强飞秒激光烧蚀石英玻璃的超快时间分辨光学诊断

采用超快时间分辨的光学诊断技术对飞秒激光脉冲烧蚀石英玻璃的动态过程进行了实验研究.首先,对烧蚀过程中石英玻璃外部的物质喷射进行了数字全息记录,获得了不同延迟时间下探测光的二维相位差分布图,并由此推断出了喷射物的成分和结构.此外,对石英玻璃内部的现象进行了时间分辨阴影图记录,从阴影图中观察到了石英玻璃内部的两个应力波的演化过程.这两个应力波与目标靶外部的两次物质喷射相关. 关键词： 脉冲数字全息 飞秒激光烧蚀 超快时间分辨 应力波     

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     

飞秒激光烧蚀硅表面产生等离子体的发射光谱研究

对中心波长为800 nm,脉宽为100 fs的激光脉冲烧蚀空气中硅(111)产生的等离子体发射光谱进行了时间和空间分辨研究. 结果表明,在等离子体羽膨胀初期(小于50 ns时间范围内),等离子体发射光谱主要由连续光谱构成,此后连续光谱强度逐渐减弱,线状光谱开始占主导地位;在羽体膨胀过程中离子谱线的存在时间短于原子谱线的存在时间. 由时间分辨发射光谱发现在羽体膨胀过程中等离子体辐射波长存在红移现象,波长红移量随时间演化呈二次指数衰减. 最后给出等离子体发射光谱谱线强度的时空演化规律. 关键词： 飞秒激光 脉冲激光烧蚀 等离子体 发射光谱     

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。     

超短脉冲激光烧蚀石墨产生的喷射物的时间分辨发射光谱研究

本文使用不同激光能流(18 J/cm²–115 J/cm²)和脉冲宽度(50 fs–4 ps)的超短脉冲激光在真空中(4×10^-4 Pa)烧蚀高定向热解石墨. 通过测量烧蚀喷射物的时间分辨发射光谱研究喷射物的超快时间演化. 在喷射物发射光谱中, 观察到了C₂基团的天鹅带光谱系统, 416 nm附近C₁₅基团的由电子能级¹Σ_u⁺ 和¹Σ_g⁺之间的振动跃迁产生的光谱峰以及连续谱. 50 fs, 115 J/cm²的脉冲激光烧蚀产生的喷射物的连续谱的强度衰减分为快速下降和慢速下降两个阶段(以20 ns时间延迟为分界). 这表明连续谱是由两种不同的组分贡献的. 快速下降阶段, 连续谱主要由碳等离子体通过韧致辐射产生; 慢速下降阶段, 连续谱主要由烧蚀后期产生的大颗粒碳簇的热辐射贡献. 实验结果还揭示了激光能流的提高, 会明显增加喷射物中碳等离子体和激发态C₂的含量, 但对质量稍大的C₁₅的影响较小; 此外, 50 fs脉冲激光烧蚀产生的连续谱的存在时间会随着激光能流的减小而增大, 这说明低能流更有利于在烧蚀后期产生碳簇. 脉宽主要影响喷射物连续谱的时间演化. 4 ps脉冲激光烧蚀产生的连续谱的整个时间演化过程明显慢于50 fs脉冲产生的连续谱.     

纳秒激光单脉冲烧蚀硅表面的超快成像研究

利用ICCD可以在纳秒时间尺度下成像的特点,以飞秒准连续激光产生的超短脉冲光为探测光,对纳秒激光单脉冲烧蚀硅靶表面的演化过程进行动态监测。在能量密度为50J／cm^2时,捕获了纳秒单脉冲激光烧蚀硅靶面过程中等离子体演化的时间分辨图像。图像表明,纳秒激光烧蚀硅靶产生的等离子体开始时密度大,膨胀速度快,当纳秒激光脉冲过后,等离子体不再产生,并且其膨胀速度不再增加,直至完全消失。     

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.     

