飞秒激光烧蚀金属靶的冲击温度

在明确飞秒激光与物质相互作用过程冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝靶和铜靶过程中的冲击温度与其他物理量的关系, 利用飞秒激光烧蚀金属的双温模型提取了冲击温度的绝对值, 基于非傅里叶热传导模型计算了冲击温度的分布. 此项研究结果对飞秒激光安全加工含能材料有借鉴意义. 关键词： 飞秒激光 含能材料 烧蚀 冲击温度     

单脉冲飞秒激光烧蚀炸药过程的热效应研究

飞秒激光能够在极短时间内烧蚀炸药产生高温高压等离子体。可以利用飞秒激光对含能材料或含能元器件进行精密加工。深入认识飞秒激光烧蚀炸药过程中,炸药内部的热效应是发展飞秒激光加工炸药技术的基础。建立了单脉冲飞秒激光烧蚀炸药过程的流固耦合计算模型,考虑了在高温高压等离子体和炸药自热反应的共同作用下,炸药内部的热效应。对飞秒激光烧蚀TNT炸药过程进行了流体力学数值模拟。计算结果表明:TNT炸药中未烧蚀区域产生了热效应,峰值温度高于TNT炸药的点火温度,但由于炸药内热效应区域极小,高温持续时间极短,因此炸药内温度迅速下降,没有发生点火现象。     

飞秒激光烧蚀氟化钙晶体表面特性

采用飞秒激光对氟化钙晶体表面进行加工。通过调控激光参数,采用静止聚焦和动态扫描两种方式在晶体表面加工出一系列微结构(烧蚀孔和烧蚀线)。分别对两种加工方式烧蚀后的氟化钙晶体表面微结构进行系统研究,包括参数依赖关系、材料表面烧蚀阈值等。计算结果表明:在静止聚焦情况下,累积因子为0.0033;在动态扫描情况下,当扫描方向与激光偏振方向垂直或平行时,累积因子分别为0.0043和0.0052。飞秒激光加工过程中的脉冲累积效应能够对晶体的烧蚀产生重要影响。     

基于双相延迟模型的飞秒激光烧蚀金属模型

为了分析飞秒激光烧蚀过程,在双相延迟模型的基础上建立了双曲型热传导模型.模型中考虑了靶材的加热、蒸发和相爆炸,还考虑了等离子体羽流的形成和膨胀及其与入射激光的相互作用,以及光学和热物性参数随温度的变化.研究结果表明:等离子体屏蔽对飞秒激光烧蚀过程有重要的影响,特别是在激光能量密度较高时;两个延迟时间的比值对飞秒激光烧蚀过程中靶材的温度特性和烧蚀深度有较大的影响;飞秒激光烧蚀机制主要以相爆炸为主.飞秒激光烧蚀的热影响区域较小,而且热影响区域的大小受激光能量密度的影响较小.计算结果与文献中实验结果的对比表明基于双相延迟模型的飞秒激光烧蚀模型能有效对飞秒激光烧蚀过程进行模拟.     

气流对飞秒激光加工炸药装药过程的热安全性影响分析

由于炸药具有热传导系数小、对温度极其敏感的特点,在使用多脉冲飞秒激光对其进行持续加工时,极有可能在炸药内形成热累积,从而导致点火、燃烧等危险事件的发生。为了降低激光加工材料过程中的热效应,人们普遍采取在材料加工表面施加气流的方法。为了研究加载气流条件下,炸药装药在飞秒激光作用下产生的烧蚀产物的运动规律以及炸药装药内部的温度变化,建立了加载气流条件下飞秒激光加工炸药装药过程的二维流固耦合计算模型,对在单侧、双侧不同入射角度的亚音速气流作用下,飞秒激光加工奥克托今（HMX）炸药装药的过程进行了数值模拟计算。计算结果表明：单侧气流会在炸药加工表面形成漩涡流,导致烧蚀气体产物在炸药表面做旋转运动,加重了烧蚀产物对炸药的热影响;双侧气流会在远离炸药加工表面的地方形成较大的漩涡流,从而使烧蚀气体产物迅速离开炸药加工表面,有效降低了炸药的温度,提高了飞秒激光加工炸药装药过程的安全性。     

