Showing papers in "Indian Journal of Physics in 2017"

Analysis of heat and mass transfer with MHD and chemical reaction effects on viscoelastic fluid over a stretching sheet

Abstract: This article addresses the mass and heat transfer analysis over an electrically conducting viscoelastic (Walters B′) fluid over a stretching surface in presence of transverse magnetic field. The impact of chemical reaction, as well as non-uniform heat source, are also taken into account. Similarity transformations are employed to model the equations. The governing equations comprises of momentum, energy, and concentration which are modified to a set of non-linear differential equations and then solved by applying confluent hypergeometric function known as “Kummer’s function”. The exact solution for heat equation is obtained for two cases i.e. (1) Prescribed surface temperature, (2) Prescribed wall heat flux. Physical behavior of all the sundry parameters are against concentration, temperature, and velocity profile are presented through graphs. The inclusion of magnetic field is counterproductive in diminishing the velocity distribution whereas reverse effect is encountered for concentration and temperature profiles.

A new analytical approach to solve some of the fractional-order partial differential equations

Abstract: The aim of the present paper is to present an analytical method for the time fractional biological population model, time fractional Burgers, time fractional Cahn–Hilliard, space–time fractional Whitham–Broer–Kaup, space–time fractional Fokas equations by using the generalized tanh–coth method. The fractional derivative is described in the sense of the modified Riemann–Liouville derivatives. The method gives an analytic solution in the form of a convergent series with easily computable components, requiring no linearization or small perturbation. We have obtained the exact solutions for the aforementioned nonlinear fractional equations. A generalized fractional complex transform is appropriately used to convert these fractional equations to ordinary differential equations which subsequently resulted into number of exact solutions.

Thermoelectric and mechanical properties of gapless Zr 2 MnAl compound

Abstract: We present the study of elastic and magnetic properties of Zr2MnAl full-Heusler alloys within the first-principles density functional theory. The lattice constant, magnetic moment, bulk modulus and density of states are calculated using the full potential linearized augmented plane wave method in the generalized gradient approximation scheme. The thermoelectric properties are studied between the temperature range of 50–800 K. Seebeck coefficient (S) measurements indicate the material as n-type with large S value of −83.06 μV/K at 400 K. The material shows higher efficiency for thermoelectric use with figure of merit equal to 0.92 at 400 K relatively higher in comparison for the other full Heusler compounds in these temperature ranges. The behaviour of gapless character is mainly responsible for the anomalous transport properties of the material required for the thermoelectric applications.

Compact star modeling with quadratic equation of state in Tolman VII space–time

Abstract: In present article we extend one of our earlier works Bhar et al. (Astrophys. Space Sci. 359: 13, 2015) by considering quadratic equation of state for the matter distribution. The solution has its distinct feature as the EoS chosen is quadratic and is presenting for the first time in Tolman VII background. The solution is well behaved in nature in all respects and satisfies energy conditions. The solution is also free from central singularities and satisfies Buchdahl condition. Using this solution, we optimized the masses and radii of few well-known compact stars namely Her X-1, RX J1856.5-3754, PSR B0943 + 10, PSR B1913 + 16 and Cyg X-2 with their experimentally observed values.

Phononic crystals with one-dimensional defect as sensor materials

Abstract: Recently, sensor technology has attracted great attention in many fields due to its importance in many engineering applications. In the present work, we introduce a study using the innovative properties of phononic crystals in enhancing a new type of sensors based on the intensity of transmitted frequencies inside the phononic band gaps. Based on the transfer matrix method and Bloch theory, the expressions of the reflection coefficient and dispersion relation are presented. Firstly, the influences of filling fraction ratio and the angle of incidence on the band gap width are discussed. Secondly, the localization of waves inside band gaps is discussed by enhancing the properties of the defected phononic crystal. Compared to the periodic structure, localization modes involved within the band structure of phononic crystals with one and two defect layers are presented and compared. Trapped localized modes can be detected easily and provide more information about defected structures. Such method could increase the knowledge of manufacturing defects by measuring the intensity of propagated waves in the resonant cavities and waveguides. Moreover, several factors enhance the role of the defect layer on the transmission properties of defected phononic crystals are presented. The acoustic band gap can be used to detect or sense the type of liquids filling the defect layer. The liquids make specific resonant modes through the phononic band gaps that related to the properties of each liquid. The frequency where the maximum resonant modes occur is correlated to material properties and allows to determine several parameters such as the type of an unknown material.

