Effect Of Laser Intensity Research Articles

Effect of laser wavelength and intensity on the divergence of hot electrons in fast ignition

Recently, the short wavelength laser is believed to have a promising prospect in fast ignition for reducing the conflict between laser energy requirement and electron stopping range. Here we investigate the influence of laser wavelength and intensity in the angular dispersion of hot electrons. Both our theoretical model and numerical simulations show that the angular dispersion would increase rapidly with the shortening of laser wavelength due to the Weibel instability, while the laser intensity has little effect on it. These results have important implications for fast ignition.

Light scattering from dense cold atomic media

We theoretically study the propagation of light through a cold atomic medium, where the effects of motion, laser intensity, atomic density, and polarization can all modify the properties of the scattered light. We present two different microscopic models: the "coherent dipole model" and the "random walk model", both suitable for modeling recent experimental work done in large atomic arrays in the low light intensity regime. We use them to compute relevant observables such as the linewidth, peak intensity and line center of the emitted light. We further develop generalized models that explicitly take into account atomic motion. Those are relevant for hotter atoms and beyond the low intensity regime. We show that atomic motion can lead to drastic dephasing and to a reduction of collective effects, together with a distortion of the lineshape. Our results are applicable to model a full gamut of quantum systems that rely on atom-light interactions including atomic clocks, quantum simulators and nanophotonic systems.

Investigation of ionization speed in field ionization with laser–plasma interaction

AbstractThe effects of target density and laser intensity on ionization speed are studied in this paper by 1D3V particle-in-cell simulations, where the field ionization of single atom is involved basing Ammosov-Delone-Krainov model in the form of Penetrante and Bardsley. To consider the ionization speed, the evolution of plasma density for the helium target, particularly, the ion density change rate near the target front surface, are discussed. The results show that not only the laser intensity, but also the target density will affect field ionization and further affect the plasma formation. This work will be helpful for further understanding of plasma formation in intense laser pulse. Also, it may be benefit for the setup of initial parameters before the simulation of laser–plasma interaction.

Effect of laser intensity on radio frequency emissions from laser induced breakdown of atmospheric air

The studies on the effect of input laser intensity, through the variation of laser focusing geometry, on radio frequency (RF) emissions, over 30–1000 MHz from nanosecond (ns) and picosecond (ps) laser induced breakdown (LIB) of atmospheric air are presented. The RF emissions from the ns and ps LIB were observed to be decreasing and increasing, respectively, when traversed from tight to loose focusing conditions. The angular and radial intensities of the RF emissions from the ns and ps LIB are found to be consistent with sin2θ/r2 dependence of the electric dipole radiation. The normalized RF emissions were observed to vary with incident laser intensity (Iλ2), indicating the increase in the induced dipole moment at moderate input laser intensities and the damping of radiation due to higher recombination rate of plasma at higher input laser intensities.

Laser intensity effects in carrier-envelope phase-tagged time of flight-photoemission electron microscopy

A time of flight-photoemission electron microscope is combined with a single-shot stereographic above-threshold ionization phase meter for studying attosecond control of electrons in tailored plasmonic nanostructures spatially and energetically via a carrier-envelope phase tagging technique. First carrier-envelope phase-resolved measurements of gold nanoparticles on gold plane and surface roughness from a gold film show an apparent carrier-envelope phase modulation with a period of π. This modulation is found to originate from an intensity dependence of the photoelectron spectra and the carrier-envelope phase measurement rather than from an intrinsic carrier-envelope phase dependence, which is confirmed by simulations. This useful finding suggests that intensity tagging should be considered for phase tagging experiments on plasmonic nanostructures with low carrier-envelope phase sensitivity in order to correct for the intensity-related carrier-envelope phase artifact.

Second harmonic generation of self-focused Cosh-Gaussian laser beam in collisional plasma

This paper presents theoretical investigation of effect of self-focusing of Cosh-Gaussian (ChG) laser beam on second harmonic generation (SHG) in collisional plasma. Due to non uniform intensity distribution along the wave front of ChG laser beam non uniform Ohmic heating of plasma electrons takes place. Non uniform heating of plasma the laser beam leads to self-focusing of the laser beam which in turn produces strong density gradients in the transverse direction. The generated density gradients excite electron plasma wave at pump frequency that interacts with the incident laser beam to produce its second harmonics. Following moment theory approach in Wentzel–Kramers–Brillouin (W.K.B) approximation second order differential equation governing the evolution of spot size of laser beam with distance of propagation has been derived. Numerical simulations have been carried out to investigate the effect of laser intensity as well as plasma parameters on self-focusing of the laser beam and also on yield of second harmonics. Results predict that ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics.

