Metal nanoplasmas as bright sources of hard X-ray pulses

We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and "lightning rod" effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities approximately 10(14)-10(15) W cm(-2). Our experiments show a 13-fold enhancement in hard x-ray yield (10-200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions.     

超短超强激光与液体的相互作用研究

超短超强激光与液体靶相互作用时表现出许多有趣的特点，这明显区别于激光脉冲与固体或气体靶的相互作用情况．文章分别介绍了激光诱发等离子体所产生的高压冲击波、激光空泡、X射线、高能超热电子以及白光，对它们的产生机制及其各自的显著特征进行了综合描述．文章最后对超短超强激光脉冲与各种不同形态的液体靶相互作用的应用前景作了简单介绍。     

X-ray emission from short-pulse laser plasmas

Highly intense picosecond and subpicosecond laser pulses interacting with solids can create hot and dense plasmas which emit x-ray pulses in a broad spectral range from 100 eV up to MeV. The duration of these x-ray pulses depends on the transient behaviour of the relaxation and recombination mechanisms, as well as on the lifetime of energetic electrons produced via nonlinear processes in the plasma. This paper reports experiments using a 1.5-ps laser pulse with high constrast ratio (up to 10¹⁰) and intensities up to 10¹⁸ W cm^-2 irradiating solid targets. Both the line spectrum characteristics of a magnesium plasma, recorded using crystal spectrometers with high spectral resolution, and kinetic calculations have allowed the deduction of plasma parameters in the process of plasma evolution. In addition, hard x-ray pulses from a tantalum plasma were measured and their scaling was explained as bremsstrahlung emission from energetic electrons. Absolute dose values of x-ray pulses are given.     

High repetition rate plasma mirror for temporal contrast enhancement of terawatt femtosecond laser pulses by three orders of magnitude

We present a plasma mirror configuration that improves the temporal pulse contrast of femtosecond terawatt laser pulses by a factor of thousand using a single antireflection coated glass target. The device provides ultra-high contrast for experiments with a maximum repetition rate of 10 Hz. A third-order cross-correlator has been used to measure the temporal pulse contrast for several different plasma mirror targets. It is shown that the ASE can be suppressed to a level of 10⁻¹¹. A comparison between a triggered and an untriggered plasma mirror reveals differences in the intensity distribution of the focused beam. The triggered plasma mirror produces a slightly larger focus due to the expansion of the triggered plasma mirror at −3 ps before the main pulse. We propose a cost-effective AR-coated and a blank glass target to reduce the costs of the consumable target material. High-harmonic radiation on solid surfaces has been generated with different plasma mirror targets to demonstrate the high laser contrast.     

Blow-off of aluminium films

The blow-off of Al targets from a transparent support by single pulses of intense laser irradiation through the support is investigated for target thicknesses up to some m. The blow-off process is theoretically described by calculations of the phase front dynamics in the target, adopted from Harrach's analytical solutions for laser heating and burnthrough of opaque solid slabs. Experimentally two laser beam sources were used: a Nd:YAG laser with a Gaussian intensity profile and a Nd:Glass-laser system with a homogenized flat-top profile. Two types of removal mechanism could be identified. In the first mode (low laser intensities and/or thin targets) the target is completely molten and can easily be blown-off by the vapor originating at the target-support interface. In the second mode (high laser intensities and/or thick targets) superheating of the liquid Al or a burst of the remaining solid layer of the target occurs. In both cases in the second mode the blow-off process is characterized by higher vapor pressures.     

Numerical study of picosecond soft x-ray enhancement from porous targets irradiated by double laser pulses

Simulations show enhanced free-free and free-bound x-ray emission from laser-produced plasmas for both porous and solid targets irradiated by picosecond laser pulses under different prepulse conditions. The porous targets are modeled as a thick solid substrate over-coated with a thin porous layer. Using porous targets and prepulses shows that x-ray yields can be enhanced significantly over single pulses on solid density targets. The optimum conditions of prepulse and porous layer density are investigated by simulations with a fluid and atomic physics code.     

Application of snow nanograin targets for the generation of fast ions in femtosecond laser plasma

We have measured the energy of the directed motion of multiply charged ions produced when solid targets are exposed to low-contrast (10^?3–10^?2) femtosecond laser pulses with intensities 10¹⁵–10¹⁶ W cm^?2. The measurements are based on the recording of spatially resolved X-ray spectra for H-and He-like oxygen ions in the target plane. Analysis of the He_β and Ly_α line profiles has revealed fractions of accelerated ions in plasma with energies from several to several tens of kiloelectronvolts. We show that using a layer of frozen nanometer-size water droplets as the targets leads to an effective absorption of laser pulses and a twofold rise in the energy (to 0.1 MeV) of He-like oxygen ions compared to the use of solid targets.     