New solitary wave solutions to the (2+1)-dimensional Calogero–Bogoyavlenskii–Schiff and the Kadomtsev–Petviashvili hierarchy equations

Abstract: In this paper, with the help of Wolfram Mathematica 9 we employ the powerful sine-Gordon expansion method in investigating the solution structures of the two well known nonlinear evolution equations, namely; Calogero–Bogoyavlenskii–Schiff and Kadomtsev–Petviashvili hierarchy equations We obtain new solutions with complex, hyperbolic and trigonometric function structures All the obtained solutions in this paper verified their corresponding equations We also plot the three- and two-dimensional graphics of all the obtained solutions in this paper by using the same program in Wolfram Mathematica 9 We finally submit a comprehensive conclusion

Dynamics of the positron acoustic waves in electron–positron–ion magnetoplasmas

Abstract: Dynamics of the positron acoustic waves in electron–positron–ion (e–p–i) magnetoplasmas with $$\kappa $$ -distributed hot electrons and positrons is investigated in the frameworks of the Kadomtsev–Petviashili (KP) and modified Kadomtsev–Petviashili (mKP) equations. Employing the reductive perturbation technique, the KP and mKP equations are derived. Using the bifurcation theory of planar dynamical systems, the positron acoustic solitary wave solutions, the kink and anti-kink wave solutions are obtained. Considering an external periodic perturbation in the electron–positron–ion magnetoplasmas, the perturbed KP and mKP equations are studied via some qualitative and quantitative approaches. To corroborate in the fact that the perturbed KP and mKP equations can indeed give rise to the quasiperiodic and chaotic motions, the phase plane plots, time series plots, and the Poincare section are used. The quasiperiodic and developed chaos can be observed for the perturbed positron acoustic waves. The frequency ( $$\omega $$ ) of the external periodic perturbation plays the role of the switching parameter in chaotic motions of the perturbed positron acoustic waves through quasiperiodic route to chaos. This work can be useful to understand the dynamics of nonlinear electromagnetic perturbations in space and laboratory plasmas consisting of $$\kappa $$ -distributed hot electrons and positrons.

Temperature and voltage dependence of barrier height and ideality factor in Au/0.07 graphene-doped PVA/n-Si structures

Abstract: In this study, Au/0.07 graphene-doped PVA/n-Si structures were fabricated and current conduction mechanism in these structures were investigated in the temperature range of 80–380 K through forward bias current–voltage (I–V) measurements. Main electrical parameters were extracted from I–V data. Zero-bias barrier height ( $$\overline{\varPhi }_{B0}$$ ) and ideality factor (n) were found strong functions of temperature and their values ranged from 0.234 eV and 4.98 (at 80 K) to 0.882 eV and 1.15 (at 380 K), respectively. Φ ap versus q/2kT plot was drawn to obtain an evidence of a Gaussian distribution of the barrier heights (BHs) and it revealed two distinct linear regions with different slopes and intercepts. The mean values of BH (Φ Bo) and zero-bias standard deviation (σ s ) were obtained from the intercept and slope of the linear regions of this plot as 1.30 eV and 0.16 V for the first region (280–380 K) and 0.74 eV and 0.085 V for the second region (80–240 K), respectively. Thus, the values of $$\overline{\varPhi }_{B0}$$ and effective Richardson constant (A*) were also found from the intercept and slope of the modified Richardson plot [ln(I s /T 2) − q 2 σ 2 /2k 2 T 2 vs q/kT] as 1.31 eV and 130 A/cm2 K2 for the first region and 0.76 eV and 922 A/cm2 K2 for the second region, respectively. The value of A* for the first region was very close to the theoretical value for n-Si (112 A/cm2 K2). The energy density distribution profile of surface states (Nss) was also extracted from the forward bias I–V data by taking into account voltage dependent effective BH (Φe) and n.

