光学薄膜激光诱导损伤机理研究

基于电子密度演化模型,借助数值方法,研究了飞秒激光作用下光学薄膜内的电子密度演化过程,讨论了初始电子密度Ni和激光脉冲宽度τ对光学薄膜激光损伤阈值Fth的影响,分析了激光诱导薄膜损伤过程中MPI和AI的性质和作用.研究结果表明,对应于一定的脉宽,存在一个临界初始电子密度,当Ni低于这一临界密度时,Fth不受Ni影响;当Ni高于临界密度时,Fth随Ni增加而降低.临界初始电子密度随着脉宽的减小而增加。对于FS和BBS介质薄膜,Fth随脉宽的增加而升高。初始电子密度Ni对BBS中的MPI和AI基本没有影响;同样Ni对FS中的AI基本不产生影响,但当Ni>1011 cm－3时,FS中MPI电子密度随Ni增加而降低.在所研究的脉宽范围τ∈［0.01,5］ps,AI是FS介质激光诱导损伤的主要机制.而对于BBS,当脉宽τ∈［0.03,5］ps,AI是激光诱导损伤的主要机制;当脉宽τ∈［0.01,0.03］ps,MPI在激光诱导损伤中占主导地位.     

150fs～10ps脉宽下熔石英激光损伤的仿真与分析

基于约化电子数密度增长速率方程,建立了熔石英导带电子数密度随脉冲持续时间变化的模型。利用电子数临界密度这一概念,得到了150fs~10ps脉宽下,熔石英激光损伤阈值范围。分析表明,5~10ps,雪崩电离仍然起主要作用,而光致电离提供的初始电子使雪崩电离不再依赖材料原有的初始电子;当脉宽减小到约为4ps时,光致电离与雪崩电离作用相等;之后,光致电离起主要作用。通过仿真出的损伤阈值拟合,得到了该脉宽区间下新的脉宽定律:熔石英的损伤阈值正比于脉宽的0.38次方;考虑温度对熔石英损伤阈值的影响,熔石英的损伤阈值正比于脉宽的0.34次方。     

短脉冲激光辐照下SiO2损伤微观机理简化模型

 从电子密度速率方程出发，建立短脉冲激光辐照下SiO₂材料中导带电子增长简化模型，计算了SiO₂中光致电离速率和电子雪崩速率，得到SiO₂激光损伤阈值与脉冲宽度的关系，计算分析了光致电离和碰撞电离两种电离机制在导带电子累积过程中的不同作用。结果表明：脉冲较长，碰撞电离几乎能提供全部的导带电子，激光损伤阈值与脉宽的0.5次方成正比；脉冲较短时，导带电子主要由碰撞电离产生，光致电离提供碰撞电离的初始电子，激光损伤阈值随着脉宽的减小，先增加后减小。     

激光波长及脉宽对光学材料抗损伤性能的影响

 基于能带理论,利用激光与光学材料相互作用的理论模型,研究了激光辐照下材料导带自由电子数密度的变化,讨论了材料损伤阈值与激光波长、脉冲宽度、材料禁带宽度之间的关系,数值分析了激光波长和脉冲宽度对损伤阈值的影响。结果表明:当脉宽小于1 ps时,材料损伤阈值随脉宽增大而减小;当脉宽大于1 ps时,材料损伤阈值随脉宽增大而增大;激光波长为10 fs~10 ns,损伤阈值随着波长的减小而减小。     

多脉冲激光辐照下介质损伤理论研究

针对纳秒脉冲和飞秒脉冲不同的损伤机制,分别建立了两种多脉冲激光损伤模型。脉宽小于10 ps时,损伤是由于等离子体形成造成介质发生烧蚀所致,对此建立了基于电子密度演化方程的介质击穿模型;脉宽大于100 ps时,损伤是由于热沉积造成介质发生熔融所致,对此建立了基于傅里叶热传导方程的介质热损伤模型。通过计算两种模型下激光参数和材料参数对多脉冲损伤的影响,发现由于损伤机理不同,不同参数对单脉冲损伤阈值和多脉冲损伤阈值的影响趋势不完全一致,敏感程度也不同。通过计算得到了与实验结果一致的多脉冲损伤阈值与脉冲数间关系,使定量预估多脉冲损伤阈值和元件使用寿命成为可能。     

Mechanisms of femtosecond laser-induced breakdown and damage in MgO

Single-shot laser damage threshold of MgO for 40-986 fs, 800 nm laser pulses is reported. The pump-probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold.     

