• Volume 87, Issue 6

December 2016,   pages  3-100

• Volume Contents

• Enhancement in the gain recovery of a semiconductor optical amplifier by device temperature control

We present a numerical investigation on the temperature dependence of gain recovery, of a semiconductor optical amplifier (SOA). It is shown that the decrease in temperature significantly speed-up the gain recovery of the SOA. Under typical operating conditions, a 20 K reduction in temperature of the SOA results in a decrease of 150 ps in the gain recovery time. A comparative estimation of device temperature and assisted-light power requirements for enhancing the gain recovery has also been carried out. It is found that, a decrease of 8 K in the temperature of the SOA, is as effective in enhancing the gain recovery as injection of 25 dBm assistedlight power in the counter-propagating mode. Our study shows that under moderate current biasing conditions, temperature reduction is a better and convenient option to speed-up the gain recovery of an SOA, than the use of external assisted-light injection, which requires an additional laser source and wavelength division multiplexing(WDM) components for coupling and de-coupling, leading to insertion losses in the communication channel.

• Hypersurface-homogeneous Universe filled with perfect fluid in $f (R, T)$ theory of gravity

The exact solutions of the field equations with respect to hypersurface-homogeneous Universe filled with perfect fluid in the framework of $f (R, T)$ theory of gravity (Harko et al, \emph{Phys. Rev.} D 84, 024020 (2011)) is derived. The physical behaviour of the cosmological model is studied.

• Linear analysis of degree correlations in complex networks

Many real-world networks such as the protein–protein interaction networks and metabolic networks often display nontrivial correlations between degrees of vertices connected by edges. Here, we analyse the statistical methods used usually to describe the degree correlation in the networks, and analytically give linear relation in the degree correlation. It provides a simple and interesting perspective on the analysis of the degree correlation in networks, which is usefully complementary to the existing methods for degree correlation in networks. Especially, the slope in the linear relation corresponds exactly to the degree correlation coefficient in networks, meaning that it can not only characterize the level of degree correlation in networks, but also reflects the speed that the average nearest neighbours’ degree varies with the vertex degree. Finally, we applied our results to several real-world networks, validating the conclusions of the linear analysis of degree correlation. We hope that the work in this paper can be helpful for further understanding the degree correlation in complex networks.

• Intensity correlations and anticorrelations in a three-level cascade system

We study the intensity–intensity correlations of the radiation emitted on probe transition in a three level cascade electromagnetically-induced transparency (EIT) scheme. Further, we show that by applying even a very weak incoherent pump, the nature of the emitted radiation can be switched from classical to non-classicalwithout significantly affecting the probe transparency

• Parametric study of radiofrequency helium discharge under atmospheric pressure

The parameters of radio frequency helium discharge under atmospheric pressure were studied by electrical and optical measurements using high voltage probe, current probe and optical emission spectroscopy. Two discharge modes $\alpha$ and $\gamma$ were observed within certain limits. During $\alpha$ to $\gamma$ mode transition, a decrease in voltage (280–168 V), current (2.05–1.61 A) and phase angle (76$^{\rm o}-56^{\rm o}$) occurred. The discharge parameters such as resistance, reactance, sheath thickness, electron density, excitation temperature and gas temperature were assessed by electrical measurements using equivalent circuit model and optical emission spectroscopy. In $\alpha$ mode, the discharge current increased from 1.17 to 2.05 A, electron density increased from $0.19 \times 10^{12} {\rm to} 0.47 \times 10^{12} {\rm cm}^{−3}$ while sheath thickness decreased from 0.40 to 0.25 mm. The excitation temperatures in the $\alpha$ and $\gamma$ modes were 3266 and 4500 K respectively, evaluated by Boltzmann’s plot method. The estimated gas temperature increased from 335 K in the α mode to 485 K in the γ mode, suggesting that the radio frequency atmospheric pressure helium discharge can be used for surface treatment applications.