Field dependence of state populations in pump-probe pulses

The effects of pulse width, laser wavelength and laser intensity on the state populations of NaI molecule driven by a pump-probe pulse were studied via employing the time-dependent wave packet approach. Wave packet moves periodically with roundtrip time 1000fs. That the wave packet bifurcates in the crossing region affects state populations. The excitation probability increases and dissociation probability decreases with increasing pulse width. The excitation probability approaches the maximum at the resonance pump wavelength 318nm, and the dissociation probability decreases with increasing pump wavelength. The excitation probability increases with increasing pump intensity while the dissociation probability does not vary. The probe wavelength and probe intensity have no effect on state populations. The wave packet motion and selective distribution of state populations, excitation and dissociation of molecule can be achieved by adjusting laser parameters. The results can provide some important basis for realizing quantum manipulation of molecules experimentally.

Exact Analytical Hyperbolic Temperature Profile in a Three-Dimensional Media Under Pulse Surface Heat Flux

AbstractIn this paper three-dimensional hyperbolic heat conduction equation in a cubic media with rectangular cross-section under a pulsed heat flux on the upper side has been solved analytically using the method of separation of variables and the Duhamel integral. The closed form solution of both Fourier and non-Fourier profiles are introduced with both modes of steady and pulsed fluxes. The results show the considerable difference between the Fourier and Non-Fourier temperature profiles. Then the answer procedure is used for modeling of interaction of a cubical tissue under a short laser pulse heating. The effects of pulse duration and laser intensity are studied analytically. Furthermore the results can be applied as a verification branch for other numerical solutions or laser treatments of biological tissues.

Studies of ablated plasma and shocks produced in a planar target by a sub-nanosecond laser pulse of intensity relevant to shock ignition

AbstractThe effect of laser intensity on characteristics of the plasma ablated from a low-Z(CH) planar target irradiated by a 250 ps, 0.438 µm laser pulse with the intensity of up to 1016W/cm2as well as on parameters of the laser-driven shock generated in the target for various scale-lengths of preformed plasma was investigated at the kilojoule Prague Asterix Laser System (PALS) laser facility. Characteristics of the plasma were measured with the use of 3-frame interferometry, ion diagnostics, an X-ray spectrometer, andKαimaging. Parameters of the shock generated in a Cl doped CH target by the intense 3ω laser pulse were inferred by numerical hydrodynamic simulations from the measurements of craters produced by the shock in the massive Cu target behind the CH layer. It was found that the pressure of the shock generated in the plastic layer is relatively weakly influenced by the preplasma (the pressure drop due to the preplasma presence is ~10–20%) and at the pulse intensity of ~1016W/cm2the maximum pressure reaches ~80–90 Mbar. However, an increase in pressure of the shock with the laser intensity is slower than predicted by theory for a planar shock and the maximum pressure achieved in the experiment is by a factor of ~2 lower than predicted by the theory. Both at the preplasma absence and presence, the laser-to-hot electrons energy conversion efficiency is small, ~1% or below, and the influence of hot electrons on the generated shock is expected to be weak.

Control of two IR signals from coupled parametric six-wave mixing processes in Rb vapor

Optical signals of two different wavelengths in the IR regime were generated in atomic Rb vapor using broad bandwidth laser pulses and competition between the two signals was observed. The effects of pump laser intensity, chirp of laser pulses and Rb number density on the characteristics of the two signals were investigated. The results show that the two signals were generated from two coupled parametric six-wave mixings processes and the competition between the two signals was dominantly governed by phase matching conditions related to Rb number density.