Analysis and characterization of Cu 2 CdSnS 4 quaternary alloy nanostructures deposited on GaN

Abstract: Through using spin coating technique, Cu2CdSnS4 (CCTS) quaternary alloy nanostructures were successfully deposited on GaN substrate using a wide range of spin coating speeds; 1500, 2000, 2500, 3000 and 3500 RPM at annealing temperature 300 °C. The optical properties were investigated through UV–vis which revealed the changing of energy band gap as the spin coating speed increases, in addition, to verify specific models of refractive index and optical dielectric constant. The structural properties were studied by X-ray diffraction which indicated that the number and intensity of the peaks were changed as the spin coating speed changes. The morphological and topographical studies of CCTS were elaborated by field emission-scanning electron microscopy and atomic force microscopy. The obtained results suggest that CCTS nanostructures deposited on GaN substrate are very suitable for optoelectronic applications, that are in accordance with the available theoretical and experimental data.

A hybrid space–time of Schwarzschild interior and Vaidya–Tikekar solution as an embedding class I

Abstract: A new type of embedding class-I metric representing anisotropic fluid distribution is presented in this paper. The new solution satisfies TOV equation and does represent a static stellar configuration. This new solution is an interesting hybrid solution where g tt is of Schwarzschild interior and g rr is of Vaidya–Tikekar. The new solution satisfies all the physical criteria like positive finite central pressure, density, causality condition etc. It also satisfies all the energy conditions such as SEC, WEC, NEC and DEC. Using this new solution, we have presented some compact star models for Her X-1, RX J1856.5-3754, Cyg X-2 and PSR J1614-2230 by optimizing their masses and radii.

Effect of Zn doping on the structural, electrical and magnetic properties of MnFe 2 O 4 nanoparticles

Abstract: Zinc doped manganese ferrite nanoparticles having chemical formula Mn1−xZnxFe2O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by co-precipitation method. The structural characterization of the prepared nanoparticles was done by X-ray diffraction and fourier transform infrared spectroscopy. It gave a confirmation of spinel phase formation. Electrical properties such as dielectric constant, dielectric loss tangent, dc-resistivity were studied. Substitution of zinc in manganese ferrite highly enhanced dielectric constant and at the same time it reduces the dielectric loss tangent. The magnetic characterization was carried out using a vibrating sample magnetometer and the parameters such as saturation magnetization, coercivity and retentivity of the synthesized samples were found out. It showed strong dependence on the zinc concentration. The observed behaviour of the structural, electrical and magnetic properties with varying zinc concentration was discussed.

Mechanically exfoliated 2D nanomaterials as saturable absorber for Q-switched erbium doped fiber laser

Abstract: We experimentally demonstrate a passive, stable and low cost Q-switched erbium-doped fiber laser based on molybdenum disulfide (MoS2) and black phosphorus (BP) saturable absorber (SA). Both MoS2 and BP SAs are prepared by mechanically exfoliating the crystal and fixing the acquired flakes onto the end surface of a standard FC/PC fiber connector. By integrating either MoS2 or BP SA into the laser cavity, a stable Q-switched operation is achieved at wavelengths of 1551.4 or 1552.9 nm respectively. Using the MoS2 SA, the repetition rate of the output laser increases from 14.25 to 38.43 kHz as the 980 nm pump power rises from 57 to 170 mW while its pulse width reduces from 10.7 to 5.02 μs. The maximum pulse energy was 141.3 nJ. On the other hand, with the BP SA, the repetition rate and pulse width fall in the ranges of 9.1–44.33 kHz and 20.75–7.04 μs respectively, as the pump power grows from 50 to 170 mW. The laser with the BP SA, produces the maximum pulse energy of 134 nJ. The experimental results indicate that MoS2 and BP SAs can be used to generate stable Q-switching pulses at 1.5 μm region successfully.

