热物性参量对飞秒激光烧蚀金属影响的分子动力学模拟

利用结合双温模型的分子动力学模拟方法,研究了飞秒激光与金属相互作用的烧蚀机制.采用中心波长为800 nm,能量密度从0.043 J·cm~(-2)到0.40 J·cm~(-2)不等,脉宽分别为70 fs和200 fs的激光烧蚀金属镍和铝材料.靶材的温度、原子位型以及内部压力随时间的演化展示了材料热物性参量特性和激光参量对烧蚀结果的影响.结果显示材料电子热传导率对飞秒脉宽激光下的影响仍然较大;对比铝和镍的结果可知,铝的电子晶格耦合系数比镍的小,故电子晶格间的温度梯度持续时间较长;铝的电子热传导系数比镍的大,所以材料上下表面电子温度耦合的时间缩短.铝薄膜表面在能量密度为0.40 J·cm~(-2)激光烧蚀下呈现纳米尺寸的晶体结构.     

飞秒激光对氟化镁烧蚀机理研究

用扫描电镜(SEM)研究了氟化镁在800nm超短脉冲激光作用下的单枪表面烧蚀形貌.根据烧蚀斑面积与激光脉冲能量间的对数关系，测得烧蚀阈值与激光脉宽的关系曲线(55—750fs).计算了导带电子的双光子吸收，改进了多速率方程模型，很好地解释了实验结果. 关键词： 飞秒激光 氟化镁 烧蚀机理 双光子吸收     

多脉冲飞秒激光对锗材料的烧蚀效应

 开展了脉宽为40 fs的不同数量激光脉冲对锗材料的烧蚀效应实验,采用扫描电镜、激光共聚焦显微镜等方法对不同数量的飞秒激光脉冲作用下锗材料表面烧蚀区进行了检测,并对作用后材料烧蚀形貌演化规律进行了分析,初步分析了锗材料烧蚀区周围形成的不同环区的形貌特征及成因,对各环区烧蚀形貌特征随激光作用脉冲数的增加而产生的形貌演化过程进行了观测。并给出单脉冲飞秒激光对锗材料的烧蚀阈值为1.2 J·cm^-2,采用激光共聚焦显微镜测得该阈值条件下单个飞秒激光脉冲对锗材料的烧蚀深度约为150 nm。     

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。     

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。     

超短脉冲照射下氟化锂的烧蚀机理及其超快动力学研究

研究了超短脉冲激光照射下LiF晶体的破坏机理及其超快动力学过程,利用扫描电镜和原子力显微镜等测试手段,观测了飞秒激光照射下LiF晶体的烧蚀形貌。利用烧蚀面积与激光脉冲能量的对数关系确定了LiF晶体的破坏阈值,并利用非线性玻璃棒展宽脉宽,得到了800nm激光作用下LiF破坏阈值对激光脉宽（50～1000fs）的依赖关系;利用抽运一探针超快探测平台,探测了LiF烧蚀过程中反射率的变化。采用雪崩击穿模型,并根据晶体材料反射率与材料的介电常量的依赖关系,通过数值计算,模拟了材料烧蚀阈值与脉宽的依赖关系及材料激发过程中反射率的变化关系。结果表明,理论结果与实验结果符合较好。讨论了飞秒激光照射下LiF晶体中导带电子数密度的变化规律,并解释了相应的实验结果。     

脉冲激光烧蚀中电声弛豫时间的确定

为了提高脉冲激光制备薄膜的质量,准确掌握电声弛豫时间是关键,它对脉冲激光脉宽和能量密度的选取起着决定性的作用. 文中以铝靶材为例,利用经典的双温方程通过时域有限差分法(FDTD)得到电子、离子亚系统的温度随时间和位置演化的图像,进而得到电声弛豫时间的准确值. 这样便能准确划分热烧蚀和非平衡烧蚀,从而更好地控制激光的烧蚀过程. 同时找出了电声弛豫时间随激光脉宽以及能量密度变化的规律. 关键词： 飞秒激光 电声弛豫时间 双温方程 激光能量密度     

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     

飞秒激光打孔硅的孔洞形貌研究

在飞秒激光打孔硅材料过程中,为了得到表面等离子体效应和激光烧蚀形成的孔洞对后续激光能量在孔内分布的影响,建立单脉冲等离子体阈值理论模型及设计连续飞秒激光烧蚀硅材料实验.理论计算得到的损伤阈值为0.21J/cm2,符合实验模型测得的阈值0.20~0.25J/cm2.当载流子密度达到临界值Ncr,等离子体的激发会导致表面反射率短时间内急剧上升.入射激光通量从0.5J/cm2增大到3.0J/cm2,烧蚀深度逐渐增大并趋于约1.1μm,同时脉宽从150fs减小到50fs,烧蚀结构类似于椭圆形烧蚀轮廓.后续激光脉冲辐照在已形成的孔洞上时,基于时域有限差分法,控制光束与孔壁的夹角从79℃到49℃,激光能量越接近孔底中心,越易引发该范围内的等离子体激发;且在不同偏振态光束辐照下,孔底的能量分布不同会造成相应特殊的烧蚀形貌.增大激光通量和减小脉冲宽度获得理想的初始孔洞结构,可使后续脉冲能量集中孔底中心区域,打孔效果更好.     

