Defocusing role in femtosecond filamentation: Higher-order Kerr effect or plasma effect?

The femtosecond filamentation in the classical and high-order Kerr （HOK） models is numerically investigated by adopting multi-photon ionization （MPI） cross section with different values. It is found that in the case that the MPI cross section is relatively small, there exists a big difference between the electron density as well as clamped intensity calculated in the classical model and those calculated in the HOK one, while in the case that the MPI cross section is relatively large, the electron density and clamped intensity calculated in the two models are nearly in agreement with each other, and under this circumstance, even if the higher-order nonlinear terms do exist, the free-charge generation and the associated defocusing in a filament are enough to mask their effects. The different behaviors of the maximum intensity and on-axis electron density at the collapse position with the pulse duration provides an approach to determine which effect plays the dominant defocusing role. These results demonstrate that it is ionization that results in the difference between the two models.

Low Resistance and Long Lifetime Plasma Channel Generated by Filamentation of Femtosecond Laser Pulses in Air

An experimental investigation of the length and resistance of the laser plasma channel generated by filamentation of fs laser pulses in air was presented. It was found that the length of the plasma channel was different from that of the laser filament. This phenomenon was commensurate with a special self-guide mechanism without ionization. Through increase of the laser energy the plasma channel could be prolonged and the resistance could be reduced. To get even lower resistance, more effective control would be needed over the multi-filament. The lifetime of the plasma channel was increased by a factor of 4.5 by pulse sequences, which were generated by detuning the regenerative amplifier in the chirped pulse amplification （CPA） laser system.

Relativistic Filamentation of Intense Laser Beam in Inhomogeneous Plasma

In the paper, relativistic filamentation of intense laser beam in inhomogeneous plasma is investigated based on the nonparaxial region theory. The results show that, relativistic nonlinearity plays a main role in beam filamentation, and plasma inhomogeneity further reinforces the beam filamentation. The combination effects of relativistic nonlinearity and plasma inhomogeneity can generate particularly intense and short pulse laser. However, plasma inhomogeneity leads to obvious filamentation instability.

Dependence of Plasma Parameters on Plate Separation and Filament Location in a Double Plasma Device

A pair of stainless steel (ss) plates separates the source and target regions of a double plasma device. Two sets of tungsten filaments, placed at different distances from the ss plates, are then used to produce plasma alternately and the plasma parameters in the source and target regions for different discharge voltage, discharge current and plate separation are measured using Langmuir probes. It is found that plasma density and electron temperature are considerably affected and respond differently to changes in the plate separations and the position of the filaments.

Filamentation instability in two counter-streaming laser plasmas

The filamentation instability was observed in the interaction of two counter-streaming laser ablated plasma flows, which were supersonic, collisionless, and also closely relevant to astrophysical conditions. The plasma flows were created by irradiating a pair of oppositely standing plastic （CH） foils with Ins-pulsed laser beams of total energy of 1.7 kJ in two laser spots. With characteristics diagnosed in experiments, the calculated features of Weibel-type filaments are in good agreement with measurements.

Interactions between multiple filaments and bacterial biofilms on the surface of an apple

In this paper, the interactions between two dielectric barrier discharge(DBD) filaments and three bacterial biofilms are simulated. The modeling of a DBD streamer is studied by means of 2D finite element calculation. The model is described by the proper governing equations of air DBD at atmospheric pressure and room temperature. The electric field in the computing domain and the self-consistent transportation of reactive species between a cathode and biofilms on the surface of an apple are realized by solving a Poisson equation and continuity equations. The electron temperature is solved by the electron energy conservation equation. The conductivity and permittivity of bacterial biofilms are considered, and the shapes of the bacterial biofilms are irregular in the uncertainty and randomness of colony growth. The distribution of the electrons suggests that two plasma channels divide into three plasma channels when the streamer are 1 mm from the biofilms. The toe-shapes of the biofilms and the simultaneous effect of two streamer heads result in a high electric field around the biofilms, therefore the stronger ionization facilitates the major part of two streamers combined into one streamer and three streamers arise.The distribution of the reactive oxygen species and the reactive nitrogen species captured by time fluences are non-uniform due to the toe-shaped bacterial biofilms. However, the plasma can intrude into the cavities in the adjacent biofilms due to the μm-scale mean free path. The two streamers case has a larger treatment area and realizes the simultaneous treatment of three biofilms compared with one streamer case.

