Lattice dynamic investigation of the Raman and infrared wavenumbers of orthorhombic R2BaNiO5 (R = Y,Gd) oxides in Immm structure

A short‐range force constant model has been applied to investigate the Raman and the infrared wavenumbers in R₂BaNiO₅ (R = Y, Gd) in their orthorhombic phase of space group Immm. Calculations of zone‐center phonons are made with seven stretching and four bending force constants. The force constants are evaluated by fitting nine Raman and two infrared modes. Two Raman modes are reassigned on the basis of group theoretical calculations. The calculated Raman and infrared modes show good agreement with the observed values. The infrared values are assigned for the first time in these oxides. Copyright © 2006 John Wiley & Sons, Ltd.     

Thermoluminescence response in gamma and UV irradiated Dy2O3 nanophosphor

Low temperature solution combustion method was employed to synthesize Dy₂O₃ nanophosphors using two different fuels (sugar and oxalyl dihydrazine (ODH)). Powder X-ray diffraction confirm pure cubic phase and the estimated particle size from Scherrer's method in sugar and ODH fuel was found to be 26 and 78 nm, respectively, and are in close agreement with those obtained using TEM and W–H plot analysis. SEM micrographs reveal porous, irregular shaped particles with large agglomeration in both the fuels. An optical band gap of 5.24 eV and 5.46 eV was observed for Dy₂O₃ for sugar and ODH fuels, respectively. The blueshift observed in sugar fuel is attributed to the particles size effect. Thermoluminescence (TL) response of cubic Dy₂O₃ nanophosphors prepared by both fuels was examined using gamma and UV radiations. The thermoluminescence of sugar used samples shows a single glow peak at 377 °C for 1–4 kGy gamma irradiations. When dose is increased to 5 kGy, two more shouldered peaks were observed at 245 and 310 °C. However, in TL of ODH used samples, a single glow peak at 376 °C was observed. It is observed that TL intensity is found to be more in sugar used samples. In UV irradiated samples a single glow peak at 365 °C was recorded in both the fuels with a little variation in TL intensity. The trapping parameters were estimated by different methods and the results are discussed.     

XPS study of annealing induced effects on surface and interface electronic properties of Si/Ge nanostructures

The present study is focused on the influence of vacuum thermal treatment on surface/interface electronic properties of Si/Ge multilayer structures (MLS) characterized using X-ray photoelectron spectroscopy (XPS) technique. Desired [Si(5 nm)/Ge(5 nm)]_×10 MLS were prepared using electron beam evaporation technique under ultra high vacuum (UHV) conditions. The core-level XPS spectra of as-deposited as well as multilayer samples annealed at different temperatures such as 100 °C, 150 °C and 200 °C for 1 h show substantial reduction in Ge 2p peak integrated intensity, whereas peak intensity of Si 2p remains almost constant. The complete interdiffusion took place after annealing the sample at 200 °C for 5 h as confirmed from depth profiling of annealed MLS. The asymmetric behaviour in intensity patterns of Si and Ge with annealing was attributed to faster interdiffusion of Si into Ge layer. However, another set of experiments on these MLS annealed at 500 °C suggests that interdiffusion can also be studied by annealing the system at higher temperature for relatively shorter time duration.     

Rotational dynamics of propylene inside Na-Y zeolite cages

We report here the quasielastic neutron scattering (QENS) studies on the dynamics of propylene inside Na-Y zeolite using triple axis spectrometer (TAS) at Dhruva reactor, Trombay. Molecular dynamics (MD) simulations performed on the system had shown that the rotational motion involves energy larger than that involved in the translational motion. Therefore, rotational motion was not observed in our earlier QENS studies on propylene adsorbed Na-Y zeolite using a higher resolution spectrometer at Dhruva. Analysis of the TAS spectra revealed that the quasielastic broadening observed in propylene-loaded zeolite spectra is due to the rotational motion of the propylene molecules. This is consistent with our simulation result. Further, the rotational motion is found to be isotropic. The rotational diffusion coefficient has been obtained.      

