Optics and Spectroscopy (v.103, #6)

Artificial and biological color: The ABCs by H. J. Caulfield; J. Fu (851-854).
Faced with radiation of immeasurable complexity that bears on important distinctions among objects in the world, nature chose not to measure the spectrum, even at low resolution. Rather, biological color measures using two or more broad, spectrally overlapping sensitivity curves and uses the resulting data to compute discriminants (colors) that brains attribute to their percepts of those objects. Artificial color seeks to design sensors and process the resulting data in the same or a very similar way. Here, we seek to provide a unified view of prior and current work in this field.
Keywords: 42.66.Ne

Sol-gel holographic recording materials by P. Cheben; M. L. Calvo; F. del Monte; O. Martínez-Matos; J. A. Rodrigo (855-857).
The advances in the sol-gel holographic recording materials, namely in the photopolymerizable compositions and photorefractive glasses, are reviewed. A new sol-gel holographic material is also discussed that exploits diffusion of a high-refractive-index MA:Zr complex, with the refractive index modulation of n ∼ 10−2 and negligible scattering.
Keywords: 42.40.-i

Volume birefringence or surface relief gratings are optically inscribed on the surface of azopolymer thin films using polarized light from a moderate power laser. The inscription is done in a one step process and the images are stable over extended periods of time. Multiple holographic images can be written locally and these erasable sub-wavelength structures are used to produce a variety of optical devices.
Keywords: 78.20; 78.68

A multicomponent lattice Boltzmann model including external electric forces interactions is developed to study the behavior of a classical thermoplastic film which is used as a holographic recording material. The model incorporate also the spinodal decomposition originated in this type of polymeric fluids when a fast thermal pulse is used to develop the hologram. The surface deformation of the electrically charged thermoplastic film is simulated using the lattice Boltzmann model and the main holographic parameters, i.e., spatial frequency response (MTF) and diffraction efficiency are given and related to the hydrodynamic behavior of the fluid. The influence of the phase transition on the growth rate of the deformation and, hence, on the MTF response of the medium is studied and correlated with experimental results. The results obtained with our model are in agreement with those obtained using more complex theoretical analysis.
Keywords: 41.20.Cv; 42.40.-i; 42.70.Ln; 47.35.Bb

The fractional Fourier transform is a signal processing tool which is strongly associated with optical data manipulation. It has fast computational algorithms and it suggests solutions to interesting signal processing tasks. In this paper we review its properties as well as present a new set of its applications for blind source separation of images and for RF photonics (a field in which photonic devices are used to process RF signals).
Keywords: 02.30.Nw; 42.30.Kq

Motion estimation of magnetic resonance cardiac images using the Wigner-Ville and hough transforms by N. Carranza; G. Cristóbal; P. Bayerl; H. Neumann (877-885).
Myocardial motion analysis and quantification is of utmost importance for analyzing contractile heart abnormalities and it can be a symptom of a coronary artery disease. A fundamental problem in processing sequences of images is the computation of the optical flow, which is an approximation of the real image motion. This paper presents a new algorithm for optical flow estimation based on a spatiotemporal-frequency (STF) approach. More specifically it relies on the computation of the Wigner-Ville distribution (WVD) and the Hough Transform (HT) of the motion sequences. The latter is a well-known line and shape detection method that is highly robust against incomplete data and noise. The rationale of using the HT in this context is that it provides a value of the displacement field from the STF representation. In addition, a probabilistic approach based on Gaussian mixtures has been implemented in order to improve the accuracy of the motion detection. Experimental results in the case of synthetic sequences are compared with an implementation of the variational technique for local and global motion estimation, where it is shown that the results are accurate and robust to noise degradations. Results obtained with real cardiac magnetic resonance images are presented.
Keywords: 87.61.-c

Invariant parameters for characterizing nonuniformly partially polarized beams by R. Martínez-Herrero; P. M. Mejías (886-889).
In terms of the so-called Wigner-Stokes formalism, within the framework of the second-order irradiance moments of the field, new overall parameters are introduced to characterize the polarization structure of nonuniformly partially polarized beams. These parameters are shown to be invariant upon rotation of the transverse Cartesian coordinate axes, which define the electric field components. Moreover, such parameters are also invariant under propagation through general rotationally symmetric ABCD optical systems.
Keywords: 42.25.Ja; 42.55.-f; 42.60.Jf

Nonlocal properties of entangled two-photon generalized binomial states in two separate cavities by R. Lo Franco; G. Compagno; A. Messina; A. Napoli (890-894).
Entangled two-photon generalized binomial states of the electromagnetic field in two separate cavities are considered. The nonlocal properties of this entangled field state are analyzed by studying the electric field correlations between the two cavities. A Bell’s inequality violation is obtained using an appropriate dichotomic cavity operator that is, in principle, measurable.
Keywords: 42.50.Dv; 03.65.Ud; 03.67.Mn

