Giant photoinduced optical bleaching at room and liquid helium temperatures in Sb/As(2)S(3) multilayered films

We report the observation of giant photo induced optical bleaching in Sb/As(2)S(3) multilayered film at room and liquid He temperatures, when irradiated with 532 nm laser at moderate intensities. The experimental results show a dramatic increase in transmittance near the band gap regime at both the temperatures; however the rates at which transmission change occurs are rather slow at low temperature. The huge change in transmission is due to the photo induced intermixing of As(2)S(3) layer with Sb. Our XPS measurements show that photo induced intermixing occurs through the wrong homopolar bonds, which under actinic light illumination are converted into energetically favored hetropolar bonds. (C) 2011 Elsevier B.V. All rights reserved.

Computation of thermodynamic properties of liquid ferrous alloys from structural measurements

Relation between X-ray scattering intensities, mean square thermal fluctuations and thermodynamic properties. High temperature X-ray diffraction study of liquid Fe-Ni and Fe-Si alloys using reflection and transmission geometries. Calculation of the structure factor as a function of wave vector. Extrapolation to zero wave vector. Calculation of the concentration-concentration correlation function defined by A. B. Bhatia and D. E. Thorton. Computation of thermodynamic quantities of mixing A G, LlH and LlS for the binary alloys. Comparison with direct thermodynamic measurements reported in the literature.

Anomalous temperature dependence of phonons and photoluminescence bands in pyrochlore Er(2)Ti(2)O(7): signatures of structural deformation at 130 K

We report a Raman study of single crystal pyrochlore Er(2)Ti(2)O(7) as a function of temperature from 12 to 300 K. In addition to the phonons, various photoluminescence (PL) lines of Er(3+) in the visible range are also observed. Our Raman data show an anomalous red-shift of two phonons (one at similar to 200 cm(-1) and another at similar to 520 cm(-1)) upon cooling from room temperature which is attributed to phonon-phonon anharmonic interactions. However, the phonons at similar to 310, 330, and 690 cm(-1) initially show a blue-shift upon cooling from room temperature down to about 130 K, followed by a red-shift, indicating a structural deformation at similar to 130 K. The intensities of the PL bands associated with the transitions between the various levels of the ground state manifold ((4)I(15/2)) and the (2)H(11/2) as well as (4)S(3/2) excited state manifolds of Er(3+) show a change at similar to 130 K. Moreover, the temperature dependence of the peak position of the two PL bands shows a change in their slope (d(omega)/d(T)) at similar to 130 K, thus further strengthening the proposal of a structural deformation. The temperature dependence of the peak positions of the PL bands has been analyzed using the theory of optical dephasing in crystals.

EPR, thermo and photoluminescence properties of ZnO nanopowders

Nanocrystalline ZnO powders have been synthesized by a low temperature solution combustion method. The photoluminescence (PL) spectrum of as-formed and heat treated ZnO shows strong violet (402, 421, 437, 485 nm) and weak green (520 nm) emission peaks respectively. The PL intensities of defect related emission bands decrease with calcinations temperature indicating the decrease of Zn(i) and V(o)(+) caused by the chemisorptions of oxygen. The results are correlated with the electron paramagnetic resonance (EPR) studies. Thermoluminescence (TL) glow curves of gamma irradiated ZnO nanoparticles exhibit a single broad glow peak at similar to 343 degrees C. This can be attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centers. The trapping parameters of ZnO irradiated with various gamma-doses are calculated using peak shape method. It is observed that the glow peak intensity increases with increase of gamma dose without changing glow curve shape. These two characteristic properties such as TL intensity increases with gamma dose and simple glow curve structure is an indication that the synthesized ZnO nanoparticles might be used as good TL dosimeter for high temperature application. (C) 2011 Elsevier B.V. All rights reserved.

Evidence for the presence of remnant strain in grey‐tracked KTiOPO4

Grey tracks produced in KTiOPO4 (KTP) by applying a dc electric field have been studied through optical absorption, Raman scattering, and synchrotron x‐ray topography. A study of the optical absorption and Raman scattering from the grey‐tracked region suggests that their formation is accompanied by changes in the electronic levels of Ti4+. There is no evidence for a major structural change or disorder in the grey‐tracked region. However, the x‐ray topographs do indicate the presence of a remnant strain in the lattice, which might contribute to the observed changes in the Raman intensities.

