The inverse Bremsstrahlung absorption in the presence of Maxwellian and non-Maxwellian electrons

Inverse Bremsstrahlung absorption(IBA) of an intense laser field in plasma containing Maxwellian and nonMaxwellian(with Kappa and q-nonextensive distribution functions) electrons is studied analytically. Our results show that IBA decreases with an increase in temperature at high intensities and a decrease in plasma density for all kinds of distribution functions. Another striking result is that IBA is independent of the laser intensity at low intensity but is dependent on it when the intensity is going to rise. Also, it could be find that the behavior of the absorption as the function of laser intensity for the Kappa distribution with κ = 10 at low intensity is close to that for the Maxwellian distribution, but at high intensity it is close to that in the presence of q-nonextensive electrons with q = 0.9. These results provide insights into the inverse Bremsstrahlung absorption in the laser–plasma interactions.     

Dose estimation and shielding calculation for X-ray hazard at high intensity laser facilities

An ionizing radiation hazard produced from the interaction between high intensity lasers and solid targets has been observed. Laser-plasma interactions create "hot" electrons, which generate bremsstrahlung X-rays when they interact with ions in the target. However, up to now only limited studies have been conducted on this laser-induced radiological protection issue. In this paper, the physical process and characteristics of the interaction between high intensity lasers and solid targets are analyzed. The parameters of the radiation sources are discussed, including the energy conversion efficiency from laser to hot electrons, hot electron energy spectrum and electron temperature, and the bremsstrahlung X-ray energy spectrum produced by hot electrons. Based on this information, the X-ray dose generated with high-Z targets for laser intensities between 10¹⁴ and 10²⁰ W/cm² is estimated. The shielding effects of common shielding items such as the glass view port, aluminum chamber wall and concrete wall are also studied using the FLUKA Monte Carlo code. This study provides a reference for the dose estimation and the shielding design of high intensity laser facilities.     

Laser propulsion with a high specific impulse using a thin film propellant

A new laser propulsion scheme with a high specific impulse is proposed in this paper.An extremely thin polyimide film is used as the propellant to eliminate thermal diffusion and sputter from the target material.It is found that a high specific impulse of 1520 s can be achieved at 10 11-W/cm 2 laser intensity because of economic use of the propellant.The influences of the laser intensity and the ablation area on the specific impulse are also studied in the experiment.     

Affirming nonlinear optical coefficient constancy from z-scan measurement

Goodness of fit is demonstrated for theoretical calculation of z-scan data based on beams propagating in the nonlinear medium and the Fresnel–Kirchhoff diffraction integral in experiments with high nonlinear refraction and absorption. The constancy of nonlinear optical parameters is achieved regardless of sample thickness and laser intensity, which clarifies the physical significance of optical parameters. We have obtained γ = 2.0 × 10-19 m2/W and β = 5.0 × 10-13 m/W for carbon disulfide excited by a pulsed laser at 800 nm with pulse duration of 35 fs,which are independent of sample thickness and laser intensity. Affirming constancy of the extracted parameters to the incident light intensity may become a practice to verify the goodness of the z-scan experiment.     

Trends in ultrashort and ultrahigh power laser pulses based on optical parametric chirped pulse amplification

Since the proof-of-principle demonstration of optical parametric amplification to efficiently amplify chirped laser pulses in 1992,optical parametric chirped pulse amplification(OPCPA)became the most promising method for the amplification of broadband optical pulses.In the meantime,we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers.The output power and pulse duration of these systems have ranged from a few gigawatts to hundreds of terawatts with a potential of tens of petawatts power level.Meanwhile,the output pulse duration based on optical parametric amplification has entered the range of fewoptical-cycle field.In this paper,we overview the basic principles,trends in development,and current state of the ultrashort and laser systems based on OPCPA,respectively.     

