飞秒激光烧蚀铝靶产生喷射物的超快脉冲数字全息诊断

采用脉冲数字显微全息技术,对50 fs单脉冲激光烧蚀铝靶过程中的物质喷射以及等离子体演化的动态过程进行了实验研究,获得了高时空分辨的动态数字全息图.由全息图观察到了热弹力波引起的二次喷射现象,并且报道了大延迟下喷射物质对400 nm探测光所引起的干涉条纹的反常移动现象.通过对全息图进行数字再现,得到了不同延迟下探测光穿过等离子体后的二维相位分布,并运用逆Abel变换算法获得了等离子体折射率以及等效电子密度的时空演化动态过程.根据实验以及计算所得到的有关于喷射物的光学性质,对喷射物的结构和成分进行了分析. 关键词： 脉冲数字全息 飞秒激光烧蚀 超快时间分辨 等离子体     

记录飞秒级超快动态过程的脉冲数字全息技术

采用脉冲数字全息技术实现对飞秒级超快动态过程的数字显微全息记录.其中，全息记录系统将单脉冲分割成具有飞秒至皮秒量级时间延迟的角度相同的物光子脉冲序列和具有同样时间延迟的角度不同的参考光子脉冲序列，并以空间角分复用的方式在普通CCD的一帧图像上记录下包含多张子全息图的复合全息图，然后通过数字傅里叶变换和数字滤波的方法，分别再现出每张子全息图所记录的图像.通过对飞秒激光激发空气电离过程的全息记录，获得了具有飞秒时间分辨的等离子体形成和传播过程的动态图像. 关键词： 超短脉冲激光 飞秒全息 脉冲数字全息 空间角分复用     

应用波分复用技术实现飞秒级超快动态过程的脉冲数字显微全息记录

采用波分复用方法实现飞秒级超快动态过程的脉冲数字显微全息记录和再现。在全息记录过程中,利用BBO倍频晶体对入射激光进行倍频,将基波和谐波分光,并经时间延迟后进入迈克耳孙干涉仪,从而可应用波分复用技术,先后在CCD的一帧图像上记录两张具有不同空间频率的子全息图。采用这种方法,实现了对单脉冲飞秒激光激发空气电离的超快动态过程的全息记录,并通过数字傅里叶变换和数字滤波的方法,分别再现出每张子全息图记录的波前,从而获得了具有飞秒时间分辨的空气等离子体形成和传播过程的动态振幅和相位图像,其单次曝光时间为50 fs,曝光时间间隔为400 fs,相应的曝光频率约为2.5×1012f/s。     

飞秒脉冲烧蚀单晶硅的超快动力学

利用基于Pump-probe系统的超快时间分辨阴影图的方法,研究了空气中飞秒激光烧蚀单晶硅的动力学过程。实验采用脉宽为50 fs、平均能量密度约35 J/cm2的单脉冲激光烧蚀单晶硅,获取飞秒激光烧蚀单晶硅过程中等离子体和冲击波的形成和发展过程的时间分辨阴影图。实验结果表明:飞秒激光烧蚀单晶硅导致其表面物质喷发的过程是不连续的,分为明显的两次喷发过程。这表明飞秒激光与单晶硅作用的过程中,在不同的时间段可能由不同的机制主导,在前期可能是多光子电离为主,在后期可能是由多光子效应和雪崩效应共同作用。研究还发现,延迟时间较长时,冲击波形状发生畸变。     

纳秒激光诱导损伤熔石英玻璃的动力学过程

采用超快时间分辨阴影成像技术研究了纳秒激光诱导损伤熔石英玻璃前后表面和体内的动力学过程,对比分析了前后表面和体内的损伤差异及损伤机制。在前表面,观察了空气和材料中的等离子体和冲击波的产生与发展过程;亚纳秒激光辐照下,前表面材料内观察到三个应力波,并观察到材料体内的损伤过程。在后表面,除观察到冲击波的产生与发展过程,还观察到表面物质的烧蚀去除与喷发过程。在材料内部,损伤由自聚焦和点缺陷吸收两种机制主导,而且点缺陷吸收诱导材料体内损伤有时间先后顺序。     

飞秒激光烧蚀固体靶的冲击压强

在明确飞秒激光与物质相互作用过程中冲击压强和冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝、铜、硅三种固体靶过程中的冲击压强与其他物理量的关系, 利用飞秒激光烧蚀固体靶的时间分辨泵浦探测阴影图提取了冲击压强的绝对值. 此项研究结果对飞秒激光安全加工含能材料以及飞秒激光推进技术有重要意义. 关键词： 飞秒激光 含能材料 冲击压强 冲击温度     

大气环境下飞秒激光对铝靶烧蚀过程的研究

利用时间分辨的光阴影成像技术研究了在大气环境下飞秒激光烧蚀铝靶的动态过程. 在入射激光能量为4 mJ, 激光光斑超过1 mm时, 激光烧蚀区表面物质以近似平面冲击波形式向外喷射; 在同样激光能量下、激光光斑较小时(约0.6 mm), 激光烧蚀区以近似半球型冲击波形式向外喷射. 当激光能量比较大时(7 mJ), 发现空气的电离对于激光烧蚀靶材有着重要影响. 在光轴附近烧蚀产生的喷射物具有额外的柱状和半圆型的结构, 叠加在平面冲击波结构上.     