脉宽对中红外激光带内损伤HgCdTe材料的影响

研究了脉宽对于中红外脉冲激光带内损伤碲镉汞（HgCdTe）材料阈值的影响,使用一维自洽模型对激光辐照HgCdTe材料程中的载流子数密度,载流子对数目流,载流子对能流,载流子温度和材料晶格温度等相关参数进行仿真计算。仿真结果表明,波长2.85 μm,脉宽30 ps～10 ns单脉冲激光带内辐照HgCdTe材料的损伤阈值为200～500 mJ/cm2。其中,300 ps～3 ns脉冲激光的损伤阈值相近,均为200 mJ/cm2且低于其他脉宽激光的损伤阈值。搭建实验光路并进行相关实验验证仿真模型的正确性。实验发现,波长2.85 μm、脉宽300 ps的单脉冲激光带内辐照HgCdTe材料的损伤阈值在200 mJ/cm2左右。相同条件下,10 ns单脉冲激光带内辐照HgCdTe材料的损伤阈值约474 mJ/cm2。百皮秒脉冲激光对HgCdTe材料的损伤过程结合了热击穿和光学击穿效应,其独特的毁伤机理加剧了材料的损伤。     

Theoretical investigation of laser pulse width dependence in a thermal confinement regime

Previous molecular dynamics (MD) simulations of ultraviolet (UV) laser ablation demonstrate the distinct dependence of material ejection on laser fluence and laser pulse duration. In this paper, we examine the pulse width dependence when the laser pulse widths are appropriate for the thermal confinement regime. We perform MD simulations of laser ablation with a laser pulse duration of 1 ns and compare with a pulse width of 150 ps as in previous simulations. The simulations confirm that the pulse width in thermal confinement regime does not dramatically influence the molecular ejection mechanism. The simulations reveal differentiations, however, in plume composition and the ablation threshold value. PACS 02.70.Ns; 61.80.Az; 79.20.Ap     

Effect of pulse width and fluence of femtosecond laser on the size of nanobump array

Conical nanobump arrays were generated on gold thin film processed by interfering femtosecond laser. The transition of the height and diameter as functions of fluence and pulse width was investigated. When the fluence was 87 mJ/cm², the height and diameter were not so different at 350 fs or shorter pulse width. They decreased at longer pulse width, and no bump could be generated over 1.6 ps. The results suggest the decrease of size is due to the diffusion of electron to not-excited region, and due to heat conduction to not heated region or substrate, or change of absorbance of laser. At long pulse width of 2.4 ps and relatively higher fluence of 190 mJ/cm², nanobump had liquid-like structure as a stop motion of a water drop.     

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.     

开放空间重复频率微波脉冲击穿概率理论与实验研究

综合考虑有效初始电子产生理论、雪崩电子击穿理论等过程中的击穿延迟时间,探讨了开放空间微波脉冲的击穿延时概率分布,提出了重复频率微波脉冲击穿概率模型,定义了基于概率模型的微波脉冲击穿阈值。利用S波段微波准光学反射聚焦系统对一定气压大气击穿过程进行了模拟,监测击穿放电发光时刻作为击穿时间,分别在铯137放射源存在与否情况下开展了系列实验。研究结果表明,提高种子电子产生率相较于提高电离率是增大脉冲击穿概率更有效的方法;重复频率过程中,若存在累积效应,击穿延时概率分布曲线将左移并趋于稳定,击穿后的气体在短时间内容易再次击穿。     

Thermal characteristics of double-layer thin film target ablated by femtosecond laser pulses

Thermal characteristics of tightly-contacted copper--gold double-layer thin film target under ablation of femtosecond laser pulses are investigated by using a two-temperature theoretical model. Numerical simulation shows that electron heat flux varies significantly on the boundary of copper--gold film with different maximal electron temperature of 1.15×10³ K at 5 ps after ablating laser pulse in gold and copper films, which can reach a balance around 12.6 ps and 8.2 ps for a single and double pulse ablation, respectively, and in the meantime, the lattice temperature difference crossing the gold--copper interface is only about 0.04×10³ K at the same time scale. It is also found that electron--lattice heat relaxation time increases linearly with laser fluence in both single and double pulse ablation, and a sudden change of the relaxation time appears after the laser energy density exceeds the ablation threshold.     