CFD analysis of Newtonian and non-Newtonian droplets impinging on heated hydrophilic and hydrophobic surfaces

Abstract: In the present study, the behaviors of Newtonian and shear-thinning non-Newtonian droplets impinging on heated hydrophilic and hydrophobic surfaces have been investigated numerically using Ansys-Fluent. In this context, the volume-of-fluid technique is applied to track the free-surface of the liquid, and variable time-step is also utilized to control the Courant number. Furthermore, we have considered the dependence of viscosity, density and surface tension on temperature during the simulation. The results are compared to available experimental data at the same conditions, such as boundary conditions. The results demonstrate that there is a good agreement between the obtained results and the experimental trends, concerning normalized diameter profiles at various Weber numbers. Therefore, the focus of the present study is an assessment of the effects of variations in Weber number, contact angle and surface temperature for Newtonian and non-Newtonian liquids on dynamics behavior of droplet in collision with hydrophobic and hydrophilic surfaces. The results represent that the behaviors of Newtonian and non-Newtonian droplets are totally different, indicating the droplet sensitivity to the working parameters.

A class of traveling wave solutions for space–time fractional biological population model in mathematical physics

Abstract: The \((\frac{G'}{G})\)-expansion method is utilized for a reliable treatment of space–time fractional biological population model. The method has been applied in the sense of the Jumarie’s modified Riemann–Liouville derivative. Three classes of exact traveling wave solutions, hyperbolic, trigonometric and rational solutions of the associated equation are characterized with some free parameters. A generalized fractional complex transform is applied to convert the fractional equations to ordinary differential equations which subsequently resulted in number of exact solutions. It should be mentioned that the \((\frac{G'}{G})\)-expansion method is very effective and convenient for solving nonlinear partial differential equations of fractional order whose balancing number is a negative integer.

Analytical investigation for Lorentz forces effect on nanofluid Marangoni boundary layer hydrothermal behavior using HAM

Abstract: In this paper, semi analytical approach is applied to investigate nanofluid Marangoni convection in presence of magnetic field. Koo–Kleinstreuer–Li model is taken into account to simulate nanofluid properties. Homotopy analysis method is utilized to solve the final ordinary equations which are obtained from similarity transformation. Roles of Hartmann number and nanofluid volume fraction are presented graphically. Results show that temperature augments with rise of nanofluid volume fraction. Impact of nanofluid volume fraction on normal velocity is more than tangential velocity. Temperature gradient enhances with rise of magnetic number.

Group method analysis of mixed convection stagnation-point flow of non-Newtonian nanofluid over a vertical stretching surface

Abstract: The group method analysis is applied to study the steady mixed convection stagnation-point flow of a non-Newtonian nanofluid towards a vertical stretching surface. The model utilized for the nanofluid incorporates the Brownian motion and thermophoresis effects. Applying the one-parameter transformation group which reduces the number of independent variables by one and thus, the system of governing partial differential equations has been converted to a set of nonlinear ordinary differential equations, and these equations are then computed numerically using the implicit finite-difference scheme. Comparison with previously published studies is executed and the results are found to be in excellent agreement. Results for the velocity, temperature, and the nanoparticle volume fraction profiles as well as the local skin-friction coefficient and local Nusselt number are presented in graphical and tabular forms, and discussed for different values of the governing parameters to show interesting features of the solutions.

Application of different entropies to study of bound magnetopolaron in an asymmetric quantum dot

Abstract: An electron strongly coupled to the LO-phonon in an asymmetric quantum dot has been considered. The system has a central impurity and it is under electric and magnetic fields. The eigenenergies and eigenfunctions of the ground and the first-excited states of the electron have been calculated using the Pekar variational method. Entropy of the system for different values of Coulomb impurity parameter, electron-LO phonon coupling strength, dispersion coefficient and electric field have been studied. Two entropies, Shannon and Gaussian entropy have been employed. It is found that the entropy has the oscillatory periodic evolution as function of the time due to the confinement form. It is deduced that the entropies increase with enhancing Coulomb impurity parameter, electron-LO phonon coupling strength and dispersion coefficient. With increasing electron-LO phonon coupling strength, the entropies decrease. The control of the coherence of the system can be done with the modulation of the electric field, the Coulomb bound potential, dispersion coefficient and electron-LO phonon coupling strength.