The oscillator strengths f of the resonance transitions are calculated by the Dirac-Fock method for the first ten terms of the rubidium isoelectronic sequence. The variation of f values along the isoelectronic sequence is analyzed. The corresponding transition probabilities and lifetimes τ of the levels 5p 2 P 1/2 and 5p 2 P 3/2 are calculated. The dependence of τ on the ion charge is approximated by an analytical equation. The obtained values of f and τ are compared with experiment and other calculations.
Keywords: 03.65.-w; 32.70.Cs

The geometric symmetry group of the internal dynamics of molecules of model of description of molecules of dimethyl ether and acetone is obtained taking into account the torsional motion of identical methyl tops. On this basis, a rigorous model for the description of the spectra of these molecules in the ground electronic-vibrational state is constructed. The model yields a simple algebraic scheme for the calculation of the energy level positions in the energy spectrum and of the transition intensities between them. A new type of rotational electric dipole transitions is predicted.
Keywords: 31.15.Hz

Taking as examples the emission spectra of low-temperature dense plasma of organic composition obtained in jets of capillary and slot discharges with an evaporating wall, it is shown that some lines of singly ionized carbon that are broadened under the conditions of significant screening of plasma microfields can be used as reliable “standards” for determination of the electron concentration.
Keywords: 52.25.-b

The energy and spectral characteristics of a barrier discharge in a mixture of iodine vapor with krypton have been investigated theoretically and experimentally. The emission spectrum consists of the single I*2 molecular band D′ → A′ peaking at 342 nm, the iodine resonance line at 206.2 nm, and the group of bands corresponding to iodine emission. The highest intensity of the I*2 (342 nm) band was obtained at a mixture pressure near 450 Torr. A mean output power and an efficiency of 550 mW and 1.6%, respectively, have been achieved. It is shown that, for the I2 barrier-discharge excilamp, the homogeneous, rather than filamentary form of discharge glow is optimal from the viewpoint of the highest mean output power. The maximal calculated value of the emission efficiency for the 342-nm band was 5%. The main processes determining energy losses in plasma have been found, and ways to increase the efficiency of emission in the D′ → A′ band of the I*2 molecule have been proposed.
Keywords: 52.25.Os; 52.25.Dg

Nonadiabatic transitions upon atomic collisions by A. K. Belyaev; D. V. Vlasov; A. M. Kas’yanova (920-924).
The standard interpretation of the adiabatic approach and the t matrix method for the calculation of the probabilities of nonadiabatic transitions and inelastic cross sections in atomic collisions is analyzed, and a model problem of collisions of hydrogen atoms with neutrons is considered. It is shown that the application of the standard approach results in a physically inconsistent result—considerable values of the excitation cross sections of hydrogen atoms by neutrons. The application of the t matrix method improves the result by two to three orders of magnitude even for a restricted basis set.
Keywords: 34.10.+x; 34.50.Fa

A method for determining the near-field image of a semiconductor surface with inhomogenous distribution of carriers has been proposed. This method is based on the linear response theory. As a result, the solution to the self-consistent Lippmann-Schwinger equation is expressed in terms of the effective susceptibility. A calculation is carried out for a semiconductor surface with inhomogenous distribution of electrons under the surface. It is shown that polarizations of both the probe field and the field at the detector in the far zone significantly affect the near-field distribution. The results obtained suggest that the orientation of aggregates (defects) under the semiconductor surface can be determined. The approach developed is universal and can be used for simulation in the scanning near-field microscopy for different types of objects under the surface.
Keywords: 07.79.Fc; 72.20.Jv; 73.25.+i

Spectral study of the properties of photochromic polysubstituted lactones by Yu. P. Strokach; O. I. Kobeleva; T. M. Valova; V. A. Barachevsky; D. V. Pashchenko; B. V. Lichitskiĭ; M. M. Krayushkin (936-942).
The spectral and photothermal properties of thermally irreversible derivatives of dihetarylethenes, lactones, whose open form shows fluorescence, are studied. It is shown that the nature of substituents determines not only the absorptive but also the luminescent properties of these compounds, as well as their photocoloration and fluorescence quantum yields. The obtained dependences of the properties of lactones on their structure are important for targeted synthesis of compounds that can be used in the creation of two-photon reversible recording media with fluorescence reading of optical information suitable for application in 3D optical memory on optical disks.
Keywords: 82.50.Hp