Analysis of a cascade‐pumped 16 μm CO2‐N2 downstream‐mixing gasdynamic laser

The generation of a 16 μm laser beam through cascading in a downstream‐mixing CO2 gasdynamic laser is studied. To simulate actual lasing action, a generalized, two‐dimensional, flow‐radiation‐coupled power extraction model for a gasdynamic laser is used. Also, to model the cascade process a new four‐mode CO2‐N2 vibrational kinetic model has been proposed. The steady‐state intensity obtained for an exclusive 9.4 μm transition is of the order of 5×107 W/m2. In the cascade mode of operation the steady‐state intensities for 9.4 and 16 μm transitions of the order of 5×107 W/m2 and 1.0×106 W/m2, respectively, have been obtained.

Multiphoton absorption in CsLiB6O10 with femtosecond infrared laser pulses

Nonlinear absorption and refraction characteristics of cesium lithium borate (CsLiB6O10) crystal have been studied using Z-scan technique. Ti:sapphire laser with 110 fs pulse width operating at 800 nm wavelength and pulse repetition rate of 1 kHz is used as the source of photons. Intensity of the laser pulse is varied from 0.541 to 1.283 T W/cm2 to estimate the intensity dependence of multiphoton absorption coefficients. Using the theory of multiphoton absorption proposed by Sutherland [ Handbook of Nonlinear Optics, in 2nd ed., edited by D. G. McLean and S. Kirkpatrick, Dekker, New York (2003) ], found that open aperture Z-scan data fit well for the five-photon absorption (5PA) process. 5PA coefficients are obtained by fitting the expressions into the open aperture experimental data for various peak intensities (I00). The nonlinear refractive index n2 estimated from closed aperture Z-scan experiment is 1.075×10−4 cm2/T W at an input peak intensity of 0.723 T W/cm2. The above experiment when repeated with a 532 nm, 6 ns pulsed laser led to an irreversible damage of the sample resulting in an asymmetric open aperture Z-scan profile. This indicates that it is not possible to observe multiphoton absorption in this regime of pulse width using 532 nm laser.

XXVI. The relation between thermo-optic and piezo-optic phenomena in crystals

It is pointed out that the change in refractive index with temperature of a crystal is different from what is calculated from the accompanying change in volume and the piezo-optic coefficients. The difference, which is a pure temperature effect, is explained as being due to the change in polarizability of the atoms produced by a change in the amplitude of vibration. The polarizability (α) can be expanded as a Taylor series in the changes of the distance (r) between the atoms and it is found that while the piezo-optic coefficient depends only on ∂α/∂r, the pure temperature effect is a function of ∂ 2 a/∂r 2. Making use of the experimental data, the values of a and its first two derivatives can be determined. These values are foundto be of the same order as those deduced from the intensities of Rayleigh and Raman scattering of light. The theory predicts that dn/dT should vary as the coefficient of cubical expansion at different temperatures and this is verified to be true. Finally, calculations are made of the thermo- and piezo-optic coefficients, considering the electrostatic interaction between the atoms. These do not adequately explain the observed facts, since no provision is made for the distortion of electron atmospheres around the atoms and the consequent changes in polarizability.

Novel nonlinear opticalphenomenain nematic liquid crystals

In normal materials, the nonlinear optical effects arise from nonlinearities in the polarisabilities of the constituent atoms or molecules. On the other hand the nonlinear optical effects in liquid crystals arise from totally different processes. Also they occur at relatively low laser intensities. In a laser field a liquid crystal exhibits many novel and interesting nonlinear optical effects. In addition we also find laser field induced effects that are peculiar to liquid crystals, like structural transformations, orientational transitions, modulated structures and phase transitions, to name a few. Here we dwell upon a few of these interesting and important nonlinear optical phenomena that exist in nematic liquid crystals.

Ultrasonic degradation of butadiene, styrene and their copolymers

Ultrasonic degradation of commercially important polymers, styrene-butadiene (SBR) rubber, acrylonitrile-butadiene (NBR) rubber, styrene-acrylonitrile (SAN), polybutadiene rubber and polystyrene were investigated. The molecular weight distributions were measured using gel permeation chromatography (GPC). A model based on continuous distribution kinetics approach was used to study the time evolution of molecular weight distribution for these polymers during degradation. The effect of solvent properties and ultrasound intensity on the degradation of SBR rubber was investigated using different pure solvents and mixed solvents of varying volatility and different ultrasonic intensities. (C) 2011 Elsevier B.V. All rights reserved.