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。     

Removal of the indigo color by laser beam–denim interaction

Our experiments show that the laser fading process removes efficiently indigo-dye from denim support. We use the beams from Nd : YAG laser (1064 nm and its second harmonic 532 nm) and CO₂ (10.6 μm) lasers. Different laser pulse parameters were used in order to obtain laser power density and fluency to start the ablation process. The purpose of this work is to determine the change of denim diffuse reflectivity spectra after laser irradiation with different wavelength and different power density. The change of diffuse reflectivity coefficient was up to 17% at 450 nm wavelength (from 8% reflectivity for unirradiated denim).     

Optical transmission and reflection of aluminum film irradiated by nanosecond laser beam and experimental studying of phase-explosion

In this paper we present evidence for a phase explosion during the laser-induced ablation process by studying the optical reflectivity of the ablated plume. The ablation was produced by irradiating thin film aluminum coated on a quartz substrate with a single pulse laser beam in ambient air. The laser pulse was provided by the second harmonic of a Q-switched Nd:YAG laser with ∼10 ns pulse duration. The transmission of a low power He–Ne laser beam through the hot ablated material plume and its reflection (from the front surface, and rear surface of aluminum film) were also monitored during the duration of the ablation event. The results show that the front surface reflectivity is enhanced at an early time of ablation which is described as strong evidence for the creation of a phase explosion in this process.     

Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses

We have studied the plasma formation and ablation dynamics in fused silica upon irradiation with a single 120 fs laser pulse at 800 nm by using fs-resolved pump-probe microscope. It allows recording images of the laser-excited surface at different time delays after the arrival of the pump pulse. This way, we can extract both the temporal evolution of the surface reflectivity and transmission, at 400 nm, for different spatial positions in the spots (and thus for different local fluences) from single series of images. At fluences well above the visible ablation threshold, a fast and large increase of the reflectivity is induced by the formation of a dense free-electron plasma. The maximum reflectivity value is reached within ≈1.5 ps, while the normalized transmission decreases within ≈400 fs. The subsequent temporal evolution of both transient reflectivity and transmission are consistent with the occurrence of surface ablation. In addition, the time-resolved images reveal the existence of a free-electron plasma distribution surrounding the visible ablation crater and thus formed at local fluences below the ablation threshold. The lifetime of this sub-ablation plasma is ≈50 ps, and its maximum electron density amounts to 5.5×10²² cm⁻³.     

Different crystallization processes of as-deposited amorphous Ge2Sb2Te5 films on nano- and picosecond single laser pulse irradiation

The crystallization dynamics of as-deposited amorphous Ge₂Sb₂Te₅ films induced by nano- and picosecond single laser pulse irradiation is studied using in situ reflectivity measurements. Compared with nanosecond laser pulse, the typical recalescence phenomenon did not appear during the picosecond laser pulse-induced crystallization processes when the pulse fluence gradually increased from crystallization to ablation threshold. The absence of melting and recalescence phenomenon significantly decreased the crystallization time from hundreds to a few tens of nanoseconds. The role of pulse duration time scale on the crystallization process is qualitatively analyzed.     

飞秒双脉冲激光照射金属薄膜的热行为

通过双温方程对飞秒单脉冲与双脉冲照射金薄膜进行了计算模拟分析,得到了金靶的电子温度和晶格温度随着时间空间的变化。在同样激光能量密度下,单脉冲与双脉冲使得金膜温度的变化表明双脉冲使得更多的激光能量渗透到靶材内部,这些能量可以使得烧蚀深度更深,有利于提高激光烧蚀靶材的效率。计算结果显示随着激光能量密度的增加熔化面深度逐渐增加,单脉冲与双脉冲熔化面深度的变化明显不同。在激光能量密度高于损伤阈值附近,单脉冲的烧蚀深度大于双脉冲的烧蚀深度,随着激光能量密度增加,双脉冲的烧蚀深度将大于单脉冲的烧蚀深度。     

脉冲激光烧蚀Ge产生等离子体特性的数值模拟

 针对激光烧蚀半导体材料Ge初期的特点,建立了1维的热传导和流体动力学模型。对波长为248 nm、脉宽为17 ns、峰值功率密度为4×10⁸ W/cm²的KrF脉冲激光在133.32 Pa氦气环境下烧蚀Ge产生等离子体的特性进行了数值模拟。结果表明：单个激光脉冲对靶的烧蚀深度达到55 nm,蒸气膨胀前端由于压缩背景气体产生压缩冲击波, 波前的速度最大,温度很高。从不同时刻的电离率分布图中得出,在靶面附近区域,Ge的1阶电离始终占优势;在中心区域,脉冲作用时间内,Ge的2阶电离率比1阶电离率大,脉冲结束后,Ge的2阶电离率下降,1阶电离率逐渐变大。     

