环境气压对激光烧蚀Cu诱导发光机理的影响

利用时间分辨的光谱测量技术,测定了不同氪气压强下脉冲激光烧蚀金属Cu诱导等离子体发光羽的发射光谱及其强度随时间的分布.利用快速同步照相的方法,拍摄了不同氪气压强下的等离子体发光羽的照片.实验结果表明,等离子体发光羽的光谱主要由原子谱线构成,发光羽颜色随环境气压而变化.结合实验结果探讨了环境气压对脉冲激光烧蚀Cu诱导等离子体发光羽的发光机理的影响,认为不同环境气压下等离子体发光羽的发光机理不同,低压下以电子碰撞传能激发辐射为主,中压下以电子与原子碰撞传能激发和电子与一价离子的复合激发辐射为主,高压下以电子与一价离子的复合激发辐射为主,并用此机理定性地解释了所观察到的实验现象.     

脉冲激光制膜过程中等离子体演化规律的研究

利用有限差分法对脉冲激光沉积(PLD)技术制备KTa065Nb035O3(KTN)薄膜过程中等离子体在等温和绝热两个阶段的速度演化进行了模拟，并给出了其中主要粒子在空间的具体演化规律，对等离子体在空间膨胀的物理机制，进行了深入的讨论，给出了相应演化过程的物理图像，并揭示了等离子体羽辉在膨胀过程中呈现椭球外形的内在原因.     

充空气的同轴慢波结构高功率微波器件粒子模拟

 粒子模拟了电子碰撞空气产生的等离子体对同轴慢波结构高功率微波器件的影响，并且在充空气条件下对器件结构参数进行了进一步优化。模拟表明，气压越高，产生的二极管电流越大，二极管电压越低，频率越低。等离子体离子对电子束的空间电荷中和及等离子体电子对微波的能量吸收共同影响输出微波功率的大小。在一定的气压范围内，提高气压能够提高输出功率，此时等离子体离子对电子束的空间电荷中和起主导作用。气压高于一定值时，所产生的等离子体电子强烈吸收微波，输出功率迅速下降，甚至引起脉冲缩短。此外，由于等离子体的存在，器件最佳相互作用区长度以及最优端面反射系数均有可能发生改变。最后还对慢波结构周期数以及漂移段长度等进行了研究，优化的器件内、外导体周期数为11和8.5，慢波结构前端以及内外慢波结构末端分别接4, 17和2 mm的漂移段，在气压4 Pa下获得了1.64 GW的输出功率，效率39%。     

极不均匀电场下重复频率纳秒脉冲气体击穿实验研究

 实验研究了尖-板电极中，不同重复频率（10,100,500,1 000 Hz）、不同间隙(0.5,1 cm),不同气压（0.1～0.4 MPa）等条件下空气的绝缘特性。得到了击穿时延、重复频率耐受时间、施加脉冲个数等与施加电压、重复频率的关系。研究发现：在该实验条件下击穿时延随着场强与气压的比值减小而增加，但重频耐受时间和脉冲击穿个数并没有明显变化；随着重复频率的提高，击穿时延和重频耐受时间会减小，但脉冲击穿个数可能会增加；且重复频率条件下击穿的极性效应不明显；重复施加的脉冲产生大量的亚稳态粒子和残余电荷影响放电的发展过程；负离子的脱负或正离子碰撞阴极的过程，及亚稳态粒子的去激励作用给击穿提供了有效初始电子。     

气体脉冲放电等离子体阻抗的参量研究

在纯净氦气条件下，对高频纵向脉冲放电的等离子体动态阻抗进行了实验研究.测量了放电等离子体阻抗随放电电压、频率以及气体压强之间的变化关系，表明等离子体阻抗随频率和电压的增加而下降，随气压的升高而增大.定性分析了这些宏观参量与等离子体阻抗变化之间的关系，拟合出等离子体阻抗随电压、频率、气压变化的经验公式，并对数值模拟与实验结果进行了比较研究. 关键词： 等离子体阻抗 气体脉冲放电     