Statistical Distribution of Laser‐induced Growth of Nanoparticles: Effects of Laser Intensity and Medium–Nanoparticles Interactions#

We theoretically investigate the size distribution of nanoaggregates created by a laser‐induced monomer generation process, an example of which can be found in J. Phys. Chem. C 2008, 112, 10715. A simple analytic expression for the most probable size distribution of nanoparticles is obtained by considering statistical ensemble of nanoparticles with various sizes. When effects of the surface interaction and the defect degeneracy of the nanoparticle are negligible, the most probable size distribution of nanoparticles is given by the exponentially decaying function of the nanoparticle size. The presence of defect degeneracy drastically changes the size distribution to be like gamma distribution. The electronic surface tension of the nanoparticle and the interaction between the nanoparticle and environment make the size distribution deviate from gamma distribution, which also significantly change the mean and the variance of the size distribution of the nanoparticles.

Laser assisted nitriding of nickel–chromium-based superalloy surface: Heating and diffusion analysis

Inconel 718 is a nickel–chromium-based superalloy, and it is widely used in power industry because of its resistance to high-temperature environments. Treatment of the alloy becomes essential to prevent niobium segregation at the surface. Laser controlled melting and gas assisted nitriding is one of the methods to minimize changes in the elemental composition of the alloy surface. In general, high pressure nitrogen assisting gas is used coaxially with the laser beam to form a nitride layer and avoiding high-temperature exothermic oxidation reactions in the laser-irradiated region. The present study is carried out to model and simulate sequentially coupled thermal-diffusion process during laser assisted surface nitriding of nickel–chromium-based superalloy in line with experimental conditions. High pressure nitrogen gas jet is considered to impinge onto a workpiece surface coaxially with the laser beam during the treatment process. Finite element model is incorporated to predict the nitrogen concentration and temperature in the laser treated layer. It is found that the predictions of surface temperature and nitriding are found to be in close agreement with the experimental data. The study is extended to include the effect of laser intensity on the nitriding behavior.

LASER-INDUCED THERMAL–MECHANICAL DAMAGE CHARACTERISTICS OF CLEARTRAN MULTISPECTRAL ZINC SULFIDE WITH TEMPERATURE-DEPENDENT PROPERTIES

Laser-induced thermal–mechanical damage characteristics of window materials are the focus problems in laser weapon and anti-radiation reinforcement technology. Thermal–mechanical effects and damage characteristics are investigated for cleartran multispectral zinc sulfide ( ZnS ) thin film window materials irradiated by continuous laser using three-dimensional (3D) thermal–mechanical model. Some temperature-dependent parameters are introduced into the model. The temporal-spatial distributions of temperature and thermal stress are exhibited. The damage mechanism is analyzed. The influences of temperature effect of material parameters and laser intensity on the development of thermal stress and the damage characteristics are examined. The results show, the von Mises equivalent stress along the thickness direction is fluctuant, which originates from the transformation of principal stresses from compressive stress to tensile stress with the increase of depth from irradiated surface. The damage originates from the thermal stress but not the melting. The thermal stress is increased and the damage is accelerated by introducing the temperature effect of parameters or the increasing laser intensity.

Raman and surface-enhanced Raman spectroscopy evidence for oxidation-induced decomposition of graphite

It has been proposed that reduction of exfoliated graphite oxide could be a potential method for producing large quantities of graphene. Raman and surface-enhanced Raman spectroscopy are used to show that oxidation of graphite and exfoliated graphite significantly increases the defect structure of both materials. This would likely lead to a heavily defected graphene structure when oxygen is removed. To insure the observed decomposition is not due to the laser light, the effect of laser intensity on the materials was investigated. It was found that at the highest laser intensity (1.4 × 108 W/M2) there was a significant increase in defects. However, lower laser intensity was found which did not produce defects and was used in the studies of the effect of oxidation on the spectra.

Effect of laser intensity on the apparent isotope patterns of heme and peptide ions in MALDI-TOF MS

The apparent isotope patterns of heme ions and tryptically digested peptide ions in matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF) mass spectra (MS) were systematically investigated as a function of laser intensity. Two standard proteins, myoglobin and cytochrome c, were tryptically digested to provide heme b and heme c ions, respectively, along with several peptides. MALDI mass spectra of these materials, co-deposited with one of two matrix materials, 2,5-dihydroxybenzoic acid or α-cyano-4-hydroxycinnamic acid (CHCA), were then generated using various laser intensities. For most peptides, the relative abundance of the second isotopic peak (SIP) to that of the monoisotopic peak was similar to the theoretical relative abundances. Regardless of laser intensity, the experimental SIP abundances of heme ions were consistently higher than the theoretical SIP abundances, which is due to the overlap of isobaric species. A gradual increase in the relative SIP abundance of the heme c ion was observed within a certain laser intensity range when the CHCA matrix was used with higher loadings (80pmol) of tryptically digested cytochrome c, which is likely due to saturation of the MALDI-TOF MS detector.