Narrow band optical filter in fluorescein doped boric acid glass saturable absorber thin films

We demonstrate narrow band optical filter like frequency response with full width half maximum (FWHM) of nearly (1.75 ± 0.25) Hz in fluorescein doped boric acid glass films [10⁻⁴ M], using modulated optical phase conjugation and a nearly non-degenerate four wave mixing technique. Modulated optical phase conjugation signals are described in the limit of a weak probe and relatively strong pump beams. Both pump beams are of nearly equal intensity at a wavelength of 514.5 nm from a continuous-wave Ar⁺ laser. The probe beam frequency has been detuned with a ramp signal using a piezo electric mirror.     

Effect of absorbed pump power on the quality of output beam from monolithic microchip lasers

The dependence of the beam propagation factor (M ² parameter) with the absorbed pump power in the case of monolithic microchip laser under face-cooled configuration is extensively studied. Our investigations show that the M ₂ parameter is related to the absorbed pump power through two parameters (α and β) whose values depend on the laser material properties and laser configuration. We have shown that one parameter arises due to the oscillation of higher order modes in the microchip cavity and the other parameter accounts for the spherical aberration associated with the thermal lens induced by the pump beam. Such dependency of M ² parameter with the absorbed pump power is experimentally verified for a face-cooled monolithic microchip laser based on Nd³⁺ -doped GdVO₄ crystal and the values of α and β parameters were estimated from the experimentally measured data points.     

Analysis of fiber optic SPR sensor utilizing platinum based nanocomposites

SPR based fiber optic sensor using nanocomposite is presented. Nanocomposites comprising of Pt nanoparticles with various volume fractions embedded in dielectric matrices of TiO₂ and SnO₂ are considered. Sensitivity enhances with increase in thickness of nanocomposite and volume fraction of nanoparticles for both nanocomposites. Optimized thicknesses are obtained to be 40 and 50 nm for Pt–TiO₂ and Pt–SnO₂ nanocomposites respectively while optimized volume fraction is found to be 0.85 for both nanocomposites. 40 nm thick Pt–TiO₂ nanocomposite based sensor with 0.85 volume fraction possesses utmost sensitivity.     

Dispersion behavior of electromagnetic wave near the resonance in 1D magnetized ferrite photonic crystals

In the present article, we have analyzed the dispersion of electromagnetic wave in the one dimensional magnetized ferrite photonic crystals near the resonance in the permeability of the constituent materials for transverse magnetization in the transverse electric mode. The dispersion relation is obtained by transfer matrix method. It is observed that in the vicinity of resonant frequency, large numbers of oscillations occur in the normalized Bloch wave number. These oscillations in the Bloch wave number are strongly dependent on external magnetic fields, filling factor, and damping constant. The frequency regime of these oscillations is found to be shifted in higher frequency range with increase in the magnitude of the magnetic fields. With increase in the filling factor keeping length of periods fixed, number of oscillations is found to be increased. Near the resonance, effect of incident angle is negligible. It is demonstrated that these nearly equidistant oscillations occurring in the vicinity of resonance may be used for making filter in micro wave frequency range.     

Electronic transport across a junction between armchair graphene nanotube and zigzag nanoribbon

Based on the well known nearest-neighbor tight-binding approximation for graphene, an exact expression for the electronic conductance across a zigzag nanoribbon/armchair nanotube junction is presented for non-interacting electrons. The junction results from the removal of a half-row of zigzag dimers in armchair nanotube, or equivalently by partial rolling of zigzag nanoribbon and insertion of a half-row of zigzag dimers in between. From the former point of view, a discrete form of Dirichlet condition is imposed on a zigzag half-line of dimers assuming the vanishing of wave function outside the physical structure. A closed form expression is provided for the reflection and transmission moduli for the outgoing wave modes for each given electronic wave mode incident from either side of the junction. It is demonstrated that such a contact junction between the nanotube and nanoribbon exhibits negligible backscattering, and the transmission has been found to be nearly ballistic. In contrast to the previously reported studies for partially unzipped carbon nanotubes (CNTs), using the same tight binding model, it is found that due to the “defect” there is certain amount of mixing between the electronic wave modes with even and odd reflection symmetries. But the junction remains a perfect valley filter for CNTs at certain energy ranges. Applications aside from the electronic case, include wave propagation in quasi-one-dimensional honeycomb structures of graphene-like constitution. The paper includes several numerical calculations, analytical derivations, and graphical results, which complement the provision of succinct closed form expressions.