Properties of long light filaments in natural environment

The multiple filamentation of terawatt femtosecond （fs） laser pulses is experimentally studied in a natural environment. A more than 30-m long plasma filament with a millimeter diameter is formed by the collimated fs laser pulse freely propagating in an open atmosphere. This study provides the first quantitative experimental data about the electron density of a long range light filament in the atmosphere. The electron density of such a filament is quantitatively detected by using an electric method, showing that it is at the 1011-cm -3 level.

超快激光诱导等离子体成丝光谱

飞秒激光等离子体光丝大气中传输的独特物理过程在航天领域具有重要的应用前景。采用仿真与实验,验证了飞秒等离子体成丝的光谱展宽特征,获得了超快等离子体光丝在不同参数特征下,覆盖从可见到红外探测系统的波谱范围,及保持系统相对稳定的特征,对于借助强激光等离子体光丝光谱进行红外特征探测具有重要的启发作用。

CVD金刚石膜研究进展

金刚石由于其优异的声、光、电、热和力学性能,是重要的功能材料之一。金刚石的制备方法主要有高温高压方法和低压化学气相沉积方法。化学气相沉积法因制备得到的样品质量高、面积大,设备简单、可规模化等特性,是合成金刚石膜的重要方法。为了实现低合成压力条件下的金刚石膜的均匀、快速、大尺寸、高质量生长,目前研究人员在金刚石低压生长的控制方面做出了深入的研究。文章综述了近年来化学气相沉积法(包括热丝CVD法、离子体增强CVD法、燃烧火焰CVD法)生长金刚石膜的研究进展,包括金刚石膜的生长机理、关键设备、关键工艺参数等。此外,还详细讨论了生长过程中的关键工艺参数与金刚石膜生长速率和质量的关系,这些对化学气相沉积制备金刚石膜的研究、生产至关重要。

超短强激光大气等离子体细丝中太赫兹辐射现象及其应用

超短强激光在大气中传输形成的等离子体细丝由于能够辐射出太赫兹信号而成为一种新的太赫兹辐射源,这对实现远距离太赫兹的远程传感具有重要的意义,在太赫兹辐射产生技术领域备受关注。详细综述了超短强激光大气等离子体细丝中产生太赫兹辐射现象的研究进展和现状,并着重对等离子体细丝产生太赫兹辐射的形成机理进行了概述和初步探讨,最后对其应用前景进行了展望,进而提出目前尚未解决的问题。

飞秒超强激光在空气中光丝现象的研究

综述了飞秒超强激光脉冲在大气中传输机理的研究进展和现状 ,阐述了光丝现象的形成原理及其传输机制 ,并对其实验方法及其在实际中的应用做了详细分析 。

飞秒激光成丝诱导Cu等离子体的温度和电子密度

研究了飞秒激光成丝诱导铜击穿光谱,利用光发射光谱对产生的铜等离子体光谱强度沿着丝长度进行了测量,获得了在不同样品与聚焦透镜间距离的Cu(I)的强度分布.结果显示,由于强度钳箍效应成丝诱导的光谱在较大的透镜样品间距离范围内有较强的辐射强度.另外,利用玻尔兹曼图和斯塔克展宽计算了整个成丝繁衍距离中Cu等离子体温度和电子密度.

CVD金刚石膜制备方法及其应用

介绍了金刚石膜的应用和低压下化学汽相沉积金刚石膜的主要方法及其最新进展，并对各种方法的优缺点作了简要评述。

野外环境下太瓦飞秒激光等离子体通道特性研究

对太瓦(TW)飞秒激光在自然大气中传输时产生的超长等离子体通道的物理性质进行了研究。试验结果证实2 TW飞秒激光在大气中自由传输时实现了2km长的等离子体通道,长距离传输后通道内的等离子体电子密度约为10^11 cm^-3,仍然保持良好的导电性。高压放电试验也证实了有等离子体通道存在,可以将放电电压降低30%,说明激光诱导高压放电的有效性。本次试验研究表明长度为公里量级、长寿命大气等离子体通道的实现是可行的,将为激光引雷、大气监测、人工干预气候等实用化应用扫清技术上的障碍。