Effect of Laser Intensity on the Characteristic of Inkjet-Printed Silver Nanoparticles During Continuous Laser Sintering

The variation in electric conductivity was examined for laser irradiation with various beam intensities. A 532-nm continuous wave laser was irradiated onto inkjet-printed silver lines on a glass substrate and the electrical resistance was measured in situ during the irradiation. The results demonstrate that electrical conductivity varies nonlinearly with laser intensity, and has a minimum specific resistance of 3.1 x 10(-8) Ωm at 4 kW/cm2 irradiation. These results are interesting because the specific resistance achieved by the present laser irradiation was approximately 1.9 times lower than the best value obtainable by oven heating, even though it was still higher by 1.9 times than that of bulk silver. It is also demonstrated that the irradiation time required to complete the sintering process decreases with laser intensity. The numerical simulation of laser heating shows that the heating temperature could be as high as 250 degrees C for laser sintering, while it is limited to 250 degrees C for oven sintering. The characteristics of sintering with laser intensity based on the results of field emission scanning electron microscope images are discussed.

Optical properties of parabolic quantum dots with dressed impurity: Combined effects of pressure, temperature and laser intensity

In this paper simultaneous effects of pressure, temperature and laser radiation on the optical absorption coefficient and refractive index of a spherical quantum dot with parabolic confinement and dressed impurity are studied. By means of matrix diagonalization technique, energy eigenvalues and functions are evaluated and used to find the optical properties of the system via density operator method. It is shown that linear and nonlinear optical properties strongly depend on pressure, temperature and dressing laser intensity. The interesting point is that the influence of laser radiation depends on pressure and temperature.

Self diffraction and nonlinear optical properties for 2, 3- Diaminopyridine under cw illumination

The nonlinear absorption and refraction indices for 2,3-Diaminopyridine solution were measured using open-and closed- aperture z-scan techniques, with continuous wave (cw) irradiation. Furthermore, diffraction rings pattern as a result of nonlinear refraction was observed. The effect of concentration, wavelength and laser intensity on the nonlinear absorption, nonlinear refraction and diffraction rings are studied experimentally. It is found that the nonlinear refraction and absorption indexes in order of 10 -8 cm 2 /W and10 -3 cm/W, respectively. We suggested an opportunity to form a new nonlinear-optical media for nonlinear optical application.

The effect of pump-2 laser on Autler-Townes splitting in photoelectron spectra of K2 molecule.

We theoretically investigated Autler-Townes (AT) splitting in the photoelectron spectra of a four-level ladder K2 molecule driven by pump1-pump2-probe pulses by employing the time-dependent wave packet approach. The effect of pump-2 laser intensity and wavelength on AT splitting was studied for the first time. Triple splitting with asymmetric profiles arises because of the non-resonant excitation. The triple splitting transforms to double splitting when pump-2 detuning approaches ±1/2 times of pump-1 Rabi frequency. The splitting between two side band peaks in the triplet or doublet does not change with the pump-2 laser wavelength. The three peaks shift to a lower energy with a different shift as pump-2 wavelength increases. The magnitude of AT splitting increases with increasing pump-2 laser intensity. The asymptotic behavior of AT splitting with the pump-2 laser intensity are interesting at the threshold point of the near resonant region and far-off resonant region.

Comparison of two theories during self-focusing of Gaussian laser beam in thermal conduction-loss predominant plasmas

In the present paper, self-focusing phenomenon occurring as a result of non-linear interaction of intense laser beam with thermal conduction-loss predominant plasmas is studied by following both approaches viz. paraxial theory approach and moment theory approach. Non-linear differential equations for the beam width parameters of laser beam have been set up and solved numerically in both cases to study the variation of beam width parameters with normalized distance of propagation. Effects of laser intensity as well as plasma density on the beam width parameters have also been analyzed. It is observed from the analysis that in case of moment theory approach, strong self-focusing of laser beam is observed as compared to paraxial theory approach.