飞秒激光辐照对GaAs太阳能电池损伤的试验研究

开展了飞秒激光对GaAs太阳能电池表面辐照损伤的试验研究。研究结果表明,在能量密度0.27J/cm2时,飞秒激光对GaAs太阳能电池表面开始造成损伤,其损伤程度随着激光能量密度的增加而增加;在相同激光能量密度的条件下,其损伤程度随着扫描速度的增加而减小。采用光学显微镜对其表面损伤形貌进行分析发现,其烧蚀面积随着激光能量密度的增大而不断增大,这是因为飞秒激光的光斑能量成高斯型分布。根据烧蚀区域光斑直径与脉冲能量的关系,对GaAs太阳能电池表层TiO2/SiO2薄膜的损伤阈值进行了理论计算,其阈值为0.35J/cm2大于实验测试结果0.27J/cm2。这可能是由于理论计算中对激光光斑模式的简化以及材料特性均化处理等不确定度因素影响的结果。     

多脉冲飞秒激光精修面齿轮的烧蚀形貌研究

针对材质为18Cr2Ni4WA的面齿轮,采用飞秒脉冲激光烧蚀来提高其表面质量。建立电子-自旋-晶格三温复耦合模型,预测分析面齿轮材料电子、自旋和晶格单脉冲能量效应下的温度变化过程;结合多脉冲能量累积效应,进行面齿轮材料在激光能量密度为1.035~5.252 J/cm2下烧蚀凹坑直径和深度演变规律的预测与实验分析。研究结果表明：脉冲数的增加并不会持续增大面齿轮表面的平衡温度,基本在30个脉冲时就保持不变;能量密度的增加会使烧蚀凹坑直径和深度持续变大,但变大的速率会持续降低;烧蚀凹坑形貌在能量密度为2.467 J/cm2时最好;过大的扫描道间距会使被加工表面粗糙度值增加,扫描道间距为25 μm时粗糙度值最低为0.185 μm。     

飞秒激光烧蚀面齿轮材料的齿面表层形态研究

针对面齿轮材料18Cr2Ni4WA,研究飞秒激光辐照面齿轮材料的热力效应,建立飞秒激光烧蚀面齿轮温度-应力耦合模型,分析多脉冲时不同能量密度下电子温度、晶格温度以及热应力的变化过程。结果表明：电子温度、晶格温度以及热应力随激光能量密度的增大而增大。实验和仿真的对比结果说明,烧蚀齿面表层为残余压应力,烧蚀深度和凹坑直径随激光能量密度的增加而增大,较大的激光能量密度会产生较多的熔融物,降低飞秒激光加工质量,当能量密度为1.78 J/cm2时,齿面表层形态较好。本文为提高飞秒激光精微烧蚀面齿轮质量提供了研究基础。     

飞秒激光烧蚀MgAl2O4透明陶瓷的实验研究

实验研究了800nm飞秒激光与MgAl2O4透明陶瓷的相互作用，得到其在单脉冲、多脉冲情况下的损伤阈值和损伤面积，用CCD成像技术和扫描电镜观察了烧蚀点的形貌特征，用显微红外光谱仪测试了烧蚀区域的透过光谱．结果表明；单脉冲烧蚀条件下，烧蚀面积与脉冲能量近似为线性关系，而在多脉冲烧蚀条件下，烧蚀面积随着脉冲数量的增加呈近似波尔兹曼（Boltzmann）增大；当激光功率接近损伤阈值时，烧蚀后的区域在波数为2500-7000cm^-1范围内的红外透过率由82％提高到86％，当激光功率超过损伤阈值后，透过率降低20％左右．     

脉冲激光参数在烧蚀固体材料过程中的影响

介绍长脉冲激光和短脉冲激光与固体材料相互作用时的物理模型。介绍激光作用过程中,脉宽、波长等激光参数对材料烧蚀阈值以及加工质量的影响。     

自变频激光晶体Nd3+:GdAl3(BO3)4的研究

采用熔盐法生长出尺寸为30mm的Nd3+:GdAl3(BO3)4优质晶体,进行了吸收光谱和荧光光谱的测定研究,计算得到晶体发射截面为σe1061.9=2.9×10-19cm2和σe1338mm=5.5×10-20cm2.采用染料激光器作为泵浦源,对晶体进行了自变频激光实验研究,在紫外可调谐(378-382nm)、绿光531nm、蓝光(436-443nm)、红光(669nm)和红外可调谐(1305-1365nm)波段实现了激光输出,输出的最大功率分别为:105μJ/脉冲、119.5μJ/脉冲、445μJ/脉冲、19μJ/脉冲和31μJ/脉冲.     