飞秒激光烧蚀典型金属表面产生冲击波膨胀研究

利用时间分辨阴影成像技术进行飞秒激光烧蚀金属表面研究可以直观获取飞秒激光烧蚀金属表面产生冲击波膨胀过程的图像。通过对比飞秒激光烧蚀金属铝、铜、铁靶表面的冲击波膨胀形式发现:金属铜、铁表面的冲击波均以球面波形式传播;由于受到烧蚀物质喷溅的影响,铝靶表面竖直方向的冲击波传播形式由球面波转化为柱面波。     

飞秒激光烧蚀石英玻璃的实验与理论研究

实验研究了800nm飞秒激光作用下石英玻璃的破坏机理和烧蚀规律，给出了破坏阈值与脉冲宽度的关系.发展了雪崩击穿模型，计算了材料的烧蚀阈值与脉冲宽度的依赖关系，烧蚀深度、烧蚀体积与脉冲能量的依赖关系，研究了导带电子的扩散对材料中激光能量的沉积、分布，以及材料的破坏阈值和烧蚀规律的影响. 关键词： 飞秒激光脉冲 破坏机理 石英玻璃 电子扩散     

飞秒激光脉冲作用下氧化镁的烧蚀及其超快动力学过程

研究了双面抛光氧化镁单晶（111）表面800 nm飞秒激光单脉冲烧蚀阈值和激光脉宽的依赖关系.利用泵浦-探针技术，测量不同能量和脉宽作用下飞秒激光烧蚀的时间分辨反射率的演化.通过扫描电镜观察其烧蚀形貌，发现大量的沿氧化镁［100］晶向开裂的裂纹.讨论了表面裂纹的形成机理，并解释了飞秒激光烧蚀氧化镁的超快动力学过程.     

Time-resolved shadowgraphs of material ejection in intense femtosecond laser ablation of aluminum

The dynamic process of intense 50 fs laser ablation of aluminum is investigated by ultrafast time-resolved microscopy. A stripe pattern preceding phase explosion is clearly seen in the shadowgraph of 1 ns time delay. Intermittent material ejections are observed within the ejected plume after 2.5 and 7 ns time delay, respectively, which may be attributed to the material response to the generation of an extremely strong thermoelastic wave. Similar processes are also recorded in the ablation of silicon and glass samples, except for the glass samples, the intermittent material ejections are not found.     

Time-resolved shadowgraphs and morphology analyses of aluminum ablation with multiple femtosecond laser pulses

Aluminum ablation by multiple femtosecond laser pulses is investigated via time-resolved shadowgraphs and scanning electron microscope(SEM) images of the ablation spot. The spatial distribution of the ejected material and the radius of the shock wave generated during the ablation are found to vary with the increase in the number of pulses. In the initial two pulses, nearly concentric and semicircular stripes within the shock wave front are observed, unlike in subsequent pulses. Ablation by multiple femtosecond pulses exhibits different characteristics compared with the case induced by single femtosecond pulse because of the changes to the aluminum target surface induced by the preceding pulses.     

Femtosecond laser-assisted three-dimensional microfabrication in silica

We demonstrate direct three-dimensional (3-D) microfabrication inside a volume of silica glass. The whole fabrication process was carried out in two steps:(i) writing of the preprogrammed 3-D pattern inside silica glass by focused femtosecond (fs) laser pulses and (ii) etching of the written structure in a 5% aqueous solution of HF acid. This technique allows fabrication of 3-D channels as small as 10mum in diameter inside the volume with any angle of interconnection and a high aspect ratio (10mum -diameter channels in a 100mum -thick silica slab).     

Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses

By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of 4-mu;m diameter were more than 200 microm deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.     

Optical seizing and merging of voids in silica glass with infrared femtosecond laser pulses

We demonstrate that one can seize and translate voids formed by IR femtosecond laser pulses inside silica glass and can also cause two voids to merge into one. We also present clear evidence of a void and its surrounding region by showing scanning electron microscope images of cleaved voids, which we produced by cleaving through the glass along a plane that included a thin laser-ablated line on the surface of the glass and the voids formed inside.     

Effects of the penetration height of ethylene transverse jets on flame stabilization behavior in a Mach 2 supersonic crossflow

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900 K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through combustion pressure measurements. Simultaneously, time-resolved images of CH* chemiluminescence and shadowgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combustion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH* chemiluminescence images and the shock parameters. The results indicated that the penetration height of nitrogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH* chemiluminescence images, shadowgraphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2 mm. Fuel penetration heights above the cavity aft wall were expected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM.     

Influence of self-focusing effect on femtosecond collinear nondegenerate optical Kerr measurements

We performed the femtosecond nondegenerate optical Kerr shutter (OKS) measurements in a 10-mm-thick optical glass. The time-resolved OKS signals showed an obvious asymmetry and a large broadening with respect to that for thin samples. The time-resolved OKS signals in samples with different thicknesses showed that, the temporal broadening of the OKS signals was attributed to the group velocity mismatch between pump and probe beams of different wavelengths. By imaging the filament induced inside the glass, we attributed the asymmetry of the time-resolved OKS signals to the focusing position change of pump beam caused by self-focusing and re-focusing effect.