Exact solutions for nonlinear foam drainage equation

Abstract: In this paper, the modified simple equation method, the exp-function method, the soliton ansatz method, the Riccati equation expansion method and the $$\left( G^{\prime }/G\right)$$ -expansion method are used to construct exact solutions with parameters of the nonlinear foam drainage equation. When these parameters are taken to be special values, the solitary wave solutions and the trigonometric function solutions are derived from the exact solutions. The obtained results confirm that the proposed methods are efficient techniques for analytic treatments of a wide variety of nonlinear partial differential equations in mathematical physics. We compare our results together with each other yielding from these integration tools. Also, our results have been compared with the well-known results of others.

Investigation of the production of promethium-147 via particle accelerator

Abstract: In the present paper, we mainly aim to extend nuclear data of production of radionuclide promethium-147 used in nuclear battery technology due to its weak experimental measurement and theoretical calculation. Therefore, the cross-section for charged particle induced reactions on Nd target is calculated, and moreover, the reaction processes are simulated by particle accelerator with the energy range $${\text{E}}_{\text{particle}} = 50 \to 5$$ MeV and in the particle beam current of 20 mA to figure out yield, activity of reaction and integral yield. For a proper understanding of investigation, the obtained results are also discussed to determine the most suitable reaction and target material for the production of radionuclide promethium-147 via particle accelerator on the basis of process.

Electronic structure and optical properties of iron based chalcogenide FeX 2 (X = S, Se, Te) for photovoltaic applications: a first principle study

Abstract: In present work, the electronic structure and optical properties of the FeX2 (X = S, Se, Te) compounds have been evaluated by the density functional theory based on the scalar-relativistic full potential linear augmented plane wave method via Wien2K. From the total energy calculations, it has been found that all the compounds have direct band nature, which determined by iron 3d states at valance band edge and anion p dominated at conduction band at Γ-point and the fundamental band gap between the valence band and conduction band are estimated 1.40, 1.02 and 0.88 eV respectively with scissor correction for FeS2, FeSe2 and FeTe2 which are close to the experimental values. The optical properties such as dielectric tensor components and the absorption coefficient of these materials are determined in order to investigate their usefulness in photovoltaic applications.

The electrical properties of photodiodes based on nanostructure gallium doped cadmium oxide/ p -type silicon junctions

Abstract: Gallium doped cadmium-oxide (CdO: Ga) thin films were successfully deposited by sol–gel spin coating method on p-type Si substrate. The electrical properties of the photodiode based on nanostructure Ga doped n-CdO/p-Si junctions were investigated. The current–voltage (I–V) characteristics of the structure were investigated under various light intensity and dark. It was observed that generated photocurrent of the Au/n-CdO/p-Si junctions depended on light intensity. The capacitance–voltage and conductance–voltage measurements were carried out for this diode in the frequency range between 100 and 1000 kHz at room temperature by steps of 100 kHz. The capacitance decreased with increasing frequency due to a continuous distribution of the interface states. These results suggested that the Au/n-CdO/p-Si Schottky junctions could be utilized as a photosensor. Furthermore, the voltage and frequency dependence of series resistance were calculated from the C–V and G/ω–V measurements and plotted as functions of voltage and frequency. The distribution profile of R S –V gave a peak in the depletion region at low frequencies and disappeared with increasing frequencies.