片状石墨增强树脂基复合材料的耐激光烧蚀性能研究

采用刷涂的方法制备了一种新型的片状石墨增强钡酚醛树脂基复合材料, 并采用重频激光辐照的方法, 对其耐烧蚀性能进行了研究. 研究结果表明: 片状石墨增强钡酚醛树脂基复合材料在平均功率密度为1700 J/cm²的重频激光辐照下的热烧蚀率为32.8 μg/J, 耐激光烧蚀性能明显高于碳纤维增强的钡酚醛树脂基复合材料和钡酚醛树脂; 片状石墨增强钡酚醛树脂基复合材料中的片状石墨呈近平行的层状分布方式, 在激光辐照过程中能对入射激光起到平面反射作用, 从而有效地降低激光辐照的能量沉积; 片状石墨的片型对复合材料的耐烧蚀性能有影响, φ0.5 mm的片状石墨增强的复合材料的耐激光烧蚀性能最好.     

正弦脉冲激光激发声波的热膨胀机制理论

阐述了激光与液体媒质作用通过热膨胀机制激发平面光声源的理论;针对激光脉冲为正弦波形的情况,求解出δ脉冲和长脉冲分别在约束界面和自由界面下产生声脉冲的解析解,通过仿真得到了声脉冲剖面,并对每种情况下产生的声波进行了对比分析;然后对液体中激光致声的光声转换效率进行了理论研究,对正弦波形δ脉冲和长脉冲激光分别在约束界面和自由界面下产生声脉冲的光声转换效率分别进行了理论分析和仿真,并讨论了液体媒质特性参数和激光参数对转换效率的影响,提出了提高转换效率的方案。     

飞秒激光与金膜作用的脉冲累积效应

采用1kHz飞秒激光（脉宽148fs，中心波长775nm）对石英衬底的金膜的烧蚀过程进行了研究．单脉冲与多脉冲的烧蚀阈值可以通过烧蚀点的直径平方与所用的激光能流的关系曲线获得．通过累积能流和烧蚀脉冲数的关系，可以得到金膜的脉冲累积因子．采用飞秒激光加工材料的一些特点可以合理解释单脉冲阈值附近获得的一些实验现象．     

基于三维双温模型的飞秒激光烧蚀面齿轮材料形貌研究

在传统双温模型基础上,建立考虑变离焦量效应和随电子温度变化的动态吸收率效应的三维双温模型。分析烧蚀过程中电子、晶格温度的变化情况和烧蚀凹坑的形貌。对面齿轮材料18Cr2Ni4WA进行飞秒激光烧蚀实验结果表明：随着单脉冲飞秒激光能量密度的增大,电子的峰值温度升高,烧蚀凹坑的深度和直径增大;单脉冲飞秒激光的脉宽对烧蚀形貌的影响并不显著;烧蚀凹坑的直径和深度不会随脉冲数目一直增大,脉冲数目存在阈值,超过该阈值对于烧蚀凹坑的形貌影响反而不利。     

Method for determination of the volume of material ejected as molten droplets during visible nanosecond ablation

A novel method is presented for determining the volume of molten material ejected from a substrate as a result of visible pulsed-laser ablation. A 100-microm-wide pulsed-laser light sheet (tau approximately 5 ns, lambda = 532 nm) was used in conjunction with a CCD camera to provide high-speed cross-sectional images of single-pulse ablation of aluminum with a visible nanosecond laser source. Computational analysis of the two-dimensional gray-scale images was used to determine the total volume of material ejected from the substrate in the form of molten droplets. Ablation with dual-wavelength (511- and 578-nm) pulses of 30-ns duration was characterized under various fluence conditions (0-25 J cm(-2)), allowing a quantitative threshold for explosive melt ejection in aluminum to be established at approximately 10 J cm(-2). The temporal evolution of the ejected material showed that, for an incident fluence of approximately 40 J cm(-2), molten-droplet ejection commenced at approximately 400 ns and ceased after approximately 2 micros.     

Direct-laser writing of striplines on AgCl single crystals

Experiments on a localized and controlled photo-reduction of silver were carried out on a AgCl single crystal that was exposed to a UV-laser light. Basic parameters such as the peak power and the total energy dose were varied. The study showed that for a pulsed-laser beam with a high peak power (>20 W) the ablation threshold of the AgCl was exceeded, while at lower peak powers no ablation was detected, only a change in the surface morphology. The degree of silver reduction was monitored using EDS microanalyses of the irradiated trace and the crystal matrix. We showed that the formation of photo-reduced silver aggregates decreases the ablation threshold and that the formation of a continuous silver trace on the AgCl single crystal is possible when the power density of the laser beam is further reduced.     

Existence of phase explosion during laser ablation and its effects on inductively coupled plasma-mass spectroscopy

A sudden increase in crater depth was observed during high irradiance (> 10(10) W/cm2) laser ablation of silicon, and it is attributed to the phenomenon of phase explosion. The threshold irradiance for phase explosion showed a dependence on two laser parameters: laser beam spot size and wavelength. For a larger beam size and longer incident wavelength, a higher laser irradiance was required to generate phase explosion. The rapid increase of crater depth above the phase explosion threshold irradiance correlated with a significant increase in the ICPMS signal intensity. The ratio of crater volume to ICPMS intensity, which represents entrainment efficiency, remained the lowest at laser irradiances slightly above the phase explosion threshold. However, this ratio increased at irradiances well above the threshold (> 10(11) W/cm2). Chemical analysis using laser ablation at irradiance above 10(11) W/cm2 provides increased sensitivity via improved entrainment and transport efficiency and increased ablation rate.     

