Studies on the growth and optical characterization of dysprosium gadolinium oxalate single crystals

Preparation and optical characterization of dysprosium gadolinium oxalate (DGO) single crystal is reported. The crystals were grown using silica gel technique, by the controlled reaction of rare earth nitrates with oxalic acid. Crystals were characterized using X‐ray powder diffraction, optical absorption and fluorescence studies. Radiative transition probability, fluorescence branching ratio and radiative lifetime of Dy³⁺ in the crystal are evaluated by the parameterization of the absorption spectrum by the Judd‐Ofelt theory. The recorded fluorescence spectrum showed two well resolved peaks at 480 nm and 571 nm and are assigned to the transitions from ⁴F_9/2 → ⁶H_15/2 and ⁶H_13/2 of Dy³⁺. Stimulated emission crossection and optical gain of these transitions are also evaluated. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurements of plasma temperature and electron density in laser-induced copper plasma by time-resolved spectroscopy of neutral atom and ion emissions

Plasma produced by a 355 nm pulsed Nd:YAG laser with a pulse duration of 6 ns focussed onto a copper solid sample in air at atmospheric pressure is studied spectroscopically. The temperature and electron density characterizing the plasma are measured by time-resolved spectroscopy of neutral atom and ion line emissions in the time window of 300–2000 ns. An echelle spectrograph coupled with a gated intensified charge coupled detector is used to record the plasma emissions. The temperature is obtained using the Boltzmann plot method and the electron density is determined using the Saha-Boltzmann equation method. Both parameters are studied as a function of delay time with respect to the onset of the laser pulse. The results are discussed. The time window where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the laser-induced breakdown spectroscopy (LIBS) analysis of samples, is deduced from the temporal evolution of the intensity ratio of two Cu I lines. It is found to be 700–1000 ns.     

A quantile based test for comparing cumulative incidence functions of competing risks models

In the present note, we develop a nonparametric testing procedure for testing equality of cumulative incidence functions of competing risks models using quantile functions. Asymptotic properties of the test statistic are discussed. Simulation studies and real data examples illustrate the practical utility of the procedure.     

Spread-space spread-spectrum technique for secure multiplexing

A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals' energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented.     

Evaluation of flame retardancy and shear resistivity characteristics of organoclay within acrylate polymer

The present work focuses on the effect of organically modified nanoclays on the fire resistance and processability characteristics of poly(methyl methacrylate) (PMMA). PMMA nanocomposite of cloisite 30B (methyl tallow bis-2 hydroxyethyl quarternary ammonium salt modifier) (C30B) was prepared by melt blending technique. A comparative study of flame retardant characteristics of virgin and filled matrix was carried out using cone calorimeter at a heat flux of 50 kW m^?2, and the service temperature range was analyzed from thermogravimetric analysis. Parallel plate rheometry revealed the variation in processability as a function of C30B reinforcement and corresponding melt strength of the samples was evaluated through complex viscosity, storage, and loss modulus. Similarly, application-level modulus values were evaluated through viscoelastic properties by dynamic mechanical analysis. Supportive information for the observations from the above characterization was conducted using morphological studies by X-ray diffraction and transmission electron microscopy.     

Evidence for the quantum birth of our universe

We present evidence for a nonsingular origin of the Universe with intial conditions determined by quantum physics and relativistic gravity. In particular, we establish that the present temperature of the microwave background and the present density of the Universe agree well with our predictions from these intial conditions, after evolution to the present age using the Einstein-Friedmann equation. Remarkably, the quantum origin for the Universe naturally allows its evolution at exactly the critical density. We also discuss the consequences of these results to some fundamental aspects of quantum physics in the early Universe.     

Fluorescence quenching in dyes under an optically thin approximation

Kinetic analysis of the energy level scheme of a dye molecule yields expression for the fluorescence quantum yield. The quantum yield is dependent on molecular parameters and laser pulse characteristics. Calculations show quenching of fluorescence quantum yield due to excited state absorption. Earlier studies on quenching of fluorescence have been discussed quantitatively under an optically thin approximation.     

A satellite-borne experiment to study the fifth force

An experiment with a satellite-borne torsion balance capable of measuring the fifth force which couples to baryon number or lepton number with a strengtr gravity and range ~ 10⁹ cm is described.     

Naked singularities in spherically symmetric gravitational collapse: A critique

One of the most important questions in the physics of gravitation phenomena is whether gravitational collapse can lead to the formation of singularities which are not hidden by an event horizon. The Cosmic Censorship Conjecture (CCC) represents the hope that such a drastic event cannot happen in realistic physical situations. However, in the recent past several counter examples to the CCC were demonstrated by several researchers in situations of spherically symmetric gravitational collapse. The disturbing aspect about these counter examples is that they are strong naked singularities—they can crush matter to zero volume and can have a disastrous influence on causal physics. We examine these counter examples for their physical content by working through the dynamical collapse of inhomogeneous dust and argue that these are not physically acceptable counter examples. Our main result is that the singularities when naked are weak and when strong, strongly censored. The strong naked singularities in the counter examples do not arise from dynamical collapse; they result from the intrinsically singular nature of the initial density distributions chosen. The CCC seems to remain robust as far as spherically symmetric collapse is concerned.