Micro-patterning for polymer electrolyte fuel cells: Single pulse laser ablation of aluminum films from glassy carbon

Microfuel cells are a possible replacement for batteries as energy sources in portable devices. At PSI a micropolymer electrolyte fuel cell was developed, whose flow fields consist of micro-structured glassy carbon plates. Micro-structuring of glassy carbon is carried out in a multi-step process. A sputtered aluminum mask is selectively removed by single pulse laser ablation from glassy carbon thereby defining micro-channels subsequently etched by reactive ion etching.A pulsed XeCl excimer laser (308 nm) is used for the single pulse patterning of a metal mask on the glassy carbon. The influence of the excimer laser typical pulse to pulse energy fluctuations on the micro-structuring process must be known to minimize defects during RIE etching of the micro-channels. To obtain a better understanding of the processes occurring during ablation, ns-shadowgraphy was performed. The formation of a shockwave was observed, followed by a similar but much slower perturbation, and the subsequent release of fragments. The calculated velocities can be correlated with the energy release during ablation. The post-ablation examination of the samples by profilometry, optical microscopy, SEM and EDX is used to measure the amount of removed material, the quality of the aluminum mask edges and aluminum residues on the glassy carbon surface. Such criteria allow us to classify the laser irradiation as a function of laser fluence: no ablation, partial ablation, complete ablation, and over-ablation.     

Laser irradiation effects on gold

Investigations on the laser irradiation effects on gold are explored in terms of plasma-plume dynamics and morphological and crystallographic changes. Annealed 4N gold samples were irradiated with a Q-switched Nd:YAG laser (53 mJ, 21 MW, 532 nm, and pulse width 6–8 ns) for plume dynamics using 10-ns gated fast photography. A Q-switched pulsed Nd:YAG laser (10 mJ, 1.1 MW, 1064 nm, and pulse width 9 ns) was used to irradiate the surface of the samples for morphological and crystallographic studies of laser-irradiated gold in a vacuum ～10^?3 Torr. The annealed samples were exposed to 50 shots of a Nd:YAG laser (10 mJ, 1.1 MW, 1064 nm, and pulse width 9 ns). The investigation on the plume was done by using an intensified charged-couple device ICCD-5760/IR-UV camera. The morphological investigation of the irradiated surface was carried out by analyzing micrographs obtained using an Hitachi S 3000 H scanning-electron microscope (SEM). The crystallographic studies of the irradiated samples were performed by analyzing the XRD patterns obtained using an X’ Pert Pro Pan Analytical X-ray diffractometer. The investigation on gated ICCD images of the plume reveal that, at very earlier times, the plasma-plume expansion has a linear trend, whereas, at later times, the plasma-plume expansion is nonuniform. SEM micrographs exhibit the primary mechanisms of pulsed-laser ablation (PLA), such as hydrodynamic sputtering, thermal sputtering, exfoliation sputtering, and splashing. The surface morphology was explained in terms of crater formation, swelling, burning, nucleation, grain growth, and nonsymmetric heat conduction. The nonuniform thermal expansion of gold due to thermal-energy transfer is also studied by SEM micrographs, which was supported by XRD analysis. The structural analysis on the basis of XRD shows that the composition of the irradiated samples is not disturbed even after laser irradiation. The grain sizes also changed due to laser irradiation.     

Selective ablation of atherosclerotic lesions with less thermal damage by controlling the pulse structure of a quantum cascade laser in the 5.7-µm wavelength range

Cholesteryl esters are the main components of atherosclerotic plaques, and they have an absorption peak at the wavelength of 5.75 µm. To realize less-invasive ablation of the atherosclerotic plaques using a quasi-continuous wave (quasi-CW) quantum cascade laser (QCL), the thermal effects on normal vessels must be reduced. In this study, we attempted to reduce the thermal effects by controlling the pulse structure. The irradiation effects on rabbit atherosclerotic aortas using macro pulse irradiation (irradiation of pulses at intervals) and conventional quasi-CW irradiation were compared. The macro pulse width and the macro pulse interval were determined based on the thermal relaxation time of atherosclerotic and normal aortas in the oscillation wavelength of the QCL. The ablation depth increased and the coagulation width decreased using macro pulse irradiation. Moreover, difference in ablation depth between the atherosclerotic and normal rabbit aortas using macro pulse irradiation was confirmed. Therefore, the QCL in the 5.7-µm wavelength range with controlling the pulse structure was effective for less-invasive laser angioplasty.