphire titanate) laser with an output energy of several hundred millijoules. Collisional excitation of molybdenum ions in a longitudinal-pumping scheme was used for x-ray (λ=18.9 nm) generation. The generated radiation exhibited a low divergence and a strong dependence on the delay between picosecond and femtosecond pumping radiations.

Generation of harmonics of femtosecond radiation from the surface of aluminum targets by R. A. Ganeev; A. Ishizawa; T. Kanai; T. Ozaki; H. Kuroda (295-299).
The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I∼2×1017 W cm−2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<1017 W cm−2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.

Different schemes of fiber ring interferometers (FRIs) with a broadband nonmonochromatic radiation source manufactured on the basis of air-silica microstructured single-mode optical fibers (SMOFs) are considered. This source is close in spectral characteristics to a white light source, because the width of its emission spectrum is comparable to the mean wavelength. It is shown that an increase in the width of the spectrum of the radiation source can lead to either a substantial decrease or an increase in the zero drift, depending on the radiation polarization at the FRI entrance. The latter fact has defied explanation within simple phenomenological models of random coupling between polarization modes in SMOFs of an FRI circuit. The observed increase in the zero drift of the FRI can be explained in terms of the dependence of the parameter of polarization conservation (the parameter h) on the light wavelength for highly anisotropic SMOFs. This dependence is weak for nonmonochromatic radiation sources with a relatively small spectral width, for example, superluminescent diodes, which are traditionally used in FRIs. In contrast, for substantially more broadband radiation sources (including air-silica SMOFs), the above dependence is well pronounced and can lead to a number of undesirable effects in FRIs. Different variants of the FRI design are analyzed. It is demonstrated that the zero drift can be noticeably decreased with an increase in the width of the spectrum of the radiation source for an arbitrary radiation polarization at the entrance of an FRI with a depolarizer of nonmonochromatic radiation and a circuit fabricated from a weakly anisotropic SMOF, for which the parameter h does not depend on the light wavelength. The numerical estimates are obtained.

The propagation of a spikelike ultrashort laser pulse in a mirror waveguide of rectangular cross section with perfectly conducting walls is considered. An algorithm for the calculation of the propagation of a field pulse in such a waveguide is designed. The general approach is illustrated by calculation of the propagation of a p-polarized field pulse shorter than the dimensions of the waveguide cross section.

In commemoration of the 80th birthday of Ivan Prokhorovich Zapesochnyi by Professor O. B. Shpenik; A. N. Zavilopulo (316-317).