Absorption of short laser pulses on solid targets in the ultrarelativistic regime

We report the first direct measurements of total absorption of short laser pulses on solid targets in the ultrarelativistic regime. The data show an enhanced absorption at intensities above 10(20) W/cm(2), reaching 60% for near-normal incidence and 80%-90% for 45 degrees incidence. Two-dimensional particle-in-cell simulations demonstrate that such high absorption is consistent with both interaction with preplasma and hole boring by the intense laser pulse. A large redshift in the second harmonic indicates a surface recession velocity of 0.035c.     

Characteristics of the Spectra of a Plasma near the Surface of Solid and Liquid Targets Exprosed to a Laser Pulse of Complex Form

The results of investigation of the dynamics of the emission spectra of a plasma generated at the surface of liquid and solid targets by a laser pulse of a complex time form are presented. It is shown that the shock wave arising as a result of breakdown on the surface of the solid and developing to heights of the order of 4 mm is a laser-supported detonation shock wave. In the case of solid targets, we recorded additional laser plasma light pulses coming after the main pulse. Two additional light pulses have been recorded for the emission line AlI 396.1 nm. In the case of the liquid, we have not detected additional pulses for the investigated emission lines of the elements at the laser radiation power densities used.     

Extreme ultraviolet interferometry of warm dense matter in laser plasmas

We demonstrate that interferometric probing with extreme ultraviolet (EUV) laser light enables determination of the degree of ionization of the "warm dense matter" produced between the critical and ablation surfaces of laser plasmas. Interferometry has been utilized to measure both transmission and phase information for an EUV laser beam at the photon energy of 58.5 eV, probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness 350 nm) irradiated by a 300 ps duration pulse of wavelength 438 nm and peak irradiance 10(12) W cm(-2). The transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominant ion species. We show that refractive indices η below the solid parylene N (η(solid) = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in C(+).     

Nd-Glass laser with plasma mirror

A high power pulse Nd-glass laser system with plasma mirror is studied. Plasma is created on the surface of a solid target and the action is superradiantly triggered. Long trains of modelocked pulses are generated if the carbon or metal targets are used and Q-switched pulses are observed with dielectric targets like PVC. Tens of joules of light energy are extracted from the laser medium and absorbed by plasma in both regimes. Spatial and temporal structures of the laser beam are very reproducible, the laser action is insensitive to variations in air pressure inside the target chamber. Stimulated Brillouin back-scattering in the underdense plasma is discussed as the nonlinear mechanism governing the plasma reflectivity.     

High repetition rate laser ablation for vapor–liquid–solid nanowire growth

A high repetition rate (500 kHz) solid state laser was used for the ablation process in a Pulsed Laser Deposition (PLD)/Vapor–Liquid–Solid (VLS) growing. A ZnO target was ablated with laser powers between 0.6 W and 1.2 W, and a variable number of pulses per train and trains frequencies. ZnO structures were grown on gold patterned and unpatterned substrates surfaces. Enhanced growth of the nanostructures could be noticed on the catalyst patterned surfaces. Better nanowire morphologies were also observed for bigger number of laser pulses per train. The enhancements are more evident for low laser powers. Based on plume expansion investigations, by using a high speed camera and a Particle-In-Cell (PIC) Monte-Carlo based simulations, the nanostructure morphology variations could be understood on the basis of the plume particles diffusion process and thin film versus VLS growing competition.     

Jamming, two-fluid behavior, and "self-filtration" in concentrated particulate suspensions

We study the flow of model hard-sphere colloidal suspensions at high volume fraction Phi driven through a constriction by a pressure gradient. Above a particle-size dependent limit Phi(0), direct microscopic observations demonstrate jamming and unjamming-conversion of fluid to solid and vice versa-during flow. We show that such a jamming flow produces a reduction in colloid concentration Phi(x) downstream of the constriction. We propose that this "self-filtration" effect is due to a combination of jamming of the particulate part of the system and continuing flow of the liquid part, i.e., the solvent, through the pores of the jammed solid. Thus we link jamming in colloidal and granular media with a "two-fluid-like" picture of the flow of concentrated suspensions. Results are also discussed in the light of the original experiments of Reynolds on dilation in granular materials.     

