جستجوی مقالات مرتبط با کلیدواژه "laser" در نشریات گروه "فیزیک"

In this study, the amplification of Ti: sapphire laser pulses in a regenerative amplifier with Z geometric arrangement has been demonstrated based on chirped pulse amplification method. Two pocket cells were used to transfer the seed pulses to the amplifier cavity and extract the amplified pulses. Build up time of the amplified pulses according to the energy of the pump laser has been studied, and the dynamic evolution of the pulses inside the amplifier cavity has been investigated.  The pulse duration of the generated pulses inside the cavity without seed pulses is about 80 ns, and the buildup time is 38 ns. The 2 mJ amplified femtosecond laser pulses at a repetition rate of 10 Hz are obtained after 17 round trips using pump energy of 15 mJ at 532 nm. The amplified laser wavelength is 800 nm with 30 nm spectral bandwidth, which is 30 nm narrower than the oscillator bandwidth. The amplification efficiency relative to the pump energy is obtained at about 13%, and the amplification coefficient in this scheme reached 106.

In this article using a modified Drude model, which includes all dominant electron energy scattering mechanisms and interparticle interactions, the relativistic dynamics of a linear chain of spherical nanoparticle interacting with a laser beam propagating parallel to the orientation of nanoparticles array is investigated. In the point-dipole description of nanoparticles, taking into account interaction of two nearest neighbors, analytical solutions are obtained for the momentum equations of conducting electrons of nanoparticles related to the first, second and third harmonics of laser fields. Formulae are simplified for some asymptotic cases. Numerical analysis is carried out for a linear chain including 10 of 10nm radius Au nanoparticles. It is shown that the interparticle separation has a key role in the nonlinear dynamics of the system. Dipole-dipole interaction of nanoparticles causes a blueshift for the main and third harmonics plasmon resonance, whereas it leads to a redshift for the plasmon resonance of the second order longitudinal displacements of nanoparticles. Related to the different orders, linear and nonlinear polarization of each particle is obtained and permittivity, first and second order susceptibility, linear and nonlinear refractive index of each nanoparticle are analytically obtained which these findings can have direct application in nano-optics and nano-plasmonics.

Acceleration of high energy charged particles is one of the most important applications of high-power lasers interaction with plasma. Using targets with densities close to critical densities, because of better laser-plasma coupling, the transferred energy from the laser to plasma will be increased. Today, due to the construction of foam-structured targets, with a density close to critical density, it is possible to use such targets in order to study more about the acceleration process of particles with densities close to the critical density. In this paper, using particle simulation in a cell, a realistic form of foam structure of laser interaction with targets has been studied in more detail. The optimal state for porous foam in terms of dimensions and distances between holes is proposed based on the maximum energy of the protons produced after the laser pulse is completed and it is found out that for a foam with a density of 3.6 critical density and 16-micrometer length, the existence of holes with 0.5-micrometer radius and 1.5-micrometers for the distance between the centers can produce protons with 40 MeV energy but if the holes are not in proper conditions, protons’ maximum energy will be decreased about 20 percent relative to the condition without porous foam.

Plasmonic production of the nanoparticles is an interesting field of research. A significant part of the plasmonic nanoparticle generation process is based on the creation of the nanobubbles by pulsed laser irradiation in the liquid medium.  Here, the generation of Ag nanoparticles produced in vicinity of the nanobubbles created by pulsed femtosecond laser is investigated. This investigation is performed on theoretical and computer simulation procedures. Our simulation in comparison with empirical results in previous reports has an acceptable agreement. We investigate the parameters such as: relationship between specific heat of the nanoparticles and temperature, variation of the nanoparticles’ volume expansion with the laser fluence, the temperature dependence of the nanoparticle average size and the ratio of nanoparticle/nanobubble volume versus the laser fluences. In the simulation we utilize the equations with spherical symmetry, the separation of the time and space parts and eventually solve the coupled equations in the Laplace space.

In this paper sound generation by laser pulse irradiated on a solid block is investigated. After an introduction to known mechanisms and main equation of laser acoustics, the laser pulse profile of NdYAG laser by straight radiation on a photo-resistor was recorded and the profile was substituted as an optoacoustic source in related equation of sound evolution. Finally the solution of numerically solved equation and measured generated sound were compared. Results show that there exists direct relationship between generated sound pulse width and laser pulse width. Furthermore there exists reverse relationship between generated sound pulse width and solid block thickness. In addition, results of simulation and experimental data measured by laser microphone with photo-resistor detector are compared by obtained sound spectrum from Spectrogram which confirm previous theoretical and experimental data. Finally general rule of the similarity of sound pulse profile and sound generator laser pulse profile are observed in this experiment.

