超短脉冲激光烧蚀绝缘体材料机理的耦合理论模型

基于电子能带理论,以动力论的Fokker_Planck方程为基础,从微观层次对超短脉冲激光烧蚀绝缘体材料的机理进行分析研究.源项中分别考虑了雪崩电离、多光子电离机制,并考虑了电子能量与散射机制对电子弛豫时间的影响.建立了绝缘体烧蚀机理的耦合数学模型,其计算的激光烧蚀临界能量密度阀值与实验结果很好的吻合.定量描述了超短脉冲激光对绝缘体材料烧蚀微观过程的影响.     

A simplified predictive model for high-fluence ultra-short pulsed laser ablation of semiconductors and dielectrics

Ultra-short pulsed laser ablation is a very complicated process and a predictive model is very desirable for process design and optimization in practical applications. However, the molecular dynamics or hydrodynamic models, although they are powerful and necessary tools for the study of the fundamental physics, are time-consuming and difficult to apply for practical applications. In this paper, a predictive, simplified and easy to apply model has been developed for high-fluence ultra-short laser ablation of semiconductors and dielectrics. Unlike many other simplified models, this model does not involve any free adjustable variables. The model predictions agree well with experimental measurements for femtosecond laser ablation, while the model is not very applicable for pulse durations more than ∼10 ps.     

Mechanisms of ultra-short laser pulse ablation from ionic crystals

In ultra-short laser pulse ablation from dielectric crystals two different ablation regimes are observed: 1) At laser intensities well below the single shot damage threshold, Coulomb explosion upon multiphoton surface ionization is the dominant process, yielding electrons and fast positive ions. 2) At higher laser intensities, ablation exhibits signs of hyper-thermal emission (phase explosion) as a result of rapid hot electron thermalization. This regime is characterized by massive etching, mostly of neutral particles. The transition from regime one to two is associated with the appearance of an additional, slower group of positive ions, saturation in the positive ion yield, and the occurrence of negative ions. PACS 61.80.Ba; 61.82.Ms; 79.20.Ds     

A simple model for high fluence ultra-short pulsed laser metal ablation

The ultra-short laser metal ablation is a very complex process, the complete simulation of which requires applications of complicated hydrodynamics or molecular dynamics models, which, however, are often time-consuming and difficult to apply. For many practical applications, where the laser ablation depth is the main concern, a simplified model that is easy to apply but at the same time can also provide reasonably accurate predictions of ablation depth is very desirable. Such a model has been developed and presented in this paper, which has been found to be applicable for laser pulse duration up to 10 ps based on comparisons of model predictions with experimental measurements.     

Plasma-mediated ablation of biofilm contamination

Ultra-short pulsed laser removal of thin biofilm contamination on different substrates has been conducted via the use of plasma-mediated ablation. The biofilms were formed using sheep whole blood. The ablation was generated using a 1.2 ps ultra-short pulsed laser with wavelength centered at 1552 nm. The blood contamination was transformed into plasma and collected with a vacuum system. The single line ablation features have been measured. The ablation thresholds of blood contamination and bare substrates were determined. It is found that the ablation threshold of the blood contamination is lower than those of the beneath substrates including the glass slide, PDMS, and human dermal tissues. The ablation effects of different laser parameters (pulse overlap rate and pulse energy) were studied and ablation efficiency was measured. Proper ablation parameters were found to efficiently remove contamination with maximum efficiency and without damage to the substrate surface for the current laser system. Complete removal of blood contaminant from the glass substrate surface and freeze-dried dermis tissue surface was demonstrated by the USP laser ablation with repeated area scanning. No obvious thermal damage was found in the decontaminated glass and tissue samples.     

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。     

Modeling of UV pulsed-laser ablation of metallic targets

A model to describe the laser ablation of metallic targets is presented. It accounts for the main physical processes involved in the laser–solid–plasma interaction by considering the photon absorption and the ionization mechanisms that are active in the plasma, as well as the laser-produced plasma kinetics. The model is used to simulate the laser ablation of aluminum targets irradiated with a 6-ns UV laser pulse at 0.35 μm, and the results are compared with experimental findings. Calculations show that all the investigated plasma parameters strongly depend on the laser intensity until a roll-off is reached at irradiance ≥1.5 GW cm^-2. The satisfactorily good agreement between model predictions and experimental findings confirms that laser–plasma interaction processes and plasma kinetics play a relevant role during nanosecond laser ablation of metals in the laser intensity range of concern in this study. Received: 12 February 1999 / Accepted: 12 April 1999 / Published online: 7 July 1999     

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.     