Carrier-envelope phase effects on high-harmonic generation driven by mid-infrared laser field

The influence of the carrier-envelope phase on high-harmonic generation is investigated, both experimentally and theoretically, for three different interaction gas media, driven by mid-infrared, few-cycle and CEP-stabiUzed laser pulses. Different patterns of harmonic spectra with varying CEP for the three interaction gas media are observed. Furthermore, in comparing our experiment results to the previous works driven by near-infrared laser pulses, different phenomena are found. Through numerical simulation, we find that for the two different kinds of driving fields, i.e. mid-infrared and near-infrared laser pulses, different kinds of electron trajectories contribute to the generation of high harmonics.     

Characteristics of ultrafast K line hard x—ray source from femtosecond terawatt laser—produced plasma

Theoretical studies and analytical scalings were carried out to find the optimized laser parameters and target conditions so that ultrashort hard x-ray pulses and high x-ray power could be achieved.The dependence of laser intensity and wavelength on the yield of K-shell x-ray emission was studied.We propose an optimal design for a foil target for producing high-yield hard x-ray pulses of customizing duration.     

Analysis of electromagnetic pulses generated from ultrashort laser irradiation of solid targets at CLAPA

Electromagnetic pulses(EMPs)produced by the interaction of a TW femtosecond laser with solid targets at the Compact Laser Plasma Accelerator(CLAPA)are measured and interpreted.The statistical results confirm that the intensities of the EMPs are closely related to both target material and thickness.The signal of the titanium target is more abundant than that of the copper target with the same thickness,and the intensity of EMP is positively correlated with the target thickness for aluminium foil.With the boosted EMP radiations,the energy of accelerated protons is also simultaneously enhanced.In addition,EMPs emitted from the front of the target exceed those from the rear,which are also pertinent to the specific target position.The resonant waveforms in the target chamber are analyzed using the fast Fourier transform,and the local resonance and the attenuation lead to changes of the frequency spectra of EMPs with variation of detecting positions,which is well supported by the modeling results.The findings are beneficial to gaining insight into the mechanism of EMP propagation in a typical target chamber and providing more information for EMP shielding design.     

Effect of Time-Dependent Ionization on Propagation of a Few-Cycle Circularly Polarized Laser Pulse in Two-Level Medium

Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly （elliptically） polarized laser pulses in an open two-level medium （two-level plus continuum model） is investigated based on the conventional two-level model proposed by Slavcheva and Hess （Phys. Rev. A 72 （2005） 053804）, and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the dosed two-level model.     

Laser-produced plasma helium-like titanium Kα x-ray source and its application to Rayleigh-Taylor instability study

Several experiments are performed on the ShenGuang-II laser facility to investigate an x-ray source and test radiography concepts.X-ray lines emitted from laser-produced plasmas are the most practical means of generating these high intensity sources.By using a time-integrated space-resolved keV spectroscope and pinhole camera,potential helium-like titanium Kα x-ray backlighting (radiography) line source is studied as a function of laser wavelength,ratio of pre-pulse intensity to main pulse intensity,and laser intensity (from 7.25 to～11.3×10 15 W/cm 2).One-dimensional radiography using a grid consisting of 5 μm Au wires on 16 μm period and the pinhole-assisted point projection is tested.The measurements show that the size of the helium-like titanium Kα source from a simple foil target is larger than 100 μm,and relative x-ray line emission conversion efficiency ξ x from the incident laser light energy to heliumlike titanium K-shell spectrum increases significantly with pre-pulse intensity increasing,increases rapidly with laser wavelength decreasing,and increases moderately with main laser intensity increasing.It is also found that a gold gird foils can reach an imaging resolution better than 5-μm featured with high contrast.It is further demonstrated that the pinhole-assisted point projection at such a level will be a novel two-dimensional imaging diagnostic technique for inertial confinement fusion experiments.     