强流电子束碰撞电离背景气体研究

利用数值计算与粒子模拟两种方法，结合实际的实验数据，对高功率微波二极管中相对论电子束与背景气体相互作用碰撞产生的等离子体密度进行了研究.研究结果表明：碰撞产生的等离子体密度数值计算结果与粒子模拟结果基本一致，背景气压在0.01 Pa—0.05 Pa时，碰撞产生的等离子体密度在4—12×10⁹cm^-3，即便在考虑电子离子复合的情况下，数值计算结果与粒子模拟结果依然符合得很好.另外，粒子模拟结果表明：随着气压的增加，等离子体密度呈现先增大再减小然后又逐渐增大的过程， 关键词： 相对论电子束 等离子体 数值计算 粒子模拟     

等离子体填充0.14 THz相对论返波管模拟

模拟了0.14 THz相对论返波管中电子束与氩气相互作用产生等离子体的过程, 研究了在不同气压条件下, 等离子体对相对论返波管的输出功率、频率以及起振时间的影响. 模拟结果表明, 等离子体背景能引起太赫兹波段真空电子器件脉冲缩短, 并出现新的频率分量; 适当地注入等离子体能减少0.14 THz相对论返波管的起振时间, 提高器件的输出功率. 关键词： 太赫兹 相对论返波管 粒子模拟 等离子体     

Ar环境气压对脉冲激光烧蚀制备纳米Si晶粒平均尺寸的影响

采用XeCl脉冲准分子激光器，烧蚀高阻抗单晶Si靶，在1—500 Pa的Ar气环境下沉积制备了纳米Si薄膜. x射线衍射谱测量证实，纳米Si晶粒已经形成.利用扫描电子显微镜观测了所形成纳米Si薄膜的表面形貌，结果表明，随着环境气压的增加，所形成的纳米Si晶粒的平均尺寸增大，气压为100 Pa时达到最大值20 nm，而后开始减小. 从晶粒形成动力学角度，对实验结果进行了定性分析. 关键词： 纳米Si晶粒 脉冲激光烧蚀 表面形貌     

短脉冲强激光在稀薄等离子体中传播时的电磁类孤立子

 采用二维粒子模拟方法，研究了短脉冲强激光在稀薄等离子体中传播时电磁类孤立子的产生和时空演化过程。通过分析电磁场与等离子体波的非线性能量交换和激光场的频率谱结构等，给出了电磁类孤立子形成的基本物理图像，讨论了等离子体参数对电磁类孤立子形成的影响。模拟结果表明：类孤立子的形成是由于局部电磁波振荡频率减小至等离子体频率引起的，初始等离子体密度越高越容易形成空间局域结构。     

强激光和超薄等离子体薄膜相互作用产生高次谐波

通过一维粒子（particle in cell,简称PIC）模拟，研制了超薄等离子体薄膜在两列旋转方向相反的圆偏振激光脉冲作用下的高次谐波辐射。在10^21W/cm^2强度的激光作用下，可以获得200次以上的高次谐波。当激光脉冲宽度较短时，由于等离子体薄膜被压缩所导致的多普勒频移对高次谐波产生了显著的频率调制。而这种多普勒频移可以通过对抽运光脉冲引入频率啁啾来补偿。     

Expansion dynamics of the plasma plume created by laser ablation in a background gas

The dynamics of the expansion of the plasma plume induced by laser ablation of a copper target at a fluence of 17 J/cm² was investigated theoretically by means of a Monte Carlo simulation. When the expansion occurs under a relatively high pressure, the ambient gas particles may be involved in the collective motion of the plume. The simulation allows the study of the simultaneous collective motion of different species, such as the laser-ablated and the ambient gas particles. The influence of the background gas nature and pressure on the laser-induced plasma plume expansion behavior was studied. The expansion dynamics were found to be different in the case of the expansion in ambient gases of different molecular weight. The dynamics of the plume expansion under an argon pressure of 200 Pa seem to be strongly related to the equilibration of the pressure gradients in the gas phase, and evidence of the oscillatory behavior of the plume expansion was shown from the evolution over time of the pressure profiles in the plume. This behavior has also been observed in similar conditions for a krypton atmosphere, but for a lower pressure than for argon. The vortical flow formation at the plume periphery, involving both the laser-ablated and the argon particles at moderate pressure, was also predicted from the Monte Carlo simulation.     