Numerical study for Bödewadt flow of water based nanofluid over a deformable disk: Buongiorno model

Abstract: Here the well-known Bodewadt flow problem is extended to the case where nanofluid occupies the space above a stretchable disk. Both Brownian motion and thermophoresis effects are incorporated into the transport equations. Physically realistic condition accounting for zero normal flux of nanoparticles is invoked. Similar form of governing differential system is attained through conventional Von Karman relations. An efficient Keller-box method with high accuracy is used to report numerical solutions of the problem. Our results show that hydrodynamic boundary layer becomes thinner when larger stretching rate is imposed. Negative value of axial velocity reveals downward flow which is the consequence of radial stretching. Velocity components have oscillatory decaying profiles when the radial stretching effect is absent. Larger thermophoretic force leads to thicker temperature and nanoparticle concentration profiles. Both two-and three-dimensional streamlines are plotted for a specified ratio of rotation to the stretching rate. Comparative study of present results with those of previous published results is also discussed in a special situation.

On the super freak waves in multicomponent plasmas having two-negative ions: Xe + - F - - SF 6 - and Ar + - F - - SF 6 - plasmas

Abstract: Rational solution of the nonlinear Schrodinger equation (NLSE) is considered to investigate the properties of the ion-acoustic freak (rogue) waves in multicomponent plasmas, whose constituents are electrons, warm positive ions, and two distinct groups of warm negative ions. For this purpose, the hydrodynamic basic equations are reduced to an extended Korteweg-de Vries (EKdV) or Gardner equation. This equation is transformed into a NLSE for investigating the weakly nonlinear wavepackets. The conditions of modulational instability and rogue waves formation are defined. It is found that sign of the coefficients of the Gardner equation determines the stability/instability of the propagating pulses within the critical wave number values. Under certain values of plasma parameters, the Gardner equation reduces to a modified KdV equation. So, a new stability/instability region will be pinpointed. The rogue waves characteristics and their dependence on the plasma parameters of $$Xe^{+}-F^{-}-SF_{6}^{-}$$ and $$Ar^{+}-F^{-} -SF_{6}^{-}$$ plasma experiments are highlighted.

Classical-field model of the hydrogen atom

Abstract: It is shown that all of the basic properties of the hydrogen atom can be consistently described in terms of classical electrodynamics if instead of considering the electron to be a particle, we consider an electrically charged classical wave field—an “electron wave”—which is held by the electrostatic field of the proton. It is shown that quantum mechanics must be considered not as a theory of particles but as a classical field theory in the spirit of classical electrodynamics. In this case, we are not faced with difficulties in interpreting the results of the theory. In the framework of classical electrodynamics, all of the well-known regularities of the spontaneous emission of the hydrogen atom are obtained, which is usually derived in the framework of quantum electrodynamics. It is shown that there are no discrete states and discrete energy levels of the atom: the energy of the atom and its states change continuously. An explanation of the conventional corpuscular-statistical interpretation of atomic phenomena is given. It is shown that this explanation is only a misinterpretation of continuous deterministic processes. In the framework of classical electrodynamics, the nonlinear Schrodinger equation is obtained, which accounts for the inverse action of self-electromagnetic radiation of the electron wave and completely describes the spontaneous emissions of an atom.

Swift heavy ion induced optical and structural modifications in RF sputtered nanocrystalline ZnO thin film

Abstract: In the present study, 100 MeV Ag7+ ion beam-induced structural and optical modifications of nanocrystalline ZnO thin films are investigated. The nanocrystalline ZnO thin films are grown using radio frequency magnetron sputtering and irradiated at fluences of 3 × 1012, 1 × 1013 and 3 × 1013 ions/cm2. The incident swift heavy ions induced change in the crystallinity together with the preferential growth of crystallite size along the c axis (002) orientation. The average crystallite size is found to be increased from 10.8 ± 0.7 to 20.5 ± 0.3 nm with increasing the ion fluence. The Atomic force microscopy analysis confirms the variation in the surface roughness by varying the incident ion fluences. The UV–visible spectroscopy shows the decrement in transmittance of the film with ion irradiation. The micro-Raman spectra of ZnO thin films are investigated to observe ion-induced modifications which support the increased lattice defects with higher fluence. The variation in crystallinity indicates that ZnO-based devices can be used in piezoelectric transduction mechanism.