Space environment radiation affects the operation and life-time of optical and electronic devices on satellites payloads. LIDAR payloads include electro-optical components such as laser section. LIDAR payloads are usually setup on various platforms. Space-borne remote sensing technology provides global data set of uniform quality and rapid data acquisition and also specially provides key information for evaluating the local, regional of aerosol irradiative forcing in global climate system, cloud properties and precipitation, Air pollution and hydrologic cycle. Also, global coverage collected data is updated in every time range. On the other hand, high expenditures of design, manufacture and using advanced technology are caused an increase in design and manufacture of high reliability LIDAR payloads, the mission duration and its quality in space. Due to space standards of NASA and ESA, one of the effective factors on degradation of space systems is radiation damages. In this research, effect of Total Ionizing Dose (TID) radiation on the operation of LIDAR subsystem orbiting 500 km altitude orbit with a 3-year mission is simulated. Radiation simulations on gain medium (Nd:YAG) and pump (laser diode) are performed by GEANT4 and C programming.
Laser subsystem in LIDAR payload includes 3 different parts. These parts are Nd:YAG, mirrors and laser diodes pump. Space radiation sources are classified in three types: Van Allen radiation belts, Galactic cosmic rays (GCR), periodic gradual activities of sun. The sun emits neutralized plasma (almost protons and electrons) to the space. Some energetic particles originated from solar winds and GCRs are trapped in Earth magnetic field. These belts are expanded from 1000km to 65000 km and consist of electrons up to 7Mev and protons up to 300Mev. GCR particles are mainly protons with very high speed originated from out of the galaxy. The most populated particles in 500km altitude are electrons and protons. In order to simulate the radiation effects, Input data such as average flux number of incident particles, total number of particles passing through the sensitive volume are needed. These input data will be entered in GEANT4 toolkit by C. Most important input data are flux, total number of incident particles during the mission, Energy range, geometry, materials definition and their properties, sensitive volume definition are needed physics and initial position of particles. The most important radiation damages related to lasers and solid state optical devices are total ionizing dose and displacement damage. TID is resulted from passing charged particles through matter, while displacement damage is due to collision of passing particles with atoms of matter. TID effects depend on two factors. The first factor is generation of electron-hole pairs in dielectric layers (such as oxides). The second factor is trapping sites (oxide and interface). This damage results in degradation and possible failure, such as threshold voltage shift, decrease in drive current, switching frequencies, leakage current, noises, etc. In bipolar transistors, hfe degradation, leakage current, offset voltage, changes in offset current, bias current and gain degradation in analog devices are possible effect. In other devices, frequency shift in crystals, mechanical degradations and changes in dielectric parameters are considered.
The results demonstrated that the absorbed dose from different space radiation sources for Nd:YAG and laser diode are important. Also, differences between the absorbed doses in 2 cases were simulated. The first case was absorbed dose with no shield against space radiation and the second one was calculating the absorbed dose with 2 mm Al7075 shielding the sensitive volumes against incoming particles. GEANT4 simulations determined that the absorbed dose in Nd:YAG when no shield was used to protect the laser parts against space radiation, was reached 1951 rad. In order to decrease the absorbed dose, the shield was used and it made considerable changes. The absorbed dose was reduced to 275 rad. Calculation showed that approximately 7×1012 protons would pass through the Nd:YAG, during the mission period. The number of passing particles through the sensitive volume depends on particle flux rate, sensitive volume sizes, mission duration. This value for GCR particles was 4×108 particles in each square centimeter. Results related to simulation of inner Van Allen radiation belt particles effect on Nd:YAG showed that the absorbed doses from the inner belt electrons and protons were about 68 rad and 187.5 rad respectively while these values for GCR protons and alpha particles were about 11 rad and 8.5 rad respectively. These values were about 481 rad, 128.5 rad, 7.5 rad and 6 rad for laser diode respectively. The total values for Nd:YAG and laser diode reached 275 rad and 623 rad. It is estimated that absorbed doses for radiation sensitive parts will be increased about 15 percent and according to the threshold current, the optical power versus threshold current curve will be changed.

In this paper we studied the interaction of femtosecond laser pulse with atomic cluster of Ar. Ionization, heating and expansion of the cluster was the results of this interaction. Following this process, hot and energetic electrons and high charge state of ions were produced. By changing the characteristics of the laser and cluster, it is possible to achieve high efficiency products. Simulation of this work done with the numerical method in Nano plasma model and the evolution of charge state of ions for the various intensity of laser, different pulse shape and different initial ion density are calculated.

In this paper، the acceleration of an electron in the interaction with a plasma wave and a magnetized ion-channel is analyzed. The electron dynamics is studied treated employing complete three-dimensional Lorentz force equations. A relativistic three dimensional single particle code is used to obtain the electron-trajectories. The results of numerical calculation show that the electrons can be accelerated in the magnetized channel. Furthermore، the electron energy gain with axial magnetic field is compared to that without axial magnetic field.

In this paper، stimulated Raman scattering (SRS) and electron heating in laser plasma propagating along the plasma fusion is investigated by particle-in cell simulation. Applying an external magnetic field to plasma، production of whistler waves and electron heating associated with whistler waves in the direction perpendicular to external magnetic field was observed in this simulation. The plasma waves with low phase velocities، generated in backward-SRS and dominateing initially in time and space، accelerated the backward electrons by trapping them. Then these electrons promoted to higher energies by the forward-SRS plasma waves with high phase velocities. This tow-stage electron acceleration is more efficient due to the coexistence of these two instabilities.