Formation of surface stabilized Si nanocrystal by pulsed laser ablation in hydrogen gas

We prepared silicon nanocrystallites by pulsed laser ablation (PLA) of a Si target in hydrogen background gas. A mixture of hydrogen and helium was used as a background gas and the hydrogen partial pressure was varied. The deposited nanocrystal-film system shows a hierarchical structure composed of surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The size of the primary particles was not sensitive to the hydrogen partial pressure, while the porosity of the secondary structure constituted by the aggregation of the primary particles increased with increasing hydrogen partial pressure. This indicates that the surface is stabilized and that aggregation of the primary structure is depressed by surface hydrogenation. The optical gap energy of the deposits shifted to higher energy with increasing hydrogen partial pressure due to the formation of well-isolated nanocrystallites by surface stabilization. These results indicate that PLA in hydrogen gas is a promising technique to prepare surface stabilized and controlled silicon nanocrystallites.     

A shadowed off-axis production of Ge nanoparticles in Ar gas atmosphere by pulsed laser deposition

In this work, we report on the production of Ge nanoparticles (NPs) in an inert Ar gas atmosphere by pulsed laser deposition (PLD) at room temperature (RT). The direct deposition of energetic particles/droplets resulting from the ablation process of the target material has been avoided by using an original and customized off-axis shadow mask (shadowed off-axis) deposition set-up where the NPs deposition on the substrate takes place by means of scattering between the NPs formed in the vapor phase and the background Ar atoms. It is found that the Ar gas pressure parameter has a relevant role in the crystallization process, with better crystallinity obtained as the background Ar pressure is raised for the given experimental conditions.     

Experimental and theoretical analysis of bias ionization by?α-particles in a nitrogen laser

Nitrogen laser performance with TE configuration and wedge electrodes is analyzed with background ionization in the laser discharge channel by α particles at a low exposition rate. With the bias ionization, the laser power presents two peaks as a function of gas pressure, with one at the normal low pressure, without bias ionization, and the other at high pressure generated by bias ionization. A simple theoretical model has been developed in a trial to understand this behavior. This model was first tested in later results for a TE configuration nitrogen laser, with flat electrodes, without and with bias ionization. It has been observed that due to the competition between electrode shielding by positively charged α particles and bulk ionization by impact, the laser energy is suppressed with pressure below 50 Torr and enhanced above it.     

Surface nanostructuring of metals by laser irradiation: effects of pulse duration, wavelength and gas atmosphere

Surface modifications by nanostructuring present a new laser application for improvement of surface properties such as adhesion, mechanical characteristics or corrosion protection. In this study, we report the formation of nanoparticles by laser irradiation of a steel surface. The influence of laser parameters such as pulse duration (25–30 ns, 500 fs), wavelength (248 nm, 308 nm), and the background gas pressure (10 mbar-1 bar) on the formation of this back deposition layer composed of aggregated iron oxide nanoparticles were investigated. Scanning electron microscopy and atomic force microscopy were used to characterise the irradiated steel surface and the particle morphology deposited by backward flux. In the nanosecond laser ablation regime, films are formed by aggregated nanoparticles with well developed cauliflower like structures, the size and the morphology depending on the nature and pressure of the background gas. In the femtosecond regime, we observed the formation of micrometer sized structures at the steel surface. In particular, a non-conventional mechanism of nanocluster condensation and growth is revealed since two different ablation rates corresponding to two different predominant processes are observed. These analyses demonstrate the possibility of controlling the distribution and the size of particles by varying the laser parameters and the background gas pressure and nature. PACS 52.38.Mf; 81.65.-b; 81.15.Gh.     