飞秒激光烧蚀齿曲面的复耦合模型及形貌影响

针对飞秒激光烧蚀齿曲面过程中能量累积效应与变离焦效应对齿面形貌与烧蚀尺寸的影响问题,建立飞秒激光烧蚀复耦合模型,通过有限差分法求解得到激光烧蚀面齿轮材料18Cr2Ni4WA的电子与晶格在不同的脉宽与能量密度下的温度变化分布,对烧蚀凹坑的深度及半径进行仿真。考虑激光束在加工齿面时,与被加工齿面之间存在倾斜角,根据光斑能量分布方式得到齿面底角与激光能量的定量关系,结合焦半径与折射率的变化,对飞秒激光烧蚀齿面深度、半径与形貌进行研究。通过实验得到当激光能量密度为1.783 J/cm²且被加工齿面底角过大时,激光能量降低烧蚀过程只发生在材料表面;当能量密度为2.376 J/cm²、激光脉冲数为3 000时,烧蚀凹坑的微结构细密良好。研究结果表明飞秒激光烧蚀曲面时,激光有效能量随着倾斜角的变化降低,同时激光光斑的能量分布影响了烧蚀凹坑深度的变化,可为提高飞秒激光加工齿曲面质量提供参考。     

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     

PMP泡沫的飞秒激光烧蚀及切割方法

对密度为90 mg/cm3的PMP泡沫材料的飞秒激光烧蚀结果进行了分析,推导出该材料在脉宽50 fs、波长800 nm、重复频率为1000 Hz的飞秒激光作用下的蚀除阈值为0.91 J/cm2（100个激光脉冲）,获得了烧蚀直径分别随激光功率、脉冲数及聚焦物镜数值孔径的变化规律。相同飞秒激光加工系统下,对比了铜箔上获得的烧蚀形状,确定了PMP泡沫材料本身的多孔洞及其分布不均匀是造成烧蚀区域的形状不规则的重要因素。PMP泡沫在较高能量或是较长时间的飞秒激光作用下,烧蚀区域发生碳化的原因是由热作用引发的。提出了一种基于激光束耦合的飞秒激光切割厚度大于1 mm的薄膜-泡沫材料的方法,并获得了切割厚度大于1.5 mm、切割侧壁与光束光轴夹角小于5、切割面整洁的薄片。     

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     

Kevlar-29材料的飞秒激光制孔形貌及性能研究EI北大核心CSCD

为提升飞秒激光加工芳纶纤维复合材料制孔质量,优化制孔加工参数,进行了制孔试验。用共聚焦显微镜与超景深显微镜观察圆孔边沿形貌,并测量了热影响区大小。分析了激光功率、扫描速度和重复频率对圆孔的形貌、热影响区大小以及几何精度的影响,对比了激光制孔与机械制孔后材料的拉伸强度。研究发现,一定参数范围内,激光功率增加、重复频率降低或扫描速度减小使热影响区尺寸整体上逐渐变大。但激光功率不足导致的烧蚀不充分也会产生较大的热影响区与表面损伤。热影响区最优状态对应的激光参数为激光功率5 W,扫描速度1050 mm/s,重复频率200 kHz。对比传统机械加工,飞秒激光加工后材料的拉伸强度波动性更小。结果表明,芳纶纤维复合材料的飞秒激光制孔加工不是完全的“冷”加工,由于芳纶纤维导热性差,热量累积造成纤维及纤维周边基材的热损伤,仍会导致较小的热影响区产生。采用合适激光参数的飞秒激光对芳纶纤维复合材料进行制孔加工,能有效提升制孔质量,满足相关领域精度和强度的要求。     

Atomistic investigation of ablation of amorphous polystyrene under femtosecond laser pulse

In the present work we elucidate the thermodynamic mechanisms of femtosecond(fs)laser ablation of amorphous polystyrene by means of molecular dynamics(MD)simulations.The effects of extrinsic parameter of laser pulse intensity and intrinsic parameter of molecular architecture on the laser ablation are further studied.Simulation results show that the laser ablation-induced polymeric material removal is achieved by evaporation from the surface and expansion within the bulk.Furthermore,inter-chain sliding and intra-chain change also play important roles in the microscopic deformation of the material.It is found that both the laser pulse intensity and the arrangement of phenyl groups have significant influence on the fs laser ablation of polystyrene.     

