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。     

343 nm飞秒激光制备微孔阵列以增强聚氨酯合成革透湿性

为了提高聚氨酯(PU)合成革透湿性,分别使用343 nm飞秒激光和作为对比的1030 nm飞秒激光及1064 nm纳秒激光制备微孔阵列。采用扫描电镜(SEM)和3D激光扫描显微镜对比研究了微孔形貌。结果表明,343 nm飞秒激光可以制备出效果最佳的微孔。此外,分析了3种激光与PU涂层的作用机理,揭示了343 nm飞秒激光合成革微钻孔过程仅表现为光化学烧蚀,光化学和光热烧蚀同时发生于1030 nm飞秒激光钻孔过程,而1064 nm纳秒激光只显示了光热烧蚀。激光合成革表面钻孔后,测量其透湿性和抗张力。结果显示: 微孔密度越大,皮革透湿性(WVP)越大而抗张力越低,脉冲重叠的增加会导致WVP的增加和抗张力的下降;同时,随着脉冲重叠从91.7%降到50%,微孔直径从45 μm降低到30 μm,而微孔锥度从0.7°增加到12.1°;当脉冲重叠率为91.7%,微孔密度为2550/cm2时,最大的WVP增长率为306%。     

双束延时飞秒激光调控制备二维亚波长周期阵列结构

利用偏振垂直的双束延时蓝色飞秒激光（400 nm）经柱透镜聚焦扫描钼表面,获得了二维矩形周期排列的方形和（椭）圆形阵列结构,以及六边形周期排列的三角形阵列结构。最小结构尺寸和周期达到100 nm和280 nm。研究发现双束飞秒激光的能量密度和能量比是不同类型结构形成及形貌转化的关键参数。通过优化激光参数,实现了圆形和三角形阵列结构的大面积均匀制备。两种周期排布的阵列结构的形成与双束延时飞秒激光与材料作用的超快动力学过程之间的瞬态关联作用以及作用过程中表面等离激元波的共线和非共线激发密切相关。该二维周期阵列结构加工方法及理论可拓展至其它硬质金属和半导体硅表面。     

纳秒激光单脉冲烧蚀硅表面的超快成像研究

利用ICCD可以在纳秒时间尺度下成像的特点,以飞秒准连续激光产生的超短脉冲光为探测光,对纳秒激光单脉冲烧蚀硅靶表面的演化过程进行动态监测。在能量密度为50J／cm^2时,捕获了纳秒单脉冲激光烧蚀硅靶面过程中等离子体演化的时间分辨图像。图像表明,纳秒激光烧蚀硅靶产生的等离子体开始时密度大,膨胀速度快,当纳秒激光脉冲过后,等离子体不再产生,并且其膨胀速度不再增加,直至完全消失。     

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。     

飞秒激光脉冲能量累积优化对黑硅表面形貌的影响

在黑硅表面制备的微结构可以使其获得多种表面功能,这些功能在太阳能、探测器等领域具有广泛的应用.因此,黑硅微结构的形成机理及制备条件优化一直是研究关注的焦点.本文的研究发现,随着激光辐照量(提高单脉冲能量或增加积累脉冲数)的增加会遇到形貌尺寸生长的瓶颈效应:过多的能量累积对微结构的优化和控制并没有进一步的作用.理论计算结果表明,产生这一现象的原因是前序飞秒激光脉冲诱导产生的微结构形貌对当前激光脉冲能量的吸收产生了调制,使当前激光脉冲的有效烧蚀效率降低.根据这一飞秒激光烧蚀规律,提出了一种优化表面形貌的新方案——在辐照激光总能量一定的条件下,通过改变激光能量的分配方式(单脉冲能量与脉冲数的组合)可实现表面形貌的优化.这一新的工作方式不但可以提高黑硅的制备效率,而且还有助于减少飞秒加工过程带来的表面缺陷及损伤,并降低加工过程中的能源消耗.这一研究成果对黑硅性能的进一步提升及其工程应用具有重要的意义.     

基于等离子体紫外辐射的强激光自动寻焦系统

精准定位激光束焦点位置是提高激光雕刻、切割、焊接等加工精度的重要基础，而传统测量方法不适用于自动寻找强激光焦点。基于激光烧蚀金属后等离子发光含大量紫外谱线的原理，以日盲的氮化镓肖特基光电二极管为传感器，设计了以304不锈钢靶材为耗材的红外强激光束自动寻焦方法及其装置。该方法与共聚焦显微镜检测平均烧蚀坑深的方法相比，当以脉冲宽度100 ns、重复工作频率20 kHz、平均功率10 W的1064 nm光纤激光雕刻机为实验对象时，二者定焦位置相差24 μm。     