气压对激光烧蚀Al等离子体中粒子速度的影响

在背景气压为8× 10-3— 100Pa范围内 ,通过测量脉冲激光烧蚀平面Al靶产生的等离子体辐射谱的时间分辨特征 ,比较空间不同点辐射的飞行时间轮廓的相对延迟 ,从而得到辐射粒子速度及其空间分布 .利用绝热膨胀的理论和激波模型分别对背景气压小于 0 .6Pa的结果和 5Pa时的结果作了分析 ,并得出激波的波面基本上为柱对称. With the ambient gas pressure in the range from 8×10 -3 to 10 2 Pa, Q-switched YAG laser ablates plane aluminum target and plasma are produced. Optical emission spectroscopy is used to carry out time-resolved analysis of atomic particles. Using the resonance transition of Al I 396.1 nm (3p 2P-4s 2S), the spatiol velocity distribution of Al I has been obtained under the laser energies of 160 mJ/pulse when the ablating size is about 200 μm. The velocity is at the order of 10 6 cm/s...     

蒸气压对激光辐照靶材烧蚀速率的影响

研究了强激光辐照碳/碳复合材料靶材引起的烧蚀现象及蒸气压对烧蚀速率的影响。基于傅里叶定律，建立了强激光辐照靶材的热传导模型，模拟了忽略蒸气压影响时烧蚀速率随功率的变化；通过Mott-smith近似方法描述了Knudsen层间断区域，分析了间断两侧表面粒子状态参数；结合质量连续方程和蒸气压与温度关系方程，并由气体状态方程描述蒸气流状态，对蒸气压条件下激光烧蚀碳/碳复合材料靶材的速率随功率变化的关系进行了数值模拟。结果表明，在高能激光对靶材的烧蚀过程中，蒸气压力变化会导致靶材的饱和蒸气温度发生变化，进而影响烧蚀速率且使其随功率呈非线性变化，与忽略蒸气压作用时的线性变化规律相差较大，从理论上解释了忽略蒸气压导致的实验数据与理论结果的差异。     

Nanoparticle formation by laser ablation in air and by spark discharges at atmospheric pressure

Recent promising methods of nanoparticle fabrication include laser ablation and spark discharge. Despite different experimental conditions, a striking similarity is often observed in the sizes of the obtained particles. To explain this result, we elucidate physical mechanisms involved in the formation of metallic nanoparticles. In particular, we compare supersaturation degree and sizes of critical nucleus obtained under laser ablation conditions with that obtained for spark discharge in air. For this, the dynamics of the expansion of either ablated or eroded products is described by using a three-dimensional blast wave model. Firstly, we consider nanosecond laser ablation in air. In the presence of a background gas, the plume expansion is limited by the gas pressure. Nanoparticles are mostly formed by nucleation and condensation taking place in the supersaturated vapor. Secondly, we investigate nanoparticles formation by spark discharge at atmospheric pressure. After efficient photoionization and streamer expansion, the cathode material suffers erosion and NPs appear. The calculation results allow us to examine the sizes of critical nuclei as function of the experimental parameters and to reveal the conditions favorable for the size reduction and for the increase in the nanoparticle yield.     

Background gas collisional effects on expanding fs and ns laser ablation plumes

The collisional effects of a background gas on expanding ultrafast and short pulse laser ablation plumes were investigated by varying background pressure from vacuum to atmospheric pressure levels. For producing Cu ablation plumes, either 40 fs, 800 nm pulses from a Ti: Sapphire laser or 6 ns, 1,064 nm pulses from a Nd:YAG laser were used. The role of background pressure on plume hydrodynamics, spectral emission features, absolute line intensities, signal to background ratios and ablation craters was studied. Though the signal intensities were found to be maximum near to atmospheric pressure levels, the optimum signal to background ratios are observed ~20–50 Torr for both ns and fs laser ablation plumes. The differences in laser–target and laser–plasma couplings between ns and fs lasers were found to be more engraved in the crater morphologies and plasma hydrodynamic expansion features.