激光诱导Al等离子体连续辐射的时间分布

用Ar作环境气体,压强固定在10kPa,每个激光脉冲能量为115mJ,利用时空分辨技术,采集激光烧蚀Al靶产生的等离子体辐射的时间分辨谱。分析了Al等离子体连续辐射特征。简要讨论了激光诱导等离子体连续辐射的产生机理。提出了原子对激光诱导等离子体连续辐射共振吸收理论。激光诱导等离子体的连续辐射的主要机制是轫致辐射和复合辐射,在激光脉冲作用到靶面瞬间,轫致辐射占主导地位;等离子体演化初期,复合辐射和轫致辐射共同产生等离子体连续辐射;等离子体演化后期,连续辐射主要复合辐射产生的。Al原子对连续辐射的共振吸收是选择性的,这是改变连续辐射按波长“平滑”分布的主要机制。     

脉冲激光烧蚀材料等离子体反冲压力物理模型研究与应用

分析了脉冲激光烧蚀材料等离子体等温膨胀阶段的物理特性,建立了脉冲激光烧蚀材料等离子体压力三维方程与动力学模型.应用所建模型,数值分析了单脉冲激光烧蚀青铜金刚石砂轮等离子体相关特性,得到等离子体的反冲压力最大值870 Pa出现在约25 ns后,距离砂轮表面距离约0.05 mm处.相关条件下开展脉冲激光烧蚀青铜金刚石砂轮试验,采用高速相机观测烧蚀砂轮过程中的飞溅现象;采用光栅光谱仪测量等离子体空间发射光谱,计算了等离子体电子温度、电子密度以及反冲压力.实验表明脉冲激光烧蚀青铜金刚石砂轮等离子体反冲压力可以不计,同时也验证了气体方程与动力学模型的正确性和可行性,对脉冲光纤激光烧蚀工艺优化具有启示意义.     

脉冲激光沉积动力学研究进展

脉冲激光沉积技术是现代常用的先进薄膜材料制备技术之一.文章在简要介绍脉冲激光沉积技术及其进展的基础上,较全面地介绍了脉冲激光沉积动力学的基本物理图像和动力学构架,深入地探讨了激光烧蚀靶材过程、等离子体膨胀过程和薄膜沉积过程的动力学规律,阐述了我国学者在脉冲激光沉积动力学研究方面的贡献,例如包括脉冲激光沉积三个工艺过程自洽的统一模型,等离子体膨胀的冲击波模型,基于局域能量动量守恒定律的新等离子体演化动力学模型,包括热源项、蒸发项、等离子体屏蔽效应和动态物性参数的烧蚀热传导模型,考虑电子碰撞效应和能带结构变化的修正双温模型,能统一描写从纳秒级到飞秒级脉冲激光烧蚀规律的统一双温模型等.     

基于相位共轭的动态体全息衍射特性的实时非破坏性测量

基于相位共轭技术,提出了一种动态体全息衍射特性的实时测量方法.在光学系统设计中,通过调整使得物光和参考光较强,而与参考光共轭的再现光非常弱（约为参考光的1/1 000）,借助非常微弱的共轭再现光实现了动态体全息的非破坏性实时测量.三束光的强度和偏振态可通过1/4波片、偏振片和衰减片进行调节和组合,可记录光强调制型或偏振态调制型体全息.本方法适合于光致折射率变化和光致变色材料体全息的测量.     

Neutrals ejection in intense femtosecond laser ablation

Time-resolved shadowgraphs and holograms of intense femtosecond laser ablation of aluminum are reported. The different bending trends of interference fringes are found in the ablations of aluminum and other materials at probe wavelengths of 400 and 800 nm, for the first time to our knowledge, which could be attributed to the ejection of massive neutral droplets induced by critical-point phase separation during the second material ejection.     

Probing particle synthesis during femtosecond laser ablation: initial phase transition kinetics

The impulsive superheating of matter by an intense, ultrashort laser pulse drives material expansion into vacuum (ablation) and an associated formation of nanoparticles. The underlying dynamics of particle formation are complex and direct experimental probes of the rapid material evolution are essential. Femtosecond lasers coupled to modern synchrotrons offer an important new opportunity to probe ejecta dynamics on an atomic lengthscale. Here, the impulsive heating of a semiconductor (silicon) by an intense femtosecond laser pulse leads to material ejection and time-resolved photoemission spectroscopy probes rapid solidification kinetics occurring within the ejecta. Transient photoemission peak-shifts indicate that material is ejected predominantly as liquid droplets and that solidification occurs rapidly (<50 ps). The solidification time suggests that vacuum ejection leads to significantly enhanced undercooling compared to what has been obtained by more conventional quenching techniques; this may be of interest in attempts to trap novel material states associated with extreme laser heating. Finally, a low fraction of vapor particles in the ejecta supports a view that the size-distribution of ejected particles is set by an initial fragmentation process rather than by vapor condensation. PACS 82.60.Qr; 87.64.Lg; 62.50.+p     