Ablation of subsurface tumors using an ultra-short pulse laser

This paper demonstrates the novel technique of ablating subsurface tumors with minimal thermal damage to surrounding healthy tissue using a focused laser beam from an ultra-short pulse diode laser source. Experiments were performed on anesthetized healthy mice as well as mice with mammary tumors in order to demonstrate the fundamental advantages of using a focused-beam, ultra-short pulse laser to ablate subcutaneous tissues. The technique was demonstrated through histological analysis of tissue samples after irradiation of anesthetized mice with or without mammary tumors. To demonstrate the efficacy of subsurface focusing, temperature was monitored at the subsurface tumor location and at the surface in an untranslated sample while irradiating with a focused ultra-short pulsed 1552 nm laser. Results show that temperature rise was dramatically greater at the focal depth than at the surface. Irradiation at the subsurface tumor location while translated over time across the tumor location resulted in precise ablation of the tumor. This work shows that a focused-beam ultra-short pulse 1552 nm laser results in precise ablation at the desired location with high efficacy and a minimal zone of collateral thermal and/or mechanical damage.     

激光烧蚀冲量耦合系数解析计算模型

利用激光清除空间碎片被认为是一种可行手段,冲量耦合系数是数值计算空间碎片清除效果的重要参数。建立了激光烧蚀冲量耦合系数解析计算模型,引入电离度参数,将气化机制与等离子体机制两种机制下的冲量耦合系数解析计算模型联系起来,建立统一的耦合系数解析模型。以空间碎片常见材料Al为例,计算得到冲量耦合系数、电离度、激光功率密度三者之间的变化关系。随着激光功率密度的增加,气化机制逐渐向等离子体机制过渡,电离度增加,直至完全电离,冲量耦合系数先增加后减少,并且在等离子机制占主导情况下达到最优冲量耦合。     

Model description of surface charging during ultra-fast pulsed laser ablation of materials

We present a model describing the dynamical mechanisms responsible for generating fast ion ejection under ultra-short pulsed laser irradiation. The model is based on a simplified drift–diffusion approach describing the evolution of the laser-generated charge carriers, their transport, and the electric field generated as a result of quasi-neutrality breaking in the irradiated target. The importance of different processes in generating the non-thermal material-ejection mechanisms is discussed. A common frame is applied to dielectrics, semiconductors, and metals and different dynamical behaviour is observed. The modelling results are in good agreement with fs pump–probe studies and measurements of the velocity distributions of the emitted ions.     

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。     

Guidelines for efficient direct ablation of dielectrics with single femtosecond pulses

We provide guidelines to femtosecond laser users to select ad hoc laser parameters, namely the fluence and pulse duration, in the context of the development of ablation processes at the surface of dielectrics using single femtosecond pulses. Our results and discussion are based on a comprehensive experimental and theoretical analysis of the energy deposition process at the surface of fused silica samples and of their postmortem ablation characteristics, in the range of intensities from 10¹³ to 10¹⁵ W/cm². We show experimentally and numerically that self-induced plasma transient properties at the pulse timescale dramatically determine the efficiency of energy deposition and affect the resulting ablation morphology. In practice, we determine that the precise measurement of two characteristic fluence values, namely the laser-induced ablation threshold F _th,LIAT and the fluence F _opt for maximum ablation efficiency, are only required to qualify the outcomes of laser ablation at the surface of a dielectric in an extended range of applied fluence.     

超短脉冲激光照射下氧化铝的烧蚀机理

利用烧蚀面积与激光脉冲能量的线性关系，确定了氧化铝的破坏阈值，同时采用散射光探测法，研究了800和400nm超短脉冲激光作用下氧化铝的破坏阈值对激光脉宽的依赖关系，并探讨了氧化铝的烧蚀规律. 利用雪崩击穿模型，解释了实验结果，并讨论了导带电子光吸收机理. 关键词： 飞秒激光 氧化铝 破坏阈值 雪崩模型     

Analysis of the nonlinear absorption mechanisms in ablation of transparent materials by high-intensity and ultrashort laser pulses

The mechanisms of nonlinear absorption in transparent materials under irradiation with ultrashort laser pulses are considered theoretically. Nitride semiconductor, sapphire and others transparent dielectrics were investigated. The ablation threshold for these materials is within multi-TW/cm² range. The model was used based on the tunneling absorption under the irradiation by high-intensity ultrashort pulses in terms of the theory of ionization of solid in a field of strong electromagnetic wave. The effect of the energy gap of material on the threshold of laser ablation was adequately explained.     