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (M_w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M_w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M_w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.     

Dynamics of mid-infrared femtosecond laser resonant ablation

Resonant ablation is beneficial to avoiding uncontrollable subsurface damages in the laser ablation of polymers. In this paper the dynamics of mid-infrared laser resonant ablation of polylactic acid and toluene was calculated by using fluid dynamic equations. The merits and drawbacks of mid-infrared femtosecond laser resonant ablation of high molecular weight polymers have been discussed.     

Molecular dynamic study of volumetric phase change induced by a femtosecond laser pulse

During the last decade, femtosecond laser has been used for micro-machining of various materials. In this work, the phase change phenomena during femtosecond laser ablation are investigated using molecular dynamics (MD) simulation. The process of femtosecond laser ablation of nickel is calculated. The temperature and stress history, and the generation and growth of gas bubbles are traced. For different laser pulse fluences, the material ablation process is analysed to reveal the effect of temperature and stress. PACS 02.70.Ns; 42.62.-b; 64.60.Ht     

Synthesis of nanocrystalline cubic zirconia using femtosecond laser ablation

We report on the synthesis of nanocrystalline zirconia in liquid using femtosecond laser ablation. Nanocrystalline cubic zirconia has been prepared by femtosecond laser ablation of zirconium in ammonia, while nanocrystalline tetragonal and monoclinic zirconia was synthesized in water. The physical and chemical mechanisms of the formation of nanocrystalline metastable zirconia are discussed. The intrinsic properties of femtosecond laser ablation in liquid and OH⁻¹ may be responsible for the synthesis of cubic zirconia. It is suggested that the femtosecond laser pulse can create higher temperature and pressure conditions at a localized area in the liquid than the nanosecond laser pulse and the cooling is also faster in the femtosecond laser ablation process, which determined the difference between the products synthesized with femtosecond and nanosecond-pulsed laser ablation.     

超短激光蚀除金属机制的分子动力学研究

 采用耦合电子热传导方程的分子动力学方法,研究了飞秒激光辐照下金属Ni的熔化及蚀除动力学。分析了靶材内部温度分布特征及蚀除产物的构成,主要包含单个原子及大团簇。确定了断裂位置和蚀除开始的标志,即该处温度分布出现小的峰值,且粒子数密度急剧下降。模拟结果表明：强烈的蒸发及靶材内部所产生的拉应力分别是单个原子及大团簇喷射的机制。同时,深入探讨了激光诱导压力波的传播规律,预测了压力波的波速,约为4.97 km/s。将不同脉冲能量密度下的蚀除速率同实验数据加以对比,结果相差16%~20%。预测了熔深随时间的变化规律,基本随时间的延续而呈上升的趋势。发现过热现象的存在。     

Short-pulse laser ablation of solids: from phase explosion to fragmentation

The mechanisms of laser ablation in silicon are investigated close to the threshold energy for pulse durations of 500 fs and 50 ps. This is achieved using a unique model coupling carrier and atom dynamics within a unified Monte Carlo and molecular-dynamics scheme. Under femtosecond laser irradiation, isochoric heating and rapid adiabatic expansion of the material provide a natural pathway to phase explosion. This is not observed under slower, nonadiabatic cooling with picosecond pulses where fragmentation of the hot metallic fluid is the only relevant ablation mechanism.     

Assessing the effect of the matrix in resonant infrared MAPLE

The ablation yield of polyethylene glycol is measured as a function of laser pulse energy and polymer molecular weight for both free running and Q-switched Er:YAG lasers. The results show that the deposition rate is dependent on the absorption coefficient in a way that is consistent with the blow-off ablation model but that the linear dependence of the efficiency on laser pulse energy is also consistent with a steady-state ablation model. The results for both lasers are remarkably similar given that the pulse durations are 350 μs and 85 ns, respectively. Furthermore, polystyrene thin films are deposited as well and it is found that the morphology depends on the THF/MeOH ratios in the matrices. The significance of these results is discussed as well as future directions for this research.