Laser ablation of silicon using a Bessel-like beam generated by a subwavelength annular aperture structure

Using a femtosecond laser incident to an oxide-metal-oxide film engraved with a subwavelength annular aperture (SAA) structure, we generated a Bessel-like beam to ablate silicon. Experimental results show that the silicon can be ablated with a 0.05 J/cm(2) input ablation threshold at 120 fs pulse duration. We obtained a surface hole possessing a diameter less than 1 μm. Optical performance, including depth-of-focus and focal spot of the SAA structure, were simulated using finite-different time-domain calculations. We found that a far-field laser beam propagating through a SAA structure possesses a submicrometer focal spot and high focus intensity. Our method can be easily adopted for surface machining in microfabrication applications.     

新型界面结合强度检测技术理论及实验研究

膜/基界面结合强度的测量与评价已成为硬质薄膜技术发展与应用的关键问题.提出了1种新的测量方法———激光冲击检测技术,分析了在脉冲激光作用下,薄膜脱落的机理与数学模型.以测定TiN薄膜界面结合强度为例,用650～1000 mJ的脉冲激光对膜层进行加载,并采用体式显微镜和扫描电镜对薄膜的脱落情况进行表面观察,根据反射信号检...     

Low-power resonant laser ablation of copper

We emphasize two points: (l) the properties and mechanisms of very low-fluence ablation of copper surfaces and (2) the sensitivity and selectivity of resonant laser ablation (RLA). We present results for ablation of bulk copper and copper thin films; spot-size effects; the effects of surface-sample preparation and beam polarization; and an accurate measurement of material removal rates, typically ≤ 10(-3) ? at 35 mJ/cm(2). Velocity distributions were Maxwellian, with peak velocities ≈ 1-2 × 10(5) cm/s. In addition, we discuss the production of diffractionlike surface features, and the probable participation of nonthermal desorption mechanisms. RLA is shown to be a sensitive and useful diagnostic for studies of low-fluence laser-material interactions.     

In Situ Characterization of Laser Ablation by Pulsed Photoacoustics: The Case of Organic Nanocrystal Synthesis

Here, a new methodology based on the pulsed photoacoustic (PA) technique for real-time monitoring of the ablation process used to synthesize organic nanocrystals is described. The methodology is implemented by ablating microcrystals grown from an organic chromophore with nonlinear optical properties. It was determined that the PA signal from the ablation process increases in amplitude and is time-shifted as the ablation process advances. Comparing the PA signals generated at different ablation times under different laser fluences with the nanocrystal characterization obtained through light scattering, optical microscopy, and AFM, it was demonstrated that the pulsed PA technique can be useful for monitoring the process and determining the threshold of ablation.     

Diagonal scan measurement of Cr:LiSAF 20 ps ablation threshold

We report the measurement of the 20 ps ablation threshold of pure and Cr(3+) doped LiSAF samples using a simple method that employs a single scan of the sample across a focused laser beam waist. During the scan, a profile is etched in the sample surface, and the measurement of the maximum transversal size of the profile and the pulse peak power determine the ablation threshold, without any further knowledge of the beam geometry. Also, it was possible to measure the depth of the ablation profile, to calculate its effective volume, and to identify that the maximum material removal rate per pulse does not occur at the beam waist, which is not intuitively expected.     

纤维复合材料的热膨胀系数

提出了一种利用压电光声技术测量材料热膨胀系数的实验方法,并测试了单向复合材料C/C、C/Al的横向、纵向的热膨胀系数。根据已有的理论计算方法与实验结果对该方法的测试结果进行验证,证明了该检测方法的可靠性,进而又测量了C/C、C/Al材料在任一方向上的热膨胀系数。这种方法克服了理论计算过程复杂以及常规手段无法测量任一方向上热膨胀系数的缺陷。     

高模量碳纤维增强树脂基复合材料的皮秒激光加工阈值特性

对聚丙烯腈基高模量碳纤维/改性氰酸酯树脂复合材料(M55/BS-4)和一种沥青基高导热碳纤维/树脂基复合材料(K600/5418)的皮秒激光加工阈值和形貌特性进行了研究。通过面积外延法测定并比较了这两种碳纤维复合材料的近红外皮秒激光加工阈值及其阈值孵化效应,并预测了两种复合材料的单脉冲阈值;分析了入射能量通量(0.7~25 J/cm2)及光束扫描速度(0.2~5 m/s)对切口质量的影响规律。结果表明,碳纤维热导率的巨大差异导致不同碳纤维复合材料的加工阈值及形貌存在明显定量差距。使用可获得的最高扫描速度(5 m/s)和3.2倍(~8 J/cm2)单脉冲阈值的加工参数,可使材料的碳纤维和树脂几乎协同去除,加工形貌上表现为切缝入口宽度均匀、切割边缘整齐。使用更高扫描速度并配以合适的加工能量有望进一步提高加工质量。