飞秒激光烧蚀0Cr18Ni9不锈钢精度的影响因素

为了研究影响飞秒激光烧蚀0Cr18Ni9不锈钢精度的因素,采用飞秒激光对0Cr18Ni9不锈钢进行了切割和打孔实验。利用光学显微镜、光学金相显微镜等设备,对不锈钢烧蚀区形貌和切缝显微组织进行检测,基于烧蚀过程中CCD实时采集到的不锈钢表面的激光光斑图样,采用COMSOL Multiphysic数值模拟软件,模拟了烧蚀过程中激光束的发散传播行为,并计算了光束发散角。结果表明:当激光重复频率为5kHz时,厚度为160μm的0Cr18Ni9不锈钢切缝和孔边缘被明显烧黑,切缝处晶粒明显长大,存在热影响区;烧蚀过程中,由飞秒激光超高功率密度所致的金属-空气混合等离子体使光束沿传播方向上发生散射,发散角在6°～10°之间。热影响区的存在和混合等离子体的行为是影响飞秒激光烧蚀0Cr18Ni9不锈钢精度的主要因素。     

飞秒激光湿法刻蚀微纳制造

飞秒激光微加工作为一种新型微纳制造技术,在复杂三维构型制作方面具有其独特的优势,但激光加工效率问题严重制约了飞秒激光微加工技术走向实际工程应用,提出一种飞秒激光湿法刻蚀微纳制造方法,以提高飞秒激光微加工的效率为突破口,通过调控激光与物质相互作用获得材料的目标靶向改性,进而结合化学湿法刻蚀实现硬质材料上的高效和高精度三维微加工,采用这一方法制作出的微透镜尺寸为80 μm,球冠高67 μm,表面粗糙度小于10 nm。利用这种方法,实现了不同结构与特性的高质量微透镜阵列的超精密制备,在石英内部也实现了螺旋微通道的复杂三维结构,螺旋通道直径为20 μm,长径比超过100。     

飞秒激光双光束干涉高效率地制备硅表面超亲水结构

利用飞秒激光双光束干涉技术,结合柱透镜线扫描技术,在30 s内制备了面积为10 mm×10 mm的微米-纳米复合结构,极大地提升了激光加工效率。飞秒激光刻蚀后的硅表面包含双光束干涉引起的长周期微米结构,以及飞秒激光诱导的纳米周期结构。微米-纳米复合结构极大地提升了表面粗糙度,在毛细效应的作用下,硅在空气中显示出超亲水性,接触角从40°降为0°。测量烧蚀前后硅表面的X射线光电子能谱,发现激光加工后硅表面的Si-OH和H_2O分子的含量分别增加22.3%和13.6%,这进一步增强了硅表面的亲水性。随着激光照射功率的增加以及扫描速度的下降,硅表面接触角逐步下降。本文研究为高效制备大面积超亲水结构提供了新方法,在热传导、生物芯片等领域具有潜在的应用价值。     

飞秒激光光斑灰度与几何特征的相关性分析

飞秒激光烧蚀单晶硅的加工过程中，CCD相机采集的光斑图像中有效灰度区域与背景难以区分。为了解决这一问题，提出一种主成分分析与限制对比度自适应直方图均衡化相结合的方法，对光斑图像进行增强处理，再提出去趋势互相关分析法对光斑序列图像的灰度特征与几何特征进行长程相关性分析。结果表明，光斑序列图像的灰度特征与光斑纹理对比度、光斑面积具有较强的长程相关性，与纹理同质性表现出较强的反相关关系，可根据这3项与灰度特征较强的相关关系对后续微槽加工过程中的灰度特征进行预测。     

Alcohol-assisted photoetching of silicon carbide with a femtosecond laser

Femtosecond lasers have proved to be effective tools for micromachining silicon carbide material. In the drilling process, however, when the debris around the hole was not removed efficiently, the depth of hole would not increase further. In this paper, alcohol-assisted photoetching of 6H silicon carbide was investigated using a femtosecond laser. Machining in the presence of alcohol was beneficial to the debris ejection from the hole. The alcohol flow and volatilization was also helpful to further carry away the ablation debris and reduce the ablated material redeposition. The experiment showed that photoetching assisted by alcohol produced cleaner ablation effect and deeper hole than in ambient air. Moreover, alcohol assistance would not produce additional thermal damage around the hole. Vias were formed in a 250 μm thick wafer with alcohol-assisted photoetching technique using a femtosecond laser, which demonstrated the potential for this processing technique.     