Stochastic resonance in a time-delayed bistable system driven by trichotomous noise

Abstract: This paper studies the phenomenon of stochastic resonance (SR) in a bistable system with time delay driven by trichotomous noise. Firstly, a method of numerical simulation for trichotomous noise is presented and its accuracy is checked using normalized autocorrelation function. Then the effects of feedback strength and time delay on the system responses and signal-to-noise ratio (SNR) are studied. The results show that negative feedback strength is more beneficial than positive to promote SR. The effect of time delay on SR is related to the value of feedback strength. The influence of the signal amplitude and frequency on SR is also investigated. It is found that large amplitude and small frequency of the signal can promote the occurrence of SR. Finally, the influence of the amplitude and stationary probability of trichotomous noise on SNR are discussed.

Stochastic sensitivity of a bistable energy model for visual perception

Abstract: Modern trends in physiology, psychology and cognitive neuroscience suggest that noise is an essential component of brain functionality and self-organization. With adequate noise the brain as a complex dynamical system can easily access different ordered states and improve signal detection for decision-making by preventing deadlocks. Using a stochastic sensitivity function approach, we analyze how sensitive equilibrium points are to Gaussian noise in a bistable energy model often used for qualitative description of visual perception. The probability distribution of noise-induced transitions between two coexisting percepts is calculated at different noise intensity and system stability. Stochastic squeezing of the hysteresis range and its transition from positive (bistable regime) to negative (intermittency regime) are demonstrated as the noise intensity increases. The hysteresis is more sensitive to noise in the system with higher stability.

Generation of efficient THz radiation by optical rectification in DAST crystal using tunable femtosecond laser pulses

Abstract: We report the efficient THz generation by optical rectification from an indigenously grown organic DAST crystal using the 140 fs oscillator laser pulses tunable in between 780 and 850 nm. The generated THz pulse profile and powers have been measured using the photoconductive (PC) antennas and pyroelectric detector, respectively. The highest THz peak amplitude and power is obtained at 825 nm central wavelength. We have theoretically explained the enhancement of THz radiation based on the matching of average optical group refractive index and average THz refractive index of the DAST crystal at 825 nm. In addition, the dependence of THz peak amplitude and THz power on laser power have been carried out. The measured quantum conversion efficiency (QCE) of 0.5 and 1.5 THz bands are of the order 3.7 × 10−3, 1.4 × 10−3, respectively. Finally, an attempt has been made to study the effect of polarizations on generated THz signal.

Dynamics of Bianchi I cosmologies in f(R) gravity in the Palatini formalism

Abstract: A detailed analysis of the dynamics of homogeneous and anisotropic Bianchi I geometries has been performed in f(R) gravity theory in the Palatini formalism, using dynamical systems approach. The exact solutions have been found and the behavior and stability of these solutions have been studied for three different models based on f(R) gravity. These models can produce a sequence of radiation-dominated, matter-dominated and de-Sitter periods. The analysis shows that stable solutions exist which correspond to accelerated expansion at late times. The solutions corresponding to radiation-dominated and matter-dominated era are found to be unstable. Solutions have also been found corresponding to decelerated expansion.

Black phosphorus based saturable absorber for Nd-ion doped pulsed solid state laser operation

Abstract: In this paper, the use of black phosphorus (BP) as a saturable absorber in a Q-switched Nd-ion doped solid state laser is presented. Few layers of BP in isopropyl alcohol are obtained by liquid phase exfoliation. The BP nanosheets with thicknesses in the range of 15–20 nm are deposited onto a K9 glass substrate. By inserting the BP nanosheets into a diode pumped Nd-ion doped solid state laser, stable Q-switched lasing at 0.9, 1.06, 1.3 μm is obtained. Using this approach, we have achieved a short pulse duration down to 219 ns, a high pulse energy of up to 6.5 μJ, and the corresponding peak power of 30 W. Our results show that the BP saturable absorber functions well in a Nd-ion doped solid state laser for pulsed laser generation.