Interaction of intense laser pulses with hydrogen atomic clusters

The interaction between intense femtosecond laser pulses and hydrogen atomic clusters is studied by a simplified Coulomb explosion model. The dependences of average proton kinetic energy on cluster size, pulse duration, laser intensity and wavelength are studied respectively. The calculated results indicate that the irradiation of a femtosecond laser of longer wavelength on hydrogen atomic clusters may be a simple, economical way to produce highly kinetic hydrogen ions. The phenomenon suggests that the irradiation of femtosecond laser of longer wavelength on deuterium atomic clusters may be easier than that of shorter wavelength to drive nuclear fusion reactions. The product of the laser intensity and the squared laser wavelength needed to make proton energy saturated as a function of the squared cluster radius is also investigated. The proton energy distribution calculated is also shown and compared with the experimental data. Our results are in agreement with the experimental results fairly well.     

A method to obtain pulse contrast on a single shot

A novel method for obtaining a single shot multi-point high dynamic range pulse contrast measurement is presented. We use Dammann gratings to generate multiple beamlets by division of amplitude on ultrashort laser pulses. The analysis results show that this method can achieve high dynamic range in pulse contrast measurement on a single shot by using photomultiplier tube （PMT） detectors and the long working distances to minimize cross talk between channels. Some distortion of pulse shape is also analyzed detailedly with the Dammann grating and its compensation grating, which may degrade the pulse contrast measurement in some degree by pulse stretching and spectrum clipping.     

Processes underlying the laser photochromic effect in colloidal plasmonic nanoparticle aggregates

We have studied the dynamic and static processes occurring in disordered multiparticle colloidal Ag aggregates with natural structure and affecting their plasmonic absorption spectra under pico-and nanosecond pulsed laser radiations, as well as the physical origin responsible for these processes. We have shown that depending on the duration of the laser pulse,the mechanisms of laser modification of such aggregates can be associated both with changes in the resonant properties of the particles due to their heating and melting(picosecond irradiation mode) and with the particle shifts in the resonant domains of the aggregates(nanosecond pulses) which depend on the wavelength, intensity, and polarization of the radiation.These mechanisms result in formation of a narrow dip in the plasmonic absorption spectrum of the aggregates near the laser radiation wavelength and affect the shape and position of the dip. The effect of polydispersity of nanoparticle aggregates on laser photochromic reaction has been studied.     

Theoretical study and optimization of a high power mid-infrared erbium-doped ZBLAN fibre laser

Based on the rate equations describing the erbium-doped fluoride glass(ZBLAN) fibre lasers with different pumping configurations being taken into account,this paper presents theoretical calculations related to the dynamic population density and the operation performance of a high power mid-infrared all-fibre erbium-doped ZBLAN fibre laser.It shows that the ground-state absorption,excited-state absorption and energy-transfer-upconversion processes co-exist and produce a population balance,causing the laser to operate stably at a continuous wave state.A good agreement between the theoretical results and recent experimental measurement is obtained.Furthermore,the laser structure parameters including fibre length,reflectance of output fibre Bragg grating and pumping configurations are quantitatively optimised to achieve the best performance.The results show,as expected,that the slope efficiency of the fibre laser can be improved significantly through optimisation,which then provides an important guide for the design of high-performance mid-infrared erbium-doped ZBLAN fibre lasers.     

Theoretical study on the laser gain of a multi-stage dye laser amplifier

Theoretical studies on laser gain of a multi-stage dye laser amplifier are presented in this paper. The resultsshow that the influences of amplified spontaneous emission(ASE)and nonlinear absorption of excited-state on the gain are different for different input laser energies and gains. A threshold input intensityexists for a specific system. If the imput intensity is higher than the threshold, the nonlinear absorptionof excited-state will be the main cause for gain decrease. Otherwise, the ASE is the main cause. A newscheme is proposed to calculate the gain of the amplifier by the comparison of the input intensity with theeffective saturation intensity and the choice of the calculation gain method.     

