Search

Positron annihilation lifetime spectroscopy was used to examine grown-in defects in Cd1–x Zn x Te mixed crystals as a function of Zn content (x = 0, 0.07, 0.11, 0.49, 0.9, 0.95, 1) and measuring temperature. All samples were prepared using the high-pressure modified vertical Bridgman–Stockbarger method. The crystal structure and material phase were characterized by X-ray diffraction. The positron lifetime spectra reveal the presence of both open volumes and shallow traps regardless of the sample composition. In particular, both average and bulk lifetimes are found to be much higher in ternary alloys (CdZnTe) than those in binary systems (CdTe and ZnTe). This originates from distinct differences in average electron densities and the nature of open-volume defects between binary and ternary samples. Competition in positron trapping with increasing Zn content is observed between defects characteristic for both structural systems. Moreover, a clear correlation is shown between defects and the lattice thermal conductivity of studied samples. The applicability of the positron trapping model to CdTe-based materials is discussed.

Abstract Elastic cross sections for electron scattering on tetrafluoromethane (CF$$_4$$ 4 ) from 0 up to 5 eV energy are analyzed using semi-analytical approach to the modified effective range theory (MERT). It is shown that energy and angular variations of differential, integral and momentum transfer cross sections can be parameterized accurately by six MERT coefficients up to the energy region of the resonant scattering. In particular, the model is used to determine the depth and the position of the Ramsauer–Townsend minimum as well as the s-wave scattering length. Moreover, we investigate the influence of the dipole polarizability value on the predictions of present model. To further validate our approach, the elastic data are combined with the Born-dipole cross sections for vibrational excitations as the input data for Monte Carlo simulation of electron swarm coefficients. Graphic Abstract

The nonlinear optical responses of Disperse Orange 3 (DO3) dissolved in tetrahydrofuran (THF) with different concentrations were investigated by Z-scan D4σ technique employing 12 ps and 17 ps pulses of Nd:YAG laser at 532 nm and 1064 nm, respectively. The comparison of the experimental results at both wavelengths demonstrates the strong influence of the resonant phenomena related to the linear absorption on the nonlinear response at 532 nm. In particular, DO3 solution was found to exhibit negative resonant nonlinear refraction with negligible saturable and multi-photon absorption. Moreover, the competition between negative and positive nonlinear refractive indices (n2) of the solute and the solvent was observed at low concentrations. The linear dependence of n2 on the latter quantity allowed to estimate an effective second-order hyperpolarizability of DO3 molecule at 532 nm.

In the present work we study the applicability of the binary-encounter-dipole (BED) model and its simpler version the binary-encounter-Bethe model for positron-impact direct ionization. We show that ignoring the exchange and interference effects in the theory of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] yields simple analytical formulas with no fitting parameters that can be used to estimate cross sections over a wide energy range except near-threshold region. To correct this deficiency we combine BED theory with the Wannier-type threshold law derived by Klar [J. Phys. B 14, 4165 (1981)] for positron-impact ionization. We show that such combination is necessary in order to predict cross sections at low positron energies where strong polarization-correlation effects are present during collision and the positronium formation is a dominant scattering process. The present theory is tested for a wide range of targets including helium (He), atomic hydrogen (H), neon (Ne), argon (Ar), molecular hydrogen (${\mathrm{H}}_{2}$), nitrogen (${\mathrm{N}}_{2}$), oxygen (${\mathrm{O}}_{2}$), carbon monoxide (CO), carbon dioxide (${\mathrm{CO}}_{2}$), and methane (${\mathrm{CH}}_{4}$). An extensive comparison with available experiments and theories is done.

Reliable cross-sections for electron-molecule collisions are urgently needed for numerical modeling of various processes important from technological point of view. Unfortunately, a significant progress in theory and experiment over the last decade is not usually accompanied by the convergence of cross-sections measured at different laboratories and calculated with different methods. Moreover the most advanced contemporary theories involve such large basis sets and complicated equations that they are not easily applied to each specific molecule for which data are needed. For these reasons the search for semi-empirical indications in angular and energy dependencies of scattering cross-section becomes important. In this paper we make a brief review of the applicability of the Born-dipole approximation for elastic, rotational, vibrational and ionization processes that can occur during electron-molecule collisions. We take into account the most recent experimental findings as the reference points.