飞秒激光的波长对SiC材料烧蚀的影响

利用10倍的显微物镜将近红外飞秒激光脉冲汇聚到宽带隙半导体材料6H SiC的前表面,研究样品的烧蚀及诱导微细结构。用扫描电镜(Scanning electron microscope,SEM)及光学显微镜测量烧蚀斑。利用烧蚀面积与激光脉冲能量的关系确定SiC的烧蚀阈值。给出了SiC样品的烧蚀阈值与飞秒激光波长的依赖关系。实验结果表明,可见光区随波长增加,烧蚀阈值从0.29J/cm2增加到0.67J/cm2;而在近红外区,SiC的烧蚀阈值为0.70J/cm2左右,基本上不随激光波长变化而改变。结合计算结果,可以认为在飞秒激光烧蚀SiC的过程中,在近红外区,光致电离和碰撞电离均起到了重要的作用;而在可见光区,光致电离的作用相对大一些。     

Effect of standing-wave field distribution on femosecond laser-induced damage of HfO_2/SiO_2 mirror coating

Single-pulse and multi-pulse damage behaviors of "standard"(with λ/4 stack structure) and "modified"(with reduced standing-wave field) HfO2/SiO2 mirror coatings are investigated using a commercial 50-fs,800-nm Ti:sapphire laser system.Precise morphologies of damaged sites display strikingly different features when the samples are subjected to various number of incident pulses,which are explained reasonably by the standing-wave field distribution within the coatings.Meanwhile,the single-pulse laser-induced damage threshold of the "standard" mirror is improved by about 14% while suppressing the normalized electric field intensity at the outmost interface of the HfO2 and SiO2 layers by 37%.To discuss the damage mechanism,a theoretical model based on photoionization,avalanche ionization,and decays of electrons is adopted to simulate the evolution curves of the conduction-band electron density during pulse duration.     

脉冲压缩光栅的激光损伤机理及阈值提升技术研究进展

总结了激光辐射条件下脉冲压缩光栅的激光诱导损伤机理,探究了表面形貌、加工方式、结构缺陷以及表面污染等因素对光栅损伤造成的影响,从微观损伤机理的角度阐释了产生损伤的内在原因。在脉冲压缩光栅的激光预处理、加工工艺及表面污染物的去除等方面,分析了实现光栅损伤阈值提升的内在因素,给出了提升光栅损伤阈值的技术措施。根据影响光栅损伤阈值的因素,提出在光栅运行过程中采用多种措施组合的方式来提升光栅的激光诱导损伤阈值。脉冲压缩光栅激光损伤机理和阈值的研究对脉冲压缩光栅系统的稳定运行具有实践意义,为激光装置高能量密度的输出奠定基础。最后,提出了光栅激光诱导损伤研究的科学与技术问题,为脉冲压缩光栅激光诱导损伤阈值的提升提供新的思路,服务于重大科学装置和重要技术领域的发展。     

紧聚焦飞秒脉冲与石英玻璃相互作用过程中的电子动量弛豫时间研究

本文通过数值模拟(3+1)维扩展的广义非线性薛定谔方程,研究了紧聚焦飞秒激光脉冲在诱导石英玻璃的非线性电离过程中电子动量弛豫时间对于该电离过程的影响.计算结果证明电子动量弛豫时间会直接影响入射脉冲在焦点区域所形成的峰值场强、自由电子态密度和能流等参量的分布态势,因此在与实验结果相比较后发现适合于相互作用过程的电子动量弛豫时间的理论值约为1.27 fs.进一步的研究表明,电子动量弛豫时间与逆韧致吸收效应、雪崩电离的概率、等离子体密度、等离子体的自散焦效果以及间接引起的焦平面位置的移动都有着密切的联系.当前的研究结果表明电子动量弛豫时间在飞秒激光脉冲与物质相互作用的过程中发挥着重要作用.     