Z箍缩X射线在金属表面产生电荷分离现象

以"阳"加速器(1 MA,80 ns)驱动的Z箍缩等离子体为X射线源研究X射线加载下金属表面出现的电荷分离现象,Z箍缩负载为16根直径5μm的钨丝组成的丝阵,丝阵半径3 mm。强度107W/cm2、半高宽30 ns的软X射线脉冲通过直径5 mm的限光孔辐照半径30 mm、厚3 mm的铜盘中心,在金属表面产生了脉宽相近,幅值kV的电势。测量了该电势沿金属表面的分布,观测到微弱的调制现象。电势的极性表明电子主要沿金属表面运动而不是垂直表面运动,这表明热电效应是造成电荷分离的主要机制。入射X射线强度较弱时,电子的个体行为——光电效应、康普顿效应占主导;当入射强度较大时,弱关联的集体行为——热效应占主导;进一步增大入射X射线强度将出现强关联的集体行为——电荷密度调制状态。     

High-energy negative ions from expansion of high-temperature femtosecond laser plasma

The atomic, charge and energy distributions of ions in expanding femtosecond laser plasmas have been studied by means of time-of-flight mass–charge spectroscopy. High-energy negative and positive ions with energies of up to 35 keV have been detected during the interaction of 200-fs laser pulses (I2×10¹⁶ W/cm²) with silicon, titanium and other solid targets. A high correlation between negative and positive single-charged ions of the identical atomic number is shown. Negative ions are produced as a result of collisions of fast single-charged ions and neutrals with molecules of residual gas. PACS 52.50.Jm; 52.70.Nc; 52.38.Ph     

Pulsed laser deposition: metal versus oxide ablation

We present experimental results of pulsed laser interaction with metal (Ni, Fe, Nb) and oxide (TiO₂, SrTiO₃, BaTiO₃) targets. The influence of the laser fluence and the number of laser pulses on the resulting target morphology are discussed. Although different responses for metal and oxide targets to repetitive laser irradiation could be expected due to the different band structures of metals and oxides, the optical response is quite similar for 248-nm laser irradiation. Therefore, the difference in response is largely caused by differences in thermal properties. Metal targets show periodic structures of the order of micrometers after consecutive pulses of laser radiation, while the SrTiO₃ and BaTiO₃ targets show a flat surface after ablation for relatively low fluences (1.0 Jcm^-2). The observed TiO₂ target ablation characteristics fall in between those of the ablated metals and perovskites, because ablation results in the presence of Ti-rich material, which shields the underlying stoichiometric target material from ablation. The final target morphology is dependent on fluence, number of pulses, and the movement of the target itself (rotating, scanning, or stationary). It can take between 15 and 75 pulses to reach a steady-state target morphology on a stationary target. PACS 79.20.Ds; 52.38.Mf; 81.15.Fg     

Two-surface wave decay: improved analytical theory and effects on electron acceleration

Two-surface wave decay (TSWD), i.e., the parametric excitation of electron surface waves, was recently proposed as an absorption mechanism in the interaction of ultrashort, intense laser pulses with solid targets. We present an extension of the fluid theory of TSWD to a warm plasma that treats boundary effects consistently. We also present test-particle simulations showing localized enhancement of electron acceleration by TSWD fields; this effect leads to a modulation of the current density entering into the target and may seed current filamentation instabilities. PACS 52.38.-r; 52.38.Dx     

约束靶面黑漆涂层对激光冲击波的影响

 介绍了约束靶表面的黑漆涂层对激光冲击波的影响。激光功率密度 10⁹W/cm²量级，脉宽33ns，波长1.06μm。靶材选用两种不同厚度的铜和铝，靶表面用有机玻璃约束。通过对冲击波压力的直接测量发现，涂层不仅可以增加激光冲击波压力，而且还影响冲击波的演化过程。比较有无涂层的靶面SEM照片发现，黑漆涂层能有效地保护激光辐照表面，使之不受激光烧蚀。     

Toward single attosecond pulses using harmonic emission from solid-density plasmas

We report on investigations of high-order harmonic generation from solid surfaces in the coherent wake emission regime with relativistically intense few-cycle (8 fs) laser pulses. Significant spectral broadening compared to previous experiments with many-cycle pulses and the appearance of substructures on the harmonics are observed that strongly fluctuate from shot-to-shot. Measurements in which the linear polarization was rotated or ellipticity of the laser pulse was varied exhibit a strong dependence of the harmonic emission on the polarization state of the incident pulse. We show that the observed spectral features are ultimately connected to the sub-cycle electron dynamics in the laser-solid interaction and thus proof of the few-cycle nature of the observed harmonic emission. Using a simple model we have investigated the factors that play an important role in the shape of the emitted spectrum.