激光成丝不稳定性并行数值模拟研究

实现了三维非线性流体力学与激光传播耦合的并行数值模拟程序，用来研究高强度激光在稀疏等离子体中的激光成丝不稳定性。并行数值实验结果表明，该程序在64台处理机上取得了较好的并行加速比，且能定性模拟激光成丝现象。

取向碳纳米管阵列的等离子体复合化学气相沉积法制备

采用热丝和射频等离子体复合化学气相沉积技术,用旋涂法制备负载催化剂的硅片衬底,以CH4为碳源制备出取向碳纳米管阵列薄膜.利用扫描电子显微镜对不同还原时间和不同N i(NO3)2浓度下制备的催化剂基片和取向碳纳米管阵列薄膜进行形貌分析,用透射电子显微镜和拉曼光谱对碳纳米管进行表征.结果表明,在H2-N2气氛中热还原后硅片上的催化剂粒径均匀,排列致密,利用该法制备的碳纳米管为竹节型多壁碳纳米管,管径分布均匀,管长约5μm.碳纳米管阵列薄膜垂直于硅片衬底生长,生长排列均匀致密,具有良好的取向性.

大气中飞秒激光等离子体丝上荧光特性的研究

本文通过研究超短强激光脉冲在空气中形成的等离子体丝上荧光的特性，提出了一种间接获得等离子体丝上光强分布的方法．实验研究表明，等离子体丝上荧光主要分布在290—430nm的紫外区域，该区域内的线状谱来自于N2的第二正带分子谱线和N2^＋的第一负带离子谱线．同时等离子体丝上荧光强度与入射激光能量呈非线性增长关系，当入射激光能量低于11mJ，荧光强度快速增强表明丝上光强增大，当激光能量进一步增大时，丝上的光强变化缓慢，此时成丝结构逐步由单丝向多丝演化．同时通过研究等离子体丝上光强随光脉冲传输距离的变化规律，进一步验证了等离子体成丝机制是基于光束自聚焦效应和等离子体散焦过程之间的动态平衡的正确性．

常压等离子体在尼龙染色中的应用

使用常压氦等离子射流处理尼龙6纤维,并选择分散和荧光染料对处理前后的纤维进行染色,用光学显微镜、荧光显微镜和激光共焦显微镜三种方法观察处理前后染色纤维横截面的变化,以扫描电镜观察纤维表面形态,并对其进行能谱分析。试验结果表明,常压等离子射流处理可增加纤维表面的粗糙度,增加纤维染色深度,提高尼龙6纤维的染色性能,且对纤维强度无影响。

辅助方法对热丝CVD金刚石生长速率的影响

在热丝化学气相沉积金刚石系统中 ,通过双灯丝间的热阴极放电产生等离子体 ,对衬底施加正负偏压形成电子促进 ,比较分析了它们及其组合的各种辅助方法对金刚石生长速率的影响。结果表明 ,在以丙酮为碳源、灯丝总功率不变的情况下 ,等离子体可明显增强金刚石的生长 ,其生长速率约为纯热丝法的三倍 ;而正偏压对等离子体辅助沉积金刚石不仅没有增强形核作用 ,而且抑制金刚石的生长 ;电子促进法可以显著提高金刚石的成核密度 ,但并不能提高金刚石生长速率。

局部电弧丝状放电控制激波／边界层干扰的数值研究

采用数值模拟的方法研究局部电弧丝状放电激励对激波/边界层干扰引起的气流分离的控制效果和机理。研究发现在干扰区上游和干扰区内进行电弧放电能够有效控制边界层的分离,且控制效果随着能量输入增大而增强,最大可使分离区减小40.6%,而在干扰区下游作用时对激波/边界层干扰基本没有影响。结合热阻塞效应,可得出电弧放电的作用机理是其产生的焦耳热在流场中造成局部流场阻塞,形成等离子体虚拟型面,在流场中诱导出微弱的斜激波和旋向相反的漩涡,增大了边界层内流体的动量,使其抵抗分离的能力增强,从而抑制了气流的分离。