Investigation of the optical constants of arsenic chalcogenide films in the wavelength range 0.5–2.5 μm by E. N. Kotlikov; V. A. Ivanov; V. A. Krupennikov; B. A. Tallerchik; A. N. Tropin (943-946).
Thin films of chalcogenide glasses deposited on quartz glass substrates by thermal evaporation in vacuum have been investigated. The dependences n(λ) and k(λ) for films of different composition have been determined from the transmission spectra. Expressions of the n = A + BL + CL 2 + 2 + 4 type (L = (λ 2 − 0.028)−1 and A, B, C, D, and E are constants) for calculating the refractive indices of As2Se3, AsSe4, AsS4, and AsS16.2Se16.2 films in the wavelength range from 0.5 to 2.5 μm are reported.
Keywords: 78.20.Ci

Peculiarities of the reflection spectra of PECVD nanocrystalline porous silicon films by V. A. Vikulov; A. M. Maslov; A. A. Dimitriev; V. V. Korobtsov (947-951).
Photoluminescent nanocrystalline porous silicon films have been formed on silicon substrates by plasma-enhanced chemical vapor deposition. In the visible spectral range, a nontrivial character of the reflection spectra (dependent on the film thickness) has been found. The sizes of crystallites and the root-mean-square roughness of the deposited film surface have been determined by X-ray diffraction and atomic force microscopy. It is shown that the significant decrease in the reflectance in the spectral range from 200 to 900 nm and the red shift of the characteristic peaks in the reflection spectra are related to the sizes of nanocrystallites in the films formed.
Keywords: 78.67.Bf

The spectral and temporal characteristics of the fluorescence of the anionic form of 3-hydroxyflavone in acetonitrile are studied. This form can be selected upon excitation in the region from 380 to 440 nm with the maximum near 420 nm. The fluorescence spectrum of this form has the shape of a wide structureless band peaked at about 470 nm. The lifetimes of the fluorescence of the anionic form in the region from 460 to 530 nm are measured; the average lifetimes do not depend on the recording wavelength in the entire region and are equal to 3.7 ± 0.2 ns. Addition of water to the solution leads to a gradual quenching of the fluorescence and its complete vanishing at a concentration of 10 M. This is a static quenching or quenching of the first kind according to Vavilov’s classification; i.e., it occurs in the ground state.
Keywords: 33.50.Dq

Photoinduced relaxation processes in complexes based on semiconductor CdSe nanocrystals and organic molecules by É. I. Zen’kevich; E. I. Sagun; A. A. Yarovoi; A. M. Shul’ga; V. N. Knyukshto; A. P. Stupak; C. von Borczyskowski (958-968).
Possible pathways and mechanisms of photoinduced relaxation processes in CdSe and CdSe/ZnS nanocrystals that are surface-passivated (as a result of two-point interactions) by nitrogen-containing ligands of different nature (pyridyl-substituted porphyrin molecules and their derivatives, 2,2′-dipyridyl, and 1,10-phenanthroline) are studied in toluene at 295 K by the methods of steady-state and time-resolved spectroscopy. In nanocrystal-organic ligand composites, a high luminescence-quenching efficiency of nanocrystals by molecules of tetrapyrrole compounds compared to 2,2′-dipyridyl, 1,10-shenanthroline, and pyridine can be associated with the electronic properties of a π conjugated macrocycle and anchor groups. The fundamental role that mesomeric effects and the partial overlap of HOMOs and LUMOs of porphyrin and meso-pyridyl rings play in the enhancement of nonradiative recombination of charges in a surface interface layer is substantiated.
Keywords: 78.67.Bf

To experimentally realize quantum computations on d-level basic elements (qudits) at d > 2, it is necessary to develop schemes for the technical realization of elementary logical operators. We have found sequences of selective rotation operators that represent the operators of the quantum Fourier transform (Walsh-Hadamard matrices) for d = 3–10. For the prime numbers 3, 5, and 7, the well-known method of linear algebra is applied, whereas, for the factorable numbers 6, 9, and 10, the representation of virtual spins is used (which we previously applied for d = 4, 8). Selective rotations can be realized, for example, by means of pulses of an RF magnetic field for systems of quadrupole nuclei or laser pulses for atoms and ions in traps.
Keywords: 42.50.-p

The spectrum of absorption of a probe field of arbitrary intensity by three-level atoms with the Λ configuration of levels in the field of a strong electromagnetic wave acting on an adjacent transition is studied theoretically. It is shown that the supernarrow resonance, revealed earlier by us, in the absorption spectrum of the probe field far from the resonance with the atomic transition is retained as the probe field intensity increases. The dependence of the amplitude and of the width of the supernarrow resonance on the probe-field intensity is elucidated. The resonance contrast decreases insignificantly (by only a factor of several), even when the probe-field intensity becomes comparable with the intensity of the strong field.
Keywords: 42.50.Gy; 42.50.Hz; 42.62.Fi