An EDC-based turbulent premixed combustion model

In the present study, a new turbulent premixed combustion model is proposed by integrating the Coherent Flame Model with the modified eddy dissipation concept, and relating the fine structure mass fraction to the flame surface density. First, experimental results of turbulent flame speed available from literature are compared with the predicted results at different turbulence intensities to validate the flame surface density model. It is observed that the model is able to predict the turbulent burning speeds accurately. Then, a comprehensive validation is carried out utilizing data on a turbulent lifted methane flame issuing into a vitiated co-flow. Detailed comparison of temperature and species concentrations between experiment and simulation is performed at different heights of the flame. Overall, the model is found to predict both the spatial variation and peak values of the scalars at various heights satisfactorily.

Photo, thermal, and ultrasonic degradation of EGDMA-crosslinked poly(acrylic acid-co-sodium acrylate-co-acrylamide) superabsorbents

Superabsorbent polymers (SAPs) based on acrylic acid (AA), sodium acrylate (SA), and acrylamide (AM) were synthesized by inverse suspension polymerization using ethylene glycol dimethacrylate as the crosslinking agent. The equilibrium swelling capacities and the rates of swelling of SAPs varied with the AM content and followed first-order kinetics. The photodegradation of SAPs in their equilibrium swollen state was carried out by monitoring their swelling capacity and the residual weight fraction. The SAPs degraded in two stages, wherein the swelling capacity increased to a maximum and then subsequently decreased. Thermogravimetric analysis of the SAPs indicated that the copolymeric superabsorbents had intermediate thermal stability between the homopolymeric superabsorbents. The activation energies of SAPs with 0, 20, and 100 mol % AM content were determined by Kissinger method and were found to be 299, 248, and 147 kJ mol-1, respectively. The ultrasonic degradation of the superabsorbents was carried out in their equilibrium swollen state, and the change in the viscosity with ultrasonication time was used to quantify the degradation. The ultrasonic degradation of AA/SA superabsorbent was also investigated at various ultrasound intensities. The degradation rate coefficients were found to increase with the intensity of ultrasound. The ultrasonic degradation of AA/SA/AM (20% AM) was also carried out, and degradation rate was found to be more than that of the AA/SA superabsorbent. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Percolation and connectivity in AB random geometric graphs

Given two independent Poisson point processes Phi((1)), Phi((2)) in R-d, the AB Poisson Boolean model is the graph with the points of Phi((1)) as vertices and with edges between any pair of points for which the intersection of balls of radius 2r centered at these points contains at least one point of Phi((2)). This is a generalization of the AB percolation model on discrete lattices. We show the existence of percolation for all d >= 2 and derive bounds fora critical intensity. We also provide a characterization for this critical intensity when d = 2. To study the connectivity problem, we consider independent Poisson point processes of intensities n and tau n in the unit cube. The AB random geometric graph is defined as above but with balls of radius r. We derive a weak law result for the largest nearest-neighbor distance and almost-sure asymptotic bounds for the connectivity threshold.

Basic principles of ultrafast Raman loss spectroscopy

When a light beam passes through any medium, the effects of interaction of light with the material depend on the field intensity. At low light intensities the response of materials remain linear to the amplitude of the applied electromagnetic field. But for sufficiently high intensities, the optical properties of materials are no longer linear to the amplitude of applied electromagnetic field. In such cases, the interaction of light waves with matter can result in the generation of new frequencies due to nonlinear processes such as higher harmonic generation and mixing of incident fields. One such nonlinear process, namely, the third order nonlinear spectroscopy has become a popular tool to study molecular structure. Thus, the spectroscopy based on the third order optical nonlinearity called stimulated Raman spectroscopy (SRS) is a tool to extract the structural and dynamical information about a molecular system. Ultrafast Raman loss spectroscopy (URLS) is analogous to SRS but is more sensitive than SRS. In this paper, we present the theoretical basis of SRS (URLS) techniques which have been developed in our laboratory.

Study of molecular structure and vibrational spectra of poly (beta,-1-malic acid) PMLA] using DFT approach

Poly (beta-L-malic acid) (PMLA) is a biodegradable polymer and it has various important applications in the biomedical field. In the present work the structural and spectral characteristics of PMLA have been studied by methods of infrared. Raman spectroscopy and quantum chemistry. Electrostatic potential surface, optimized geometry, harmonic vibrational frequencies, infrared intensities and activities of Raman scattering were calculated by density functional theory (DFT) using oligomeric approach employing B3LYP with complete relaxation in the potential energy surface using 6-311++G (d, p) basis set. Based on results, we have discussed the correlation between the vibrational modes and the structure of the PMLA. A complete analysis of the experimental infrared and Raman spectra has been reported on the basis of wavenumber of the vibrational bands and potential energy distribution. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The calculated infrared and the Raman spectra of the polymer based on DFT calculations show reasonable agreement with the experimental results. (c) 2012 Elsevier Ltd. All rights reserved.