Modi ed e ective range theory in analytical form is used to extrapolate elastic cross-sections down to thermal energies for positron scattering from ve atomic and molecular targets including Ar, H2, N2, C6H6 (benzene) and C6H12 (c-hexane). The derived scattering parameters are then used to estimate the annihilation rates for chosen elements using a simple formula by Gribakin. We show that a combination of two simple models has a potential for accurate parameterization of coe cients describing the interaction of thermal positrons with non-polar targets of low polarizability such as Ar and N2. DOI: 10.12693/APhysPolA.125.829 PACS: 34.80.−i, 34.80.Bm, 34.80.Uv

New experiments on the very low-energy electron and positron scattering allow to verify the old question on applicability of modified effective range theory (MERT). We perform it using an analytical solution of the Schrodinger equation with the long-range polarization potential. In this work two atomic (He, Ar) and molecular (H2, CH4) target are studied using this approach. Total cross sections were used for obtaining parameters characterizing the scattering phase shifts related to the short-range interaction potential; differential cross sections were used for comparison. Differently from previous works, we conclude that MERT with few (2-3) partial waves applies very well up to energies of few eV in all four targets studied. For positrons, reliable experimental data indicate occurrence of zeros in the s-wave phase shifts for all four targets. This should be recognized as Ramsauer-Townsend minima.

Operation of thermonuclear reactors will require knowledge of numerous cross sections for electron interaction with atoms and molecules, largely unknown at present and difficult for experiments. Th...

In this paper we briefly review experimental techniques exploiting all-optical Fourier image processing inside the 4f system in order to characterize nonlinear refractive indices. Advantages and inconveniences of imaging characterization methods are outlined and their sensitivities are compared in the context of thin films measurements (that is small nonlinear phase-shift). Comparison of imaging methods with well-known nonlinear characterization techniques such as degenerate four wave mixing, I-scan and Z-scan is also done. The influence of nonlinear absorption on sensitivity of nonlinear refraction measurements is discussed.

A simple potential model of a rigid sphere combined with an adiabatic dipole polarization (~r -4) is tested for positron-atom and positron-molecule elastic collisions. The numerical model, which is based on the analytical solution of radial Schrodinger equation for r -4 potential, depends solely upon the average dipole polarizability of the target and one adjustable parameter – the radius of a hard core. The validity of model is assessed by an extensive comparative study against numerous experimental cross-sections and theoretical phase-shifts of angular momentum partial waves for positrons scattered elastically by He, Ne, Ar, Kr, Xe, H2, N2 and CH4. In particular it is shown that this very simple approach can be used to model positron elastic collisions with targets characterized by moderate dipole polarizabilities (Ar, Kr, H2, N2) in good agreement with experiments for impact energies covering almost entire range from the positronium formation threshold down to the zero energy.

We report on third order optical nonlinear experimental characterization using Z-scan method through focal shift measurements of a converging lens. The sample is fixed in front of a lens and is illuminated by a collimated beam. The shift of the geometrical focus in the nonlinear regime using input Gaussian beams is related to the nonlinear induced dephasing owing to a simple linear relation. Numerical calculations based on the Helmholtz wave equation are performed. Good agreement is obtained between experimental and theoretical results.

We report Z-scan and eclipsing Z-scan experimental traces measuring diffraction efficiency in the image plane of the 4f system. Using nonlinearly diffracted energy, the sensitivity of both techniques is compared in order to increase the signal-to-noise ratio. The optimization of the optical signal for nonlinear characterization is proposed. A simple linear relation is provided in order to characterize nonlinear refraction from diffraction efficiency measurements. The influence of the nonlinear absorption is discussed.

Experiments performed in recent years on positron scattering from molecular hydrogen indicated a rise of the total cross section in the limit of zero energy, but essentially disagree on the amplitude of this rise. Mitroy and collaborators [J.-Y. Zhang et al., Phys. Rev. Lett. 103, 223202 (2009)] predicted a scattering length somewhat different from values deduced experimentally. Using a Markov chain Monte Carlo modified effective range theory (MCMC-MERT) we show that the prediction of Mitroy and collaborators allows one to validate the recent experimental results and determine possible uncertainties. By comparing the MERT analysis with the fixed-nuclei density functional calculations we also deduce that probably the effect of virtual positronium formation (or coupling to the virtual positronium state) determines an almost constant value of the total cross section from 3 eV up to the positronium-formation threshold.