• Structural modification and band gap engineering of sol–gel dip-coated thin films of Zn$_{0.75}$Mg$_{0.25}$O alloy under vacuum annealing

In the present study, we investigated the effect of vacuum annealing on the structural and optical properties of sol–gel dip-coated thin films of Zn$_{0.75}$Mg$_{0.25}$O alloy. XRD studies revealed that all these films werepolycrystalline with hexagonal wurtzite structure and there was no trace of additional phases other than ZnO.With increase in annealing temperature, the samples showed preferred orientation along the c-axis, (0 0 2) plane andalso peak narrowing and peak shift towards higher angles. The calculated values of mean crystallite size increased with annealing temperature indicating the improvement in crystallinity. Heat treatment caused lattice contractionand a decrease in film thickness. The optical transmittance in the visible spectral range enhanced with increase in annealing temperature while the fundamental absorption edge in the near ultra-violet region got red-shifted with annealing. The calculated values of optical energy gap of the samples showed a decrease with heat treatment due to the improvement in crystallinity during annealing and hence the subsequent decrease in quantum size effect.

• Magnetized anisotropic dark energy models with constant deceleration parameter

In this paper, we have studied the solutions of plane-symmetric Universe with variable $\omega$ in the presence and the absence of magnetic field of energy density $\rho B$. A special law of variation for Hubble’s parameterproposed by Bermann in {\it Nuovo Cimento} B 74, 182 (1983) has been utilized to solve the field equations. Some physical and kinematical properties of the models are also discussed.

• Complex plasma experimental device – A test bed for studying dust vortices and other collective phenomena

A typical device for carrying out sophisticated and complex dusty plasma experiments is designed, fabricated and made operational at the Institute for Plasma Research, India. The device is named as complex plasma experimental device (CPED). The main aim of this multipurpose machine is to study the formation and behaviour of dust vortices in the absence of external magnetic field under the effect of various plasma parameters. Further, the device is equipped with advanced imaging diagnostics for studying many other interesting phenomena such as dust oscillations, three-dimensional crystalline structures, dust rotation, etc. The device is quite flexible to accommodate many innovative experiments. Detailed design of the device, its diagnostics capabilities and theadvanced image analysis techniques are presented in this paper.

• Folding model analysis of the nucleus–nucleus scattering based on Jacobi coordinates