Analysis of avalanche mechanisms in short-pulses laser-induced damage

A theory based on the rate equation of free electron is used to analyze the process of free electron multiplication in optical materials under the laser irradiation, specially researching on the effect of avalanche ionization on the electron multiplication and material damage threshold. Numerical investigation with SiO₂ is processed using this theoretical model, and damage thresholds under different avalanche models are analyzed. The result shows that during research on the energy charge between electron and electromagnetic field, the probability of avalanche ionization should be considered. Under this assumption, the numerical threshold gets well with the experimental result.     

超短激光脉冲对宽带光学物质的微加工

通过讨论超快飞秒激光脉冲和长脉冲宽度的激光脉冲紧导致宽能隙透明电介质的损伤机理和比较超短激光脉冲与长激光脉冲对宽能隙透明电介质的损伤程度，得出超短激光脉冲是一种可对透明宽带电介质进行加工的有效工具的结论。当波长为800nm，脉冲宽度为150fs的激光脉冲紧聚焦到不同的宽能隙透明电介质（K9玻璃和ZK6玻璃）体内时，可制作不同光栅常数的光栅，并在波长为635nm的He-Ne连续激光的垂直照射下，对光栅的远场相对衍射效率和光栅的衍射效率进行了测量。     

超短超强激光脉冲辐照超薄碳膜电离状态研究

为了进一步理解极端条件下物质的电离特性, 特别是超短超强激光脉冲辐照超薄靶时等离子体的形成与分布, 本文以超薄碳膜为例, 细致研究了超短超强激光脉冲辐照下原子的离化过程. 分析和比较了强激光场直接作用电离和靶内静电场电离等两种场致电离形式, 在碰撞电离可以忽略的情况下, 发现更多的电离份额是来自靶内静电场的电离方式. 研究了激光脉冲强度对电离的影响, 发现激光脉冲强度越强, 电离速度越快, 产生的高价态离子所占比例也越高.当激光强度为1×10²⁰ W/cm²时, 尽管该强度高于电离生成C⁺⁶所需要的激光强度阈值, 但该激光脉冲并不能将整个靶电离成C⁺⁶离子, 对此本文进行了详细的分析. 在研究激光脉冲宽度的影响时, 发现激光脉宽越小, 电离速度越快, 但越小的激光脉冲电离获得的高价态离子越少.     

飞秒激光激发空气电离的阈值研究

报道在脉宽50fs—22ps，波长800nm脉冲激光作用下的空气电离阈值的研究结果.利用探测等离子体发光信号的方法，实验测量了激发空气电离所需的阈值激光强度.结果表明，当激光脉冲宽度从50fs增加到22ps时，阈值光强I_th从8.7×10¹⁴W/cm²下降到2.7×10¹³W/cm²；I_th经历了由迅速降低逐渐发展为缓慢降低的过程.在50fs—1p     

An analytical continuum-based model of time-of-flight distributions for pulsed laser ablation

An analytical model for time-of-flight (TOF) distributions of particles produced by pulsed laser ablation in vacuum has been proposed. The model takes into account the hydrodynamic expansion stage of the ablation plume and is based on a ‘sudden freeze’ model developed previously for steady-state supersonic jets. It is assumed that a continuum-like expansion of the plume takes place until a freezing time moment t _free (or, alternatively, until a ‘freezing distance’ x _free) whereupon the collisionless expansion begins. The proposed model is applied for analysis of experimental data on graphite ablation with nanosecond laser pulses. For verification, the analytical distributions are compared with calculated results obtained using a hybrid model combining a thermal model of laser-induced material heating with calculations of the plume dynamics by the direct simulation Monte Carlo (DSMC) method. It is shown that the proposed model can accurately estimate the surface temperature for conditions when the common approach fails.