Normalized experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from isobutanol (C_4H_9OH)are presented. The differential cross sections are measured at incident energies from 1 to 100 eV and scattering angles from 5° to 130°. These cross sections are compared to earlier experimental and theoretical results for isobutanol and n-butanol, as well as to results for smaller alcohols and for alkanes. Further comparisons are made with calculated cross sections for isobutylamine (C_4H_9NH_2)and for smaller amines, including ethylamine (C_2H_5NH_2), dimethylamine (CH_3NHCH_3)the two C_3H_7NH_2 isomers n-propylamine and isopropylamine, and ethylene diamine (NH_2C_2H_4NH_2). The calculated cross sections are obtained using the Schwinger multichannel method. The comparisons illuminate the role of molecular structure in determining the angular distribution of resonantly scattered electrons.

We report on the nonlinear (NL) optical properties of glassy TeO2–GeO2–K2O–Bi2O3 at λ = 800 nm and λ = 1,064 nm. Using the Kerr gate technique with a laser delivering 150 fs pulses at 800 nm, we demonstrated the fast NL response of the samples. The modulus of the NL refractive index, n 2, at 800 nm was ~10−15 cm2/W. The Z-scan technique was used to determine n 2 ≈ +10−15 cm2/W, at 1,064 nm with pulses of 17 ps. The two-photon absorption coefficient, α 2, was smaller than the minimum that we can measure (

Two chalcogenide films with composition Ge25-Sb10+xS65 (x = 0, 10) and Te20As30Se50, called 2S1G and TAS, respectively, are studied. These materials have high linear and nonlinear refractive indices and present interesting photosensitive behavior toward bandgap light. Further, these chalcogenides glasses can be deposited in an amorphous thin ?lm for optical coatings or waveguides. Their properties and limitations, including their photoinduction effects, nonlinear Kerr effect, photodiffusion of silver, and aging, are discussed. © 2010 Chinese Optics Letters.

Modified effective-range theory (MERT) was developed in the 1960s to describe electron and positron scattering from atoms. The theory was frequently used to extrapolate measured total cross sections down to the zero-energy region, which is inaccessible experimentally. However, the applicability of the model was usually limited to the very low energy range where experimental data to be extrapolated are rare. We have proposed [Idziaszek and Karwasz, Phys. Rev. A 73, 064701 (2006)] a different way of employing MERT by exploiting the properties of an analytical solution of the Schr\"odinger scattering equation with the long-range polarization potential. This alternative approach allows the validity of MERT to be extended to higher energies. At present we are applying this procedure for electron and positron scattering on He, H${}_{2}$, Ar, and CH${}_{4}$. The scattering amplitude and the effective-range parameters for $s$ and $p$ partial waves are obtained through a fitting inversion procedure applied to integral cross sections spanning most or all of the elastic region. The derived parameters are then used to obtain differential and momentum transfer cross sections; the agreement with experiments is very good.

We report a theoretical formulation for the nonlinear Kerr-induced focal shift of converging lenses in a high intensity regime. A numerical relation expressing the on-axis intensity of a focused Gaussian beam is derived in the case of a nonlinearly absorbing and diffracting lens induced in a medium. The concept of an effective Fresnel number is used to provide a simple linear relationship between the focal displacement and the nonlinear phase distortions. The influence of nonlinear absorption on the sensitivity of the focal shift measurements is also discussed.

Signal optimization is performed using Z-scan or EZ-scan techniques inside a 4-f system. Third-order nonlinear optical measurements are based on simple expressions obtained by simulation using Helmholtz wave equation through the imaging system.

International audience; Quest for photo-stable amorphous thin films in ternary GexAsySe100-x-y chalcogenide system is reported. Studied layers were fabricated using pulsed laser deposition technique. Scanning electron microscope with energy dispersive X-ray analyzer, Raman scattering spectroscopy, transmittance measurements, variable angle spectroscopic ellipsometry, and non-linear imaging technique with phase object inside the 4f imaging system were employed to characterize prepared thin films. Their photo-stability/photo-induced phenomena in as-deposited and relaxed states were also investigated, respectively. In linear regime, we found intrinsically photo-stable relaxed layers within Ge20As20Se60 composition. This composition presents also the highest optical damage threshold under non-linear optical conditions.

International audience; Determination of the composition with the best optical and physical properties in a sulfide-halide arsenic-free vitreous domain, GeS2-Sb2S3-CsI, is described. The characteristic temperatures: glass transition (Tg) and crystallization temperatures (Tx) are measured to determine the difference Tx−Tg. The nonlinear index and absorption, the linear index are measured on nine samples in the vitreous domain. With an experimental device, cartographies of each property are created. The model permits to find a composition with a good compromise for Tx−Tg, nonlinear index and nonlinear absorption. The model emphasizes the influence of each element on each property.