This paper presents the results of scattering of $^{16}O+^{209}Bi interaction near the Coulomb barrier. The interaction potential between two nuclei is calculated using the double folding model with the effective nucleon–nucleon (NN) interaction. The calculations of the exchange part of the interaction were assumed to be of finite range and the density dependence of the$NN$interaction is accounted for. Also the results are compared with thezero-range approximation. All these calculations are done using the wave functions of the two colliding nuclei in place of their density distributions. The wave functions are obtained by the$D$-dimensional wave equationusing the hyper spherical calculations on the basis of Jacobi coordinates. The numerical results for the interaction potential and the differential scattering are in good agreement with the previous works. • Pulse generation and compression using an asymmetrical porous silicon-based Mach–Zehnder interferometer configuration We propose an asymmetrical Mach–Zehnder interferometer (MZI) for efficient pulse generation and compression using porous silicon (PS) waveguide, fibre delay line and couplers. We show a pulse compression of about 0.4 ns at the output port with third-order super-Gaussian input pulse in ∼2 ns time duration and ∼40.3 W peak power level. Also, we show the possibility of obtaining compressed single- or double-pulse with judicious choice of various parameters like input peak power, delay time and input pulse width. • Plastic scintillator-based hodoscope for the characterization of large-area resistive plate chambers A scintillator-based hodoscope is fully operational at Nuclear Physics Division, Bhabha Atomic Research Centre (NPD-BARC). It was used for characterizing the resistive plate chambers (RPCs) assembled for the RE4 upgrade for the compact muon solenoid (CMS) experiment, installed during the long shut-down (LS1) using cosmic muons. It has now been employed for R & D related to gas mixtures and glass RPCs for the Indiabased neutrino observatory (INO) and muon tomography studies. The hodoscope is equipped with gas flow lines,LV, HV and VME-based DAQ with multihit TDCs. CERN-based software was adapted, implemented and along with the cosmic trigger, was used to evaluate the functional parameters for the RPCs, such as efficiency, clustersize etc. • The Klein–Gordon–Zakharov equations with the positive fractional power terms and their exact solutions In this paper, the famous Klein–Gordon–Zakharov (KGZ) equations are first generalized, and the new special types of KGZ equations with the positive fractional power terms (gKGZE) are presented. In order to derive exact solutions of the new special gKGZE, subsidiary higher-order ordinary differential equations (sub-ODEs) with the positive fractional power terms are introduced, and with the aid of the sub-ODE, exact solutions of four special types of the gKGZE are derived, which are the bell-type solitary wave solution, the algebraicsolitary wave solution, the kink-type solitary wave solution and the sinusoidal travelling wave solution, provided that the coefficients of gKGZE satisfy certain constraint conditions. • Comparison between the generalized tanh–coth and the (G'/G)-expansion methods for solving NPDEs and NODEs In this paper, we find exact solutions of some nonlinear evolution equations by using generalized tanh–coth method. Three nonlinear models of physical significance, i.e. the Cahn–Hilliard equation, the Allen–Cahn equation and the steady-state equation with a cubic nonlinearity are considered and their exact solutions are obtained. From the general solutions, other well-known results are also derived. Also in this paper, we shall compare the generalized tanh–coth method and generalized (G'/G )-expansion method to solve partial differentialequations (PDEs) and ordinary differential equations (ODEs). Abundant exact travelling wave solutions including solitons, kink, periodic and rational solutions have been found. These solutions might play important roles in engineering fields. The generalized tanh–coth method was used to construct periodic wave and solitary wave solutions of nonlinear evolution equations. This method is developed for searching exact travelling wave solutions of nonlinear partial differential equations. It is shown that the generalized tanh–coth method, with the help of symbolic computation, provides a straightforward and powerful mathematical tool for solving nonlinear problems. • Stabilization effect ofWeibel modes due to inverse bremsstrahlung absorption in laser fusion plasma using Krook collisions model In this work, the Weibel instability due to inverse bremsstrahlung absorption in laser fusion plasma has been investigated. The stabilization effect due to the coupling of the self-generated magnetic field by Weibel instability with the laser wave field is explicitly showed. The main result obtained in this work is that the inclusion of self-generated magnetic field due to Weibel instability to the inverse bremsstrahlung absorption causes a stabilizing effect of excited Weibel modes. We found a decrease in the spectral range of Weibel unstable modes.This decrease is accompanied by a reduction of two orders in the growth rate of instability or even stabilization of these modes. It has been shown that the previous analyses of the Weibel instability due to inverse bremsstrahlunghave overestimated the values of the generated magnetic fields. Therefore, the generation of magnetic fields by the Weibel instability due to inverse bremsstrahlung should not affect the experiences of an inertial confinement fusion. • A new approach to model CW CO$_2$laser using rate equations Two popular methods to analyse the operation of CW CO$_2$lasers use