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。     

Femtosecond laser drilling of crystalline and multicrystalline silicon for advanced solar cell fabrication

The rear contact solar cell concept has been implemented to increase the solar cell efficiency. Practically, it necessitates rapid fabrication of a large number of via holes to form low-loss current paths. It is not a trivial task to drill a number of microscopic holes through a typical Si wafer of ??200???m thickness at reasonable processing throughput and yield. In this research, a femtosecond laser is employed to drill via holes in both crystalline silicon (c-Si) and multicrystalline silicon (mc-Si) thin wafers of ??170???m thickness with various laser parameters such as number of laser shots and pulse energy. Since a significantly high pulse energy compared to ablation threshold is mainly applied, aiming to achieve a rapid drilling process, the femtosecond laser beam is subjected to complex non-linear characteristics. Therefore, the relative placement of the sample with respect to the laser focal position is also rigorously examined. While the non-linear effect at high pulse energy regime is complex, it also facilitates the drilling process in terms of achieving high-aspect ratio, for example, by extending the effective depth of focus by non-linear effect. Cross-sectional morphological analysis in conjunction with on-line emission and shadowgraph imaging are carried out in order to elucidate the drilling mechanism.     

Micro-ablation on silicon by femtosecond laser pulses focused with an axicon assisted with a lens

Micro-ablation of crystalline silicon was performed by irradiating a silicon substrate with femtosecond laser pulses of wavelengths 786 nm or 393 nm focused using a conical axicon assisted with a convex lens. Focusing the laser beam close to the tip of the axicon by means of the lens significantly improved the efficiency of concentration of laser pulse energy at the central spot of the resulting Bessel-Gaussian intensity distribution. As a result, micron-sized holes were formed with the diameter determined by the ablation threshold in the calculated fluence profile. It is possible to predict hole size from the laser pulse energy and the wavelength. Crystalline particles, a few tens of nanometers in size, were formed near the ablated zone.     

On the focusing limit of high-power femtosecond laser pulse propagation in air

Focused propagation of high-power femtosecond laser radiation in air is considered. Based on numerical solution of the nonlinear Schr?dinger equation for complex envelope of light wave electric field, evolution of the beam effective radius is studied. The dependence of the effective (rms) size of a focal spot and the maximally achievable intensity of laser radiation at focal waist on the initial pulse power is established. It is shown that focal spot of tightly focused intensive ultrashort laser radiation can change its size during the pulse passage through the beam waist. This is the consequence of pulse intensity clamping in region of beam focusing caused by gas photoionization and plasma producing. This may prevent laser intensity from its further growth in the focal region and arrest the transversal compression of the beam in the linear focus as a whole.     

Diffraction of a modulated laser beam in 3ω optics system

Based on diffraction optical theory, diffraction of a laser beam with periodic amplitude modulation and phase distortion is derived in 3ω optics system. Influence of defocus distance and focal length of a focusing lens on intensity distribution of diffraction light is investigated by numerical simulation. The results show that appropriate distance away from the focus spot and increase the focal length in final optical systems are beneficial to control the modulation of light intensity fluctuations and reduce the optical components damage caused by small-scale self-focusing effect.     

紧凑型大变倍比中波红外连续变焦光学系统设计

针对致冷型中波红外640×512凝视型焦平面探测器，设计了一个30×连续变焦光学系统。介绍了由无后固定组的变焦物镜组和中继透镜组组成的连续变焦系统的设计思路，不仅给出了系统在短焦、中焦、长焦3个位置的像质情况，还分析了反映全焦距范围内像质的离焦量和畸变。实验结果表明:该系统工作波段3.7 μm~4.8 μm，可以实现18 mm~540 mm连续变焦，全焦距范围内的离焦量都在焦深以内，长焦段最大畸变接近于0，短焦段最大畸变小于3%。该系统具有大变倍比、结构紧凑、变焦轨迹平滑、变焦行程短等优点，可用于红外光电探测和跟踪系统。     

Femtosecond pulsed laser ablation with spatial filtering

With the rise in demand for miniaturized features with better acute edge acuity and negligible thermal damage zone, one of the key vital areas lies in the refinement of the quality of the laser beam itself. Spatial filter is routinely used in optical micromachining systems to smoothen the Gaussian profile of the machining spot in order to obtain a feature of the desired quality. However, its profile smoothening effect has never been investigated for femtosecond pulsed laser micromachining process since the extremely high peak power of femtosecond pulses will cause damage on the filtering aperture of spatial filter. During the development of an acousto-optical micromachining system using femtosecond pulses, we found that if the damage of the filtering aperture can be circumvented, spatial filter can improve the machining quality of femtosecond pulse ablation, especially when ablation is conducted at low-fluency range (just above the ablation threshold fluency). In this paper, we investigate and demonstrate both the improvement and potential that beam refinement can bring about. In our experiment, a series of test patterns were ablated with a 400 nm second-harmonic Ti:Sapphire femtosecond laser of 150 fs duration at varying pulse energy ranging from 31 to 39 nJ. The specimen used in the experiment is a platinum- (Pt) sputtered coating of 100