International audience; We present the results of Z-scan measurements (1064 nm, 17 ps) of nonlinear refractive indices and nonlinear absorption coefficients for different compositions of chalcogenide glasses in GeS2-Sb2S3-CsI system. We show that the simple well known Boling, Glass, and Owyoung model based on the theory of the semiclassical harmonic oscillator can be a useful tool for theoretical predictions of the nonlinear refractive index in these infrared glasses. A quasi-linear behavior is observed relating the nonlinear index and the linear one. Some of the compositions reveal properties potentially useful for all optical switching applications.

A generalization of the I-scan method is considered by using one, two, three and four waves mixing experiments inside a 4-f imaging system for nonlinear optical measurements. By placing the material at the focus, the sensitivities of the measurements are compared systematically showing the same order of magnitude for all the studied configurations. Moreover the determination of the cubic optical nonlinearity is simplified by providing quadratic relations that relate the nonlinear phase shift to the measured signal. Experimental and simulated images are presented to validate our approach.

International audience; We discuss the influence of photoinduced effects (PIEs) on the measurements of nonlinear refractive indices and nonlinear absorption coefficients. A chalcogenide glass film Te20As30Se50 was studied using picosecond laser pulses at 1064 nm. The nonlinear imaging technique with phase object (NIT-PO) and the Z-scan technique were applied and their results are compared. The NIT-PO technique reveals clearly the influence of PIE on the samples' response, while by using the Z-scan technique we measured the deepness of ablated regions (holes) produced during the measurements.

We report experimental characterization of a very small rectangular phase shift (

Date du colloque: 02/08/2009; International audience; Using a 4-f imaging system for nonlinear optical measurements we deal with multi-wave mixing experiments. The complex degenerate four-wave mixing experimental setup is simplified in order to characterize nonlinear materials. Moreover, a generalization of the I-scan method is studied by considering one, two, three and four waves mixing experiments. The determination of the cubic optical nonlinearity is possible by providing quadratic relations that relate the nonlinear phase shift to the measured signal. The sensitivities of the measurements are compared systematically showing the same order of magnitude for all the studied configurations. Experimental and simulated images are presented here to validate our approach.

We analyze the influence of different objects on the sensitivity of using a 4f imaging system for nonlinear optical measurements. By defining the diffraction efficiency as the physical quantity to measure, it is possible to perform different measuring methods (Z-scan, eclipsing Z-scan, I-scan, degenerate four-wave mixing) by matching the field stop in the image plane with the object at the entry. One, two, three, and four waves mixing are considered in order to compare their related sensitivities. We provide simple quadratic relations for each object that allow the characterization of the cubic optical nonlinearity. A systematic comparison is done showing that one circular aperture object gives the highest sensitivity. The experimental measurements are performed in order to validate our simulation.

We report theoretical as well as (normalized) experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from chloroethane, C_2H_5Cl, also known as ethyl chloride. The theoretical cross sections were computed using the Schwinger multichannel variational method in the single-channel approximation, with polarization effects included via virtual excitations. Cross section measurements were made at incident energies ranging from 1 to 30 eV and at scattering angles from 10° to 125°. We compare our data to previous results for C_2H_5Cl and for the related molecule chloromethane.

We report theoretical as well as (normalized) experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from chloromethane, CH_3Cl, also known as methyl chloride. The theoretical cross sections were computed using the Schwinger multichannel variational method in the single-channel approximation, with polarization effects included via virtual excitations. Cross section measurements were made at incident energies ranging from 0.5 to 100 eV and at scattering angles from 5° to 125°. We compare our data to earlier previous results for this molecule.

A comparative study about positron annihilation in gas and liquid phases of two non-polar ring molecules: benzene (C6H6) and cyclohexane (C6H12) is presented including the most recent experimental and theoretical achievements. In addition the preliminary results of positron annihilation lifetime measurements in a liquid phase at room temperature for these two molecules are reported.