Coherent nonlinear interference effect for devices of long-term memory with femtosecond pulses by S. A. Pul’kin; M. Yu. Savel’eva; É. E. Fradkin; S. V. Uvarova (981-985).
Computer simulation of the response (polarization) of three-level systems (for example, of quantum dots) with the common upper level (the Λ-scheme) was carried out for devices of long-term memory with femtosecond driving and signal pulses. For producing an efficient response, the pulse area should be on the order of π. Under the action of the first driving pulse on the 1–2 transition and the simultaneous action of the probe pulse on the 3–2 transition, a long-lived polarization with the lifetime dependent on the lifetimes of the lower (1 and 3) levels appears owing to the nonlinear interference effect. Under the action of the second driving pulse, a signal pulse appears on the 3–2 transition, whose amplitude is proportional to the amplitude of the input probe pulse and exponentially decreases with the delay time between the first and the second driving pulses. The set of equations for the density matrix was solved numerically in the semiclassical approximation and in the rotating wave approximation. The temporal evolution of the nondiagonal matrix elements reflects the temporal evolution of the nonlinear polarization. The polarization spectrum is found by the Fourier transform of the time dependence of the nondiagonal elements. The polarization spectrum (the power spectrum) possesses an interference structure with a fringe period governed by the delay time between the first and the second driving pulses. The inverse Fourier transform reconstructs the temporal shape of the pulses.
Keywords: 78.47.+p

The use of three-photon absorption for optical limiting of laser radiation by R. A. Ganeev; A. I. Ryasnyanskiĭ; M. K. Kodirov; F. R. Akhmedzhanov; T. Usmanov (986-989).
The results of a study of nonlinear refraction and nonlinear absorption at a wavelength of 532 nm in an LGO (Li2Ge2O15) crystal by Z-scan methods are presented. It is shown that the nonlinear refraction is caused by self-focusing of the laser radiation, whereas the nonlinear absorption is three-photon in nature. The possibility of using an LGO crystal for producing the optical limiting of laser radiation of picosecond duration is demonstrated.
Keywords: 42.65.-k

A method is proposed to compensate for the influence of the group velocity mismatch on the frequency conversion efficiency for any ratio between the coefficients of square and cubic nonlinearities. The method allows the limiting efficiency of the frequency doubling to be achieved. This efficiency corresponds to the cases of phase and group synchronisms. For this purpose, it is necessary to apply a low-frequency phase modulation of the input radiation at the frequency of the first harmonic in combination with the optimal selection of the wavenumber mismatch. In practice, under certain conditions, this modulation can be obtained only with the aid of the wavenumber mismatch, whose optimal value is determined by the group velocity mismatch. The generation efficiency can be increased several times by introducing a phase shift in certain cross sections of the medium.
Keywords: 42.65.Ky

Model description of the degree of polarization of nonstationary light waves by A. L. Purtseladze; V. I. Tarasashvili (1000-1002).
A theoretical approach is developed that provides the possibility of representing partially polarized radiation in the form of a sum of mutually orthogonal polarized and incoherent components and in the form of a model device with a time-dependent circular birefringence. This approach allows one to apply the vectormatrix Jones method in calculations. The possibility of theoretically determining the degree of polarization of the light passed through a corresponding device placed in an external magnetic field is shown as an illustration.
Keywords: 42.25.Ja

Polarization-holographic reversal of the front of a reconstructed wave by V. I. Tarasashvili; A. L. Purtseladze (1003-1006).
The formation of an image by a wave conjugate to the reference wave is considered theoretically. It is shown that the polarization hologram formed upon summation of two beams with mutually orthogonal circular polarizations does not yield conjugate or ghost images in the course of reconstruction. The application of the photoinduced anisotropy in polarization-sensitive dynamic media for the correction of laser radiation with a complex polarization distribution over the wavefront is proposed and tested experimentally. The installation of polarization-holographic correctors both outside the cavity and in the intracavity volume of the laser provides the possibility of achieving divergences close to the diffraction limit at a high energy output.
Keywords: 42.25.Ja

Tunable Bragg gratings for fiber lasers by S. R. Abdullina; S. A. Babin; A. A. Vlasov; S. I. Kablukov (1007-1011).
A tuning method of a fiber Bragg grating by the bending compression-tension is considered. With this method, a relative tuning range of the resonance wavelength of the grating of about 5% has been obtained. The changes in the shape of the spectrum and in the grating reflectivity during the tuning process are studied. The reflectivity of the fiber Bragg grating has been found to considerably increase with increasing degree of compression. It is shown that such an increase is caused by compression-induced changes in the refractive index modulation amplitude in the grating. The fiber Bragg grating developed has been applied for tuning the radiation wavelength of an ytterbium fiber laser. The tuning range of the laser has been found to be 45 nm around the central wavelength 1080 nm.
Keywords: 42.55.Wd