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  

An attempt to extend helical terahertz radiation generation based on pump depletion is presented here. By considering an upward plasma density ramp, such a reasonable extension is examined to improve the results. It is claimed that the output terahertz energy is saturated which is attributed to laser pump divergence. The saturation effect is enhanced by considering suitable laser and plasma parameters. The adjustable slope of the density ramp is an important parameter for tuning terahertz generation effciency, which is restricted to the up and down limit. For the resonant excitation of terahertz radiation, the ripple wave number is suitably chosen to satisfy the phase matching condition.... 

An attempt to extend helical terahertz radiation generation based on pump depletion is presented here. By considering an upward plasma density ramp, such a reasonable extension is examined to improve the results. It is claimed that the output terahertz energy is saturated which is attributed to laser pump divergence. The saturation effect is enhanced by considering suitable laser and plasma parameters. The adjustable slope of the density ramp is an important parameter for tuning terahertz generation effciency, which is restricted to the up and down limit. For the resonant excitation of terahertz radiation, the ripple wave number is suitably chosen to satisfy the phase matching condition.... 

Propagation of a chirped laser pulse with a circular polarization through an uprising plasma density profile is studied by using 1D-3V particle-in-cell simulation. The laser penetration depth is increased in an overdense plasma compared to an unchirped pulse. The induced transparency due to the laser frequency chirp results in an enhanced heating of hot electrons as well as increased maximum longitudinal electrostatic field at the back side of the solid target, which is very essential in target normal sheath acceleration regime of proton acceleration. For an applied chirp parameter between 0.008 and 0.01, the maximum amount of the electrostatic field is improved by a factor of 2.... 

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... 

The effects of the polarized incident laser pulse on the electrons of the plasma surface and on the reflected pulse in the relativistic laser-plasma interaction is investigated. Based on the relativistic oscillating mirror and totally reflecting oscillating mirror (TROM) regimes, the interaction of the intense polarized laser pulses with over-dense plasma is considered. Based on the effect of ponderomotive force on the characteristic of generated electron nano-bunches, considerable increasing in the localization and charges of nano-bunches are realized. It is found that the circularly polarized laser pulse have N e/N cr of 1500 which is almost two and seven times more than the amounts for... 

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... 

The resonant second harmonic generation in the presence of a wiggler magnetic field by twisted laser plasma interaction is surveyed. The wiggler magnetic field provides additional momentum required for the phase matching. The Laguerre-Gaussian modes can be used to control the self-focusing and improve the second harmonic generation in laser plasma interaction. The wave equations for the fundamental and the second harmonic fields have been solved in the paraxial approximation. The generation of the second harmonic considering self-focusing is investigated. Also, the dependence of the second harmonic power on the propagation distance for different values of initial fundamental beam intensity,... 

Langmuir probes (LP) are the most common diagnostics for the local measurement of very important plasma parameters such as plasma density, electron temperature, plasma potential and the electron energy distribution function (EEDF). In this paper the applicability of the first derivative Langmuir probe method in processing the electron part of the current-voltage (I-V) characteristics in IR-T1 Tokamak is reported. The EEDF at different radial positions in the edge region and Scrape off Layer (SOL) are derived and the values of the plasma potential, electron temperature and electron density are estimated. Results comparison between the first derivative method and Stangeby method shows a... 

In this paper a new method for generating twisted terahertz radiation is proposed. The generation of twisted terahertz radiation in a plasma vortex, where the plasma density profile has helical structure, is investigated. The method is described on the basis of angular momentum exchange between plasma vortex and laser beam. The interaction of the vortex plasma and photons causes the laser angular momentum to change. Twisted terahertz radiation is produced at the beat frequency of laser and electron plasma waves. In this process, for Gaussian laser modes, the angular momentum of terahertz wave derives from the plasma vortex, while for the Laguerre-Gaussian it originates in the difference in... 

Accelerometers that work based on intensity modulation of light are more sensitive, economically feasible, and have a simpler fabrication process compared to wavelength modulation. A micro-opto-electro-mechanical-system accelerometer based on intensity modulation of light is designed and fabricated. A movable shutter that is attached to the proof mass is designed to change the intensity of light. Moreover, the mechanical part is designed to improve the overall sensitivity and linear behavior in the measurement range. The designed accelerometer is fabricated by a deep-reactive-ion-etching (DRIE) process. The DRIE process used in this report is based on a Bosch-like process, which uses SF 6...