the temperature model and the rate equation model. Among the two, the latter model directly calculates the population densities in the various vibrational levels connected with the lasing action, and provides a clearer illustration of the processes involved. Rate equation models used earlier grouped a number of vibration levels together, on the basis of normal modes of vibrations of CO$_2$. However, such grouping has an inherent disadvantage as it requires that theselevels be in thermal equilibrium. Here we report a new approach for modelling CW CO$_2$lasers wherein the relevant vibration levels are identified and independently treated. They are connected with each other through theprocesses of excitation, relaxation and radiative transitions. We use the universally accepted rate coefficients to describe these processes. The other distinguishing feature of our model is the methodology adopted for carryingout the calculations. For instance, the CW case being a steady state, all the rate equations are thus equated to zero. In the prior works, researchers derived analytical expressions for the vibration level population densities, thatbecomes quite a tedious task with increasing number of levels. Grouping of the vibration levels helped in restricting the number of equations and this facilitated the derivation of these analytical expressions. We show that insteady state, these rate equations form a set of linear algebric equations. Instead of deriving analytical expressions, these can be elegantly solved using the matrix method. The population inversion calculated in this manner alongwith the relaxation rate of the upper laser level determines the output power of the laser. We have applied the model to an experimental CW laser reported in literature. Our results match the experimentally reported power. • Real-time stress detection of monocrystalline silicon by laser irradiation using Mach-Zehnder interferometry The effect of changes in laser-induced stress upon irradiation of monocrystalline silicon was studied in detail using Mach-Zehnder interferometer, high speed camera and computer processing system, real-time detection of stress distribution and stress evolvement under different laser fluences and pulse widths. After irradiation, the changes of interference fringes were used to calculate the stress value. The results show that the stresses increased with the increase of the laser fluence. The formation of stress could be explained using thermoelastic theory. The cleavage plane's dislocation appears in the following sequence: (1 1 1) plane appears and then the dislocation slip of (1 1 0) cleavage plane appears. In addition, it is found that cleavage plane of (1 1 1) mainly exist in the spot and cleavage plane of (1 1 0) mainly exists in the vicinity of spot radius. Stress areas mainly exist in the thin layer on the surface of silicon. Furthermore, stress field was analysed by the finite-element method to get a better understanding of the formation and distribution of stress. Under the same experimental conditions, the numerical results are in agreement with the experimental values. • Quantum effects on the Rayleigh Taylor instability of stratified plasma in the presence of suspended particles The effects of quantum correction on the Rayleigh.Taylor instability (RTI) in stratified plasma layer have been investigated in the presence of suspended particles. A general dispersion relation is obtained from the linearized set of quantum hydrodynamic (QHD) equations. Two particular cases of suspended particle parameters ($f \ast$and$\alpha_0$) with and without quantum corrections are analysed. The condition of RTI is derived while the stability of the system is discussed by applying Routh.Hurwitz (RH) criterion in the polynomial equation. The results show that, in the absence of quantum term, the relaxation frequency of the suspended particles has a destabilizing effect, while the mass concentration of the suspended particles has a stabilizing effect on the growth rates of RTI. In the presence of the quantum term, the relaxation frequency of the suspended particle yields to the stability behaviour on the growth rates of RTI. • Influence of substrate temperature on certain physical properties and antibacterial activity of nanocrystalline Ag-doped In$_2$O$_3$thin films Nanocrystalline Ag-doped indium oxide (AIO) thin films, by employing a much simplified spray pyrolysis technique in different substrate temperatures (300, 350, 400 and 450$\deg$C), were fabricated for the first time. The deposited films were subjected to various characterization studies, to explore certain features like the influence of various deposition temperatures on physical and antibacterial properties. XRD results showed that all the samples exhibited preferential orientation along the (2 2 2) plane. The variation in the crystalline size with increasing substrate temperature was explained on the basis of the Zener pinning effect. The electrical sheet resistance ($R_{sh}$) was found to decrease sharply with increasing substrate temperature and attained a minimum value (62$\Omega$/$\square$) at 400$\deg\$C and then started increasing for higher deposition temperatures. Further, PL emission spectra of the samples in the visible range ascertained the possibility of applicability of the same in nanoscale optoelectronic devices. From the studies, it was found that at 400.C deposition temperature, one could expect better antibacterial efficiency against {\it Escherichia coli}. The influence of the shape and size of AIO nanograins on the antibacterial activity was analysed using scanning electron microscopy images.

• # Pramana – Journal of Physics

Volume 94, 2019
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019