Mashhad, Iran

Islamic Azad University of Mashhad is a university in Mashhad, Iran. It was established in 1982, and offers Bachelor, Master, PhD and M.D. degrees. It is famous for the quality and toughness of its undergraduate education and the success of its students in many national competitions and exams. Wikipedia.

Time filter

Source Type

Comparing simulation results with experimental findings, it is found that considering nonlinear optical gain is quite essential to accurately obtain dynamic and static characteristics of self-Assembled quantum-dot lasers (SAQDLs). In fact, the nonlinear optical gain prevents extreme decline or growth of photon population as the time increases and of output power as the injected current enhances. It also results in multi-mode lasing and increasing the number of lasing modes with elevation of the injected current. In addition, the best performance of SAQDLs, at a certain injected current, depends on homogeneous and inhomogeneous broadening. Thermal carrier excitation results in degradation of light-current characteristics. It also leads to a red shift in dominant lasing modes at low injected currents, the dominant lasing modes move toward higher energies as the current enhances until the most dominant mode becomes the central one. © 2012 Optical Society of America.

Mehmannavaz M.R.,Islamic Azad University at Mashhad | Sattari H.,Islamic Azad University at Tabriz
Laser Physics Letters | Year: 2015

We propose a model for a quintuple coupled quantum dot system based on a GaAs/AlGaAs heterostructure. Then, we analyze the optical bistability (OB) and optical multistability (OM) behaviours and transition between the regimes at a wavelength of λ = 1.550 μm. We take the benefit of consecutive and parallel interdot tunnelling and an incoherent pumping field for electrical and even optical control of the processes. It is shown that OB, OM and transition between them can be accomplished and controlled by adjusting the rate of the inter-dot tunnellings (electrical bias), probe wavelength detuning and rate of the optical incoherent pumping field. By proper choice of the controlling parameters, the bistable hysteresis loop becomes narrower, which makes it easier for the cavity field to reach saturation. We interpret the OB and OM behaviours by discussing the absorption of the active medium. We also investigate switching time between the two stable states when the output field jumps from a lower branch to an upper branch. Such a controllable OB/OM and transition between them in multiple QD molecules at a wavelength of 1.550 μm, may provide some new possibilities for technological applications in optoelectronics, solid-state quantum information science and systems dealing with signal processing. © 2015 Astro Ltd.

Barati R.,Islamic Azad University at Mashhad
KSCE Journal of Civil Engineering | Year: 2013

The Muskingum model continues to be a popular procedure for river flood routing. An important aspect in nonlinear Muskingum models is the calibration of the model parameters. The current study presents the application of commonly available spreadsheet software, Microsoft Excel 2010, for the purpose of estimating the parameters of nonlinear Muskingum routing models. Main advantage of this approach is that it can calibrate the parameters using two different ways without knowing the exact details of optimization techniques. These procedures consist of (1) Generalized Reduced Gradient (GRG) solver and (2) evolutionary solver. The first one needs the initial values assumption for the parameter estimation while the latter requires the determination of the algorithm parameters. The results of the simulation of an example that is a benchmark problem for parameter estimation of the nonlinear Muskingum models indicate that Excel solver is a promising way to reduce problems of the parameter estimation of the nonlinear Muskingum routing models. Furthermore, the results indicate that the efficiency of Excel solver for the parameter estimation of the models can be increased, if both GRG and evolutionary solvers are used together. © 2013 Korean Society of Civil Engineers and Springer-Verlag Berlin Heidelberg.

Golmakani M.E.,Islamic Azad University at Mashhad | Rezatalab J.,Islamic Azad University at Mashhad
Composite Structures | Year: 2014

In this paper, nonlinear bending behavior of the orthotropic single layered graphene sheet (SLGS) subjected to a transverse uniform load and resting on an elastic matrix as Pasternak foundation model is investigated using the nonlocal elasticity theory. The nanoplate equilibrium equations are derived in terms of the generalized displacements based on first-order shear deformation theory (FSDT) using the nonlocal differential constitutive relations of Eringen and the von Karman nonlinear strains. The differential quadrature (DQ) discretized form of the governing equations with the various types of boundary conditions are derived. The Newton-Raphson iterative scheme is employed to solve the resulting system of nonlinear algebraic equations. Numerical results obtained by the present theory are compared with available solutions in the literature and those developed by finite difference method (FDM) and dynamic relaxation method (DRM) in this work. Excellent agreement between the results of different solution method and presented results is observed. Finally, effects of small scale parameter, width ratio, thickness of plate, elastic foundation properties, load value, boundary conditions and nonlinearity are studied for both nonlocal FSDT and classical plate theory (CPT) in detail. © 2013 Elsevier Ltd.

A novel catalytic synthesis of biscoumarins from 4-hydroxycoumarin and aromatic aldehydes has been developed. The reaction occurs in ethanol in the presence of tetrabutylammonium hexatungstate [TBA] 2[W 6O 19] as catalyst to give the corresponding products in high yields. This new approach has short reaction times, clean reaction profiles, and simple experimental and workup procedures. Moreover, the catalyst can be easily recovered by filtration and used at least three times with only slight reduction in its catalytic activity.