Laser energy dependence on femtosecond laser-induced nucleation of protein

We have investigated femtosecond laser-induced nucleation of hen egg-white lysozyme (HEWL) as a function of the laser pulse energy and pulse time width. This is the first recorded study to confirm that the femtosecond laser-induced nucleation of HEWL can be induced at a specific threshold laser energy. The threshold energy is comparable to that of cavitation bubble generation. The results strongly suggest that morphological changes in the solution are key factors for protein nucleation.     

Thermal and mechanical damage of GaAs in picosecond regime

An in-depth study of the single pulse and multiple pulse laser (35 ps, 10 Hz and 1064 nm) damage for threshold fluence and greater fluence of GaAs 1 0 0 single crystal is presented. Damage which starts at a power 2×10¹¹ W/cm² in the form of pits occurs due to accumulation of laser induced microscopic defects. Effect of multiple pulse at first makes the pits more prominent in the form of Ga emission. Then the topmost layer is removed. If the number of pulses is further increased new pits are formed in the new surface (beneath the removed surface) and the above process is repeated. The thermal model is sufficient to explain this morphology. However, for larger fluences, a large cracking and fracture and the possibility of both Ga and As emission in different ratios suggest that mechanical damage is a dominant feature for higher fluences which arises due to generation of shock waves and rapid vaporization of material. Damage threshold has been calculated with the help of the thermal model given by Meyer et al. which is in good agreement with our experimental results.     

脉冲激光辐照氮化硅陶瓷损伤阈值的光谱测量

氮化硅陶瓷具有耐高温、耐腐蚀和耐磨损等优异性能, 可应用于金属材料和高分子材料难以胜任的极端工作环境。但具备这些优良特性的同时也给其加工带来了不便,传统的磨削加工方法效率低,设备损耗严重, 激光辅助加工为其提供了一种新途径。将等离子体光谱法和显微成像法相结合,对脉冲激光辐照氮化硅陶瓷的损伤阈值进行了测量,并分析了损伤机理。实验选用热压烧结氮化硅陶瓷为靶材,参考ISO21254国际损伤阈值测试标准搭建试验系统,采用1-on-1法利用Nd3+∶YAG固体脉冲激光分别在纳秒和微秒脉宽下辐照氮化硅陶瓷,两种脉宽分别选取10个能量密度梯度进行激光辐照,每个能量密度辐照10个点。利用光纤光谱仪采集光谱信息,利用金相显微镜获取显微图像信息,将光谱结果与显微成像结果对比分析, 发现纳秒脉宽下材料一旦损伤光谱上就会出现等离子体峰,通过分析光谱中等离子体峰,元素指认是否含有材料中特征元素即可判断损伤,为了区别空气电离击穿同时测量了空气等离子体光谱对比分析剔除干扰。微秒脉宽下显微图像观察到刚开始损伤时,光谱中只出现较强热辐射谱线并未出现等离子体谱线,进一步增加激光能量密度,光谱中会出现少量等离子体峰,因此不能直接以等离子体峰判断材料损伤阈值。利用金相显微镜观察损伤形貌,纳秒脉宽下在损伤区域内部观察到明显的烧蚀冲击状损伤,光谱呈现出大量等离子体谱线,说明纳秒激光辐照氮化硅损伤机制主要为等离子体冲击波引起的力学损伤效应。微秒脉宽在辐照区域边缘发现热烧蚀痕迹,损伤区内观察到大量熔融物,出现明显热辐射光谱,说明微秒激光辐照氮化硅损伤机制主要是由于长脉宽热积累引起的热损伤效应,随着能量密度增加热辐射谱上叠加有等离子体峰,等离子体峰值强度与损伤程度一致。利用零几率损伤阈值法对两种方法测得结果进行了拟合,分析发现等离子体光谱法更适用于纳秒脉宽下损伤阈值测量,得到结果为0.256 J·cm-2;显微成像法适用于微秒脉宽下损伤阈值测量,得到结果为6.84 J·cm-2。     

Laser-induced damage in SiO2 and CaF2 with picosecond and femtosecond laser pulses

Single- and multiple-shot damage thresholds and plasma-emission thresholds for fused silica and CaF₂ are reported for 790 nm photons as a function of laser pulse width (190 fs – 4.5 ps). The results are compared with single-shot plasma-emission measurements [1] and with multiple-shot damage measurements [2]. Both the damage threshold and the plasma-emission threshold are shown to decrease with decreasing pulse width over the entire pulse-width range investigated.