A study of ultrafast electron diffusion kinetics in ultrashort-pulse laser ablation of metals

Temperature dependence of the electron diffusion in metallic targets, where the electron--electron collision is the dominant process, is investigated with the help of an extended two-temperature model. In sharp contrast to the low electron temperature case, where only the electron---phonon collisions are commonly considered, the electron diffusion process underlying the high electron temperatures evolves dramatically different in both temporal and spatial domains. Calculated results of the ablation yield at different pulse durations are presented for a copper plate impinged by ultrashort laser pulses with energy fluences ranging from 0.1 J/cm² to 10 J/cm². The excellent agreement between the simulation results and the experimental data indicates the significant role of electron--electron collisions in material ablations using intense ultrashort laser pulses.     

Pharmacokinetic Monitoring of Indocyanine Green for Tumor Detection Using Photoacoustic Imaging

We report tumor detection using a photoacoustic technique for the imaging of angiogenesis and monitoring of agent pharmacokinetics on an animal model. We take 532-nm laser pulses to excite photoacoustic signals of blood vessels with acquisition by a broadband hydrophone, and the morphological characteristics of tumor angiogenesis are successfully image depicted. Furthermore, tumor pharmacokinetics is preformed and analyzed with fast multielement photoacoustic imaging of the intravenous-injected indocyanine green （ICG）. Photoacoustic signals of ICG are excited with 805 nm laser pulses and recorded by transducer array as a function of time. The difference between the photoacoustic signal from the tumor side and that from the normal side is observed, and the ICG clearance velocity in the tumor area is found to lag behind that in the normal area. Experimental results demonstrate that photoacoustie imaging of morphological parameter and pharmacokinetics with specific agent may provide high sensitive approach for tumor detection and localization.     

Scattering of light waves by electron electrostatic waves in laser produced plasmas

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov-Maxwell numerical simulation.It is found that the light waves can be scattered by electron plasma waves as well as other heavily and weakly damping electron wave modes,corresponding to stimulated Raman and Brilluoin-like scatterings.The stimulated electron acoustic wave scattering is also observed as a high scattering level.High frequency plasma wave scattering is also observed.These electron electrostatic wave modes are due to a non-thermal electron distribution produced by the wave-particle interactions.The collision effects on stimulated electron acoustic wave and the laser intensity effects on the scattering spectra are also investigated.     

PM567-Doped solid dye lasers based on PMMA

Polymers are a kind of attractive hosts for laser dyes due to their high transparency in both pumping and lasing ranges and superior optical homogeneity. In this paper solid dye samples based on polymethyl methacrylate (PMMA) doped with different concentrations of 1, 3, 5, 7, 8 -pentamethyl-2, 6-diethylpyrromethene-BF$_{2 }$ (PM567) are prepared. The absorption, fluorescence and lasing spectra of the samples are obtained. Wide absorption and fluorescence bands are obtained and a red shift of the maxima of the lasing emission spectra is observed. With the second-harmonic generation of Q-switched Nd:YAG laser (532\,nm, $\sim $20\,ns) pumping the samples longitudinally, the slope efficiencies of the samples are obtained. There is an optimal dye concentration for the highest slope efficiency when the pumping energy is lower than some typical value ($\sim $250\,mJ), and the highest slope efficiency 35.6{\%} is obtained in the sample with a dye concentration of $2\times10^{ - 4}$\,mol/L. Pumping the samples at a rate of 10Hz with a pulse energy as high as 200\,mJ (the fluence is 0.2\,J/cm$^{2})$, the output energy drops to one-half of its initial value after approximate 15500 pulses and the normalized photostability is 5.17\,GJ/mol. A kind of solid dye laser which could have some applications is built.     

Chirping and spreading of ultrashort laser pulses in underdense plasmas

A linear theory on the propagation of ultrashort pulses including only a few cycles in underdense plasmas is presented. It is shown that the dispersion in plasmas causes severe distortions in the pulse shape, including pulse chirping and spreading. The analytical calculations coincide very well with those obtained by particle-in-cell (PIC) simulations. The upper limit of the peak amplitude of the pulses, above which the linear theory breaks down due to the setting in of nonlinear effects of both the relativistic electron-mass increase and ponderomotive force, is also examined by PIC simulations. At certain high amplitudes, it is found that the ultrashort laser pulses can propagate like solitons.