基于空间光调制器的非相干数字全息单次曝光研究

菲涅耳非相干相关全息术(Fresnel incoherent correlation holography,FINCH)利用在空间光调制器(spatial light modulator,SLM)上加载双透镜模式对同一物点光分束自相干,并通过改变加载的相位因子得到不同的相移全息图.本系统利用SLM可分区编码调制特性,将FINCH成像中SLM上分三次加载的0°,120°,240°相位双透镜掩模各提取1/3组成一幅复合相移模式加载,并研究了三种相位分布方式对FINCH成像质量的影响.结果表明:三个相位在SLM上分布间隔越大,再现像越清晰.在此基础上,提出了一种新的掩模加载方式,在SLM加载透镜阵列,每一个相位因子对应一个双透镜,具有一个光轴.实验表明,通过这种加载方式,通过SLM后形成的三个相移图能够一次在电荷耦合器上记录,并且三个相移图不重叠,然后通过MATLAB编程计算将不同相移角度的全息图分别提取出来,通过三步相移计算合成一幅包含有物光波的复值全息图,最后通过数值再现算法重建待测样品.此系统可用于对光源相干性较低的实时成像系统,也为微小形变测量、动态物体的观测提供了新方法,为非相干数字全息术的发展提供了新思路.     

Application of time-resolved digital holographic microscopy in studies of early femtosecond laser ablation

We studied evolution of femtosecond laser ablation by employing novel method of time-resolved off-axis digital holographic microscopy. Phase and amplitude profiles of early shock front and ablation plume dynamics of irradiated tempered steel were reconstructed from the digital holograms. In order to gain additional information, digital holographic microscopy was combined with plasma emission imaging. By using both techniques simultaneously we studied material response to multi-pulse irradiation, shock wave propagation, ablation plume formation and plasma emission. The significant changes in ablation performance were observed when using multi-pulse irradiation if compared to widely investigated single-shot regime.     

Digital off-axis holography without zero-order diffraction via phase manipulation

By means of manipulating the phase information of the object beam in an off-axis digital holographic setup, we show that it is possible to fully eliminate the zero-order diffraction (ZOD) from numerically reconstructed holograms. Two different approaches are presented. In the first method, we introduce a ground glass on the object path beam to provide a random phase illumination. The subtraction between two holograms recorded with different positions of the ground glass yields a ZOD free hologram. In the second approach, a piece of window glass inserted on the path of the object beam produces a constant phase shift. The subtraction of two holograms, one recorded with and one without window glass generates a new hologram whose numerical reconstruction is ZOD free. Theoretical models and experimental results are shown to validate our proposals. They show that the proposed methods totally remove the ZOD without ruining the object information.     

基于数字全息的热透镜效应测量

提出一种数字全息定量测量半导体激光二极管侧面泵浦Nd∶YAG晶体热透镜效应的方法。CCD采集Nd∶YAG晶体在不同泵浦电流下的数字全息图,通过数值重建得到热透镜效应引起的相位分布及其变化过程。再由相位分布计算得到对应的热透镜焦距。实验结果表明,利用数字全息技术可以准确和有效测量热透镜效应及热透镜焦距。     

Elimination of zero-order diffraction in digital off-axis holography

A simple experimental method of eliminating zero-order diffraction in the reconstructed image of off-axis digital holography is presented. Holographic diffraction grating acting as a beam splitter, an off-axis holography system is formed. The holograms of object with different recording parameters are obtained by adjusting the reflecting mirror in the recording optical system to vary the incidence orientation of the object beam in CCD to introduce a phase shift. The zero-order image can be eliminated by numerically processing the holograms of object with different recording parameters. The theoretical analyses have been done in detail and the experimental results are also given. The zero-order image eliminated, the area of reconstructed image increases remarkably, the image quality can then be significantly improved and the better resolution obtained. The experimental results show that the method presented in this paper is feasible, simple in optical system and easy in operation and data processing.     

Electron ejection by helium metastable atoms incident on the clean and chalcogen covered Ni(100) surface

An intense, essentially photon free, helium metastable beam has been used to cause electron ejection from the clean, oxygen and sulphur covered Ni(100) surface, in a system equiped with an AES facility to monitor surface cleanness. The ejected electron energy distribution obtained for the clean Ni(100) surface is narrow and peaked at ～2 eV, unlike the distribution obtained from INS studies, and consequently indicates different de-excitation mechanisms for incident ions and excited atoms. The ejected electron distribution from the adsorbate covered surface is also narrow, but peaked at ～1 eV with structure which is essentially independent of the nature of the adsorbate. The yield of ejected electrons is found to increase linearly with coverage of both oxygen and sulphur, in contrast to the results obtained from INS. These data indicate that Auger neutralization does not occur at the surface; the possibility of Auger de-excitation is considered.