Alamatian J.,Islamic Azad University at Mashhad
Computers and Structures | Year: 2013

This paper prepares new features for applying the Dynamic Relaxation (DR) method in buckling and post-buckling structural analyses. Here, controlling the displacement's increment is the main concept so that the minimum residual displacement and the minimum kinetic energy criteria are designed for updating the load factor in DR iterations. These new formulations do not affect the DR's stability and they are successfully applied to both viscous and kinetic DR algorithms. Numerical analyses of structures with snap-through and snap-back behaviors show that the accuracy and the convergence rate of the suggested methods are higher than the well-known existing approaches such as the arc-length technique, especially, if they are combined with the kinetic DR algorithm. Calculating the buckling load is another interesting application of the proposed methods, performed here. In this manner, the suggested DR algorithms could numerically present the accurate buckling load of structures. © 2012 Elsevier Ltd. All rights reserved.

Alamatian J.,Islamic Azad University at Mashhad
Computers and Structures | Year: 2012

This paper deals with the stability and convergence rate of the kinetic damping process in Dynamic Relaxation (DR) method. Based on running an incremental analysis, a new relationship is presented for fictitious mass of the kinetic DR algorithm. This formulation is also combined with a transformational form of the Gerschgörin circles theory, proposed here. For numerical verification, some structures from the finite element models are analyzed statically and dynamically and results are compared with the well-known viscous and kinetic DR algorithms. These examples clearly show that the proposed fictitious mass considerably increases the convergence rate of the kinetic DR iterations. © 2011 Elsevier Ltd. All rights reserved.

Golmakani M.E.,Islamic Azad University at Mashhad
Composites Part B: Engineering | Year: 2013

In this paper, large deflection thermoelastic analysis of functionally graded (FG) solid and hollow rotating axisymmetric disk with uniform and variable thickness subjected to thermo-mechanical loading is studied. To achieve this goal, in addition to the uniform transverse loading, the rotating FG disk is subjected to a thermal gradient along the radial direction. Three different profiles (convex, linear and concave) are considered for variable thickness disk. The mechanical and thermal properties of FG disk are assumed to vary continuously along the radial direction by the Mori-Tanaka distribution. The nonlinear formulations are based on first order shear deformation theory (FSDT) and large deflection von Karman equations. The dynamic relaxation (DR) method combined with the finite difference discretization technique is employed to solve the equilibrium equations. Effects of grading index, angular velocity, boundary condition, thermal loading, ratios of thickness to radius and thickness profile of the disk are studied in detail. Also some linear and nonlinear analysis with different thickness-to-radius ratios is carried out based on classical plate theory (CPT) and FSDT to consider the effect of shear deformation and nonlinearity on the results. © 2012 Elsevier Ltd. All rights reserved.

Jafari A.,Islamic Azad University at Mashhad
Phytotaxa | Year: 2012

Leopoldia tijtijensis Jafari (Asparagaceae) from western of Iran (Kurdistan province) is newly described. It is similar to L. tenuiflora but differs in having dense cylindrical-conical raceme not lax cylindrical, purple-violet fertile flowers not green-brown, purple sterile flowers not pale violet, oblong urceolate fertile flower not tubular urceolate, triangular capsule not ovoid-cordate. © 2012 Magnolia Press.

Ghodsi Nahri D.,Islamic Azad University at Mashhad
Laser Physics Letters | Year: 2012

Dynamic, modulation, and output power (OP) characteristics of In(Ga)As/GaAs self-assembled quantum-dot lasers (SAQDLs) using multi-mode and multi-population rate equations analysis considering nonlinear material gain and thermal carrier escape pathways to both wetting layer and barriers are presented. I show that despite of significant effect of nonlinear material gain on time evolution of photon population, it does not affect 3-dB modulation bandwidth (MB). Thermal carrier escape processes have minor declining effects on modulation properties at around room-temperature (RT) operation and higher. Although turn-on delay increases with enhancement of temperature, in some bias currents, there is a reverse jump which is due to thermal carrier escape to barriers. In addition, it is indicated that optimum bias current to maximize MB increases as temperature enhances and that more disk-like SAQDs may provide higher MBs. Variations of some key parameters, bias current or mean quantum-dot (QD) radius, provide similar changing patterns for both MB and OP. While altering most of key parameters, average QD height, QD coverage, stripe width of the laser cavity, and temperature, leads to a tradeoff between MB and OP for a specific interval of those parameters. I show that considering our purpose, which is the highest OP or MB, we can achieve maximum possible output designing key parameters. For the present device, optimizing structural parameters, MB about 14 GHz is achieved at around RT operation under the moderate bias current 10 mA, which can be improved up to 30 GHz with decreasing carrier capture time. The results presented here may be used for designing QD lasers suitable for optical telecommunication. © 2012 by Astro, Ltd.

Loading Islamic Azad University at Mashhad collaborators
Loading Islamic Azad University at Mashhad collaborators