Sharif Digital Repository

Gupta et al. [J. Opt. Soc. Am. B. 24, No. 5 (2007)] have introduced a localized upward plasma density ramp to overcome the defocusing of a laser beam. We report that there are fundamental mistakes in their work. First, the density profile formula that they have introduced is dimensionally incorrect. Second, the calculated dielectric constant is wrong as well, which renders their main spot size equation and the quantitative results unacceptable, and it seriously falters the results of the self-focusing. We correctly derived a new equation of the spot size, and the computational curves are also presented  

Using the source dependent expansion method and paraxial approximation, the evolution of the beam spot size (BSS) in laser plasma interaction is investigated. Considering a collisionless mechanism, the self-focusing of a Laguerre-Gaussian (LG) light beam in a plasma medium is studied and the evolution of the BSS is solved analytically for the non-saturating nonlinearity state, and numerically for the saturating state. The self-focusing of a doughnut beam depends on the initial BSS, in which if the initial BSS gets bigger, the beam in the medium will be focused faster. We show that if the nonlinear effect of the plasma medium is saturated, the LG beam profile can remain stable and the BSS... 

In this paper a saturable absorber medium is employed as an optical limiter to reduce the spot size to the range of several tens of nanometres. The characteristics of a Gaussian beam are theoretically analysed upon propagation through the saturable absorber medium. Based on Maxwell equations a system of coupled nonlinear ordinary differential equations for intensity, beam radius and beam curvature is obtained. Theoretical analyses and numerical results show that the behaviour of a Gaussian beam in a saturable absorber medium strongly depends on the initial characteristics of the laser beam. Numerical results indicate that, depending on the initial conditions and a suitable saturable absorber... 

The propagation of a Gaussian beam in underdense plasma with upward increasing density ramp is analyzed. In this work are shown that the spot size oscillations of laser beam increases and its amplitude shrinks with proper plasma density ramp. This causes the laser beam to become more focused and penetrations deep into the plasma by reduction of diffraction effect. The related focusing parameters are optimized to get the best possible focusing at the relativistic threshold intensity of Nd-glass laser and the effect of the laser intensity in the self-focusing parameters is also investigated. The analytical calculations are presented and showed more reliable results in comparison to the... 

To perform active structural health monitoring, guided Lamb waves for damage detection have recently gained extensive attention. Many algorithms are used for damage detection with guided waves and among them, the delay-and-sum method is the most commonly used algorithm because of its robustness and simplicity. However, delay-and-sum images tend to have poor accuracy with a large spot size and a high noise floor, especially in the presence of multiple damages. To overcome these problems, another method that is based on sparse reconstruction can be used. Although the images produced by the sparse reconstruction method are superior to the conventional delay-and-sum method, it has the challenges... 

In this paper, the effect of basic parameters of laser and plasma on controlling self-focusing is investigated, and based on the introduced effective parameter for self-focusing (EPSF), the effects of laser wavelength, the initial spot size of laser beam, the initial intensity of laser, and the plasma electron density on self-focusing are discussed. It is found that the relativistic self-focusing strongly depend on the laser wavelength and the beam intensity. The nonlinear effect of beam intensity on the introduced parameter EPSF indicates that after an increasing region for focusing, the self-focusing effect decreases at ultrarelativistic intensities. The effect of the initial beam spot... 

Laser shock peening (LSP) is known as a post processing surface treatment which can improve the mechanical properties of some materials. Shock waves are generated by confining the laser-induced plasma to cause a large pressure shock wave over a significant surface area. In the present study, effects of LSP on the electrochemical corrosion and micro hardness properties of 316L stainless steel alloy were investigated by changing the laser parameters such as the laser spot size, the average number of impacts, and the laser intensity. Since laser parameters do not cover the desired region of LSP, we have to use the proper design of experiment method, in which the D-optimal design of MATLAB was... 

In this work we present an ellipsoid cavity regime for the production of a bunch of quasi-monoenergetic electrons. The electron output beam is more effective than the periodic plasma wave method or the plasma-channel-guided method. A hyperbola, parabola or ellipsoid path is described for the electron trajectory motion in this model. A dense bunch of relativistic electrons with a quasi-monoenergetic spectrum is self-generated here. The obtained results show a smaller width for the electron energy spectrum in comparison with the previous results. We found that there are optimum conditions to form the ellipsoid cavity. Laser beam properties (such as the spot size, power and pulse duration) and...