The elastic cross sections for electron scattering on krypton from zero energy up to 10 eV have been analyzed by the modified effective range method. A simple model based on the analytical solution of the Schrodinger equation with the polarization potential using explicitly determined scattering phase shifts for the three lowest partial waves describes the elastic differential, integral and momentum transfer cross sections up to the energy threshold of the first inelastic process well. In detail, the contribution of the long-range polarization potential to the scattering phase shift is exactly expressed, while the contribution of the short-range effects is modelled by simple quadratic expressions (the effective range expansions). The effective range parameters are determined empirically by comparison with the latest experimental differential cross sections. Presently, the calculated integral and momentum transfer cross sections are validated against numerous electron scattering experiments and the most recent quantum-mechanical theories. To complete the picture, the two-term Boltzmann analysis is employed to determine the electron transport coefficients; the agreement with the electron swarm experimental data is found to be very good.

We introduce a Z-scan technique as a tool to characterize small phase shift (

International audience; This study deals with a multiwave mixing experiment using Z-scan technique. We show that it is possible to simplify the complex degenerate four-wave mixing(DFWM) experimental setup in order to characterize nonlinear materials. One object composed of three circular apertures is used as an input in a 4 f coherent imaging system. When the nonlinear material is moved around the focus, we observe the variations of the forward fourth wave at the output as well as other diffracted waves. Experimental and simulated images are presented here to validate our approach. We show also that this method increases significantly the sensitivity of the measurement compared to DFWM. We provide a simple quadratic relation that allows the characterization of the cubic optical nonlinearity.

Modified effective range theory formulated as a Bayesian statistical model through the combination with Markov Chain Monte Carlo integration and fitting tech- niques is used to check the compatibility of different e − −Xe scattering data such as the total cross-sections, the momentum transfer cross-sections, and the differential cross-sections that were determined experimentally in the region of Ramsauer-Townsend minimum. On the basis of this predictive approach, the most probable value of the scattering length, (−6.51 ± 0.05)a0, is proposed. The present analysis suggests that the non-relativistic spinless effective range theory is suitable for the description of angu- lar and energy dependencies of e − −Xe elastic scattering

Elastic cross-sections for electron scattering on neon from 0 energy up to 16 eV are analyzed by an ana- lytical approach to the modified effective range theory (MERT). It is shown that energy and angular variations of elastic differential, integral and momentum transfer cross- sections can be accurately parameterized by six MERT coefficients up to the energy threshold for the first Feshbach resonance. MERT parameters are determined empirically by numerical comparison with large collection of available experimental data of elastic total (integral) cross-sections. The present analysis is validated against numerous electron beams and swarm experiments. The comparison of derived MERT parameters with those found for other noble gases, helium, argon and krypton, is done. The derived scattering length (for the s-partial wave) in neon, 0.227a0, agrees well with recent theories; it is small but, differently from Ar and Kr, still positive. Analogue parameters for the p-wave and the d-wave are negative and positive respectively for all the four gases compared.

Electron-scattering cross sections in methane are analysed in the very-low energy region. The correspondence between integral elastic, elastic differential and momentum transfer cross sections is checked via a novel approach to modified effective range theory, in order to determine the depth and position of the Ramsauer-Townsend minimum. Phase shifts of the two lowest partial waves are obtained explicitly and parameterized by four coefficients with the physical meaning of the scattering lengths and the effective ranges. Using recent experiments on vibrational cross sections performed over an extended (0–180°) angular range and comparing several theories, an agreement within 10% has been obtained between experimental total and present summed (elastic + vibrational) cross sections in the whole 0.1–2.0 eV energy range. An additional check for consistency is done using two-term Boltzmann analysis to derive electron diffusion coefficients. Calculated drift velocities and transversal diffusion coefficients at 0–10 Td reduced electric field agree within 5% with experiments.

Using I-scan or Z-scan techniques inside a 4-f system we perform third-order nonlinear optical characterizations. Simulations based on Helmholtz equation provide simple expressions relating the signal to the phase shift in different wave-mixing configurations.

We report on density functional calculations (DFT) of elastic integral scattering cross-sections for positron collisions with argon, krypton, nitrogen and methane. The long-range asymptotic polarization potential is described using higher-order terms going much beyond an induced dipole potential (−α / r 4) while the short-range interaction is modeled by two different forms of electron – positron correlation potential (Boronski-Nieminen and Quantum Monte Carlo potentials). The results of both approaches agree quite well with the recent theoretical and measured values. Based on the present and previous theoretical and experimental data we discuss some systematics observed in integral scattering cross-sections below the positronium formation threshold. In particular we point out on the correlation between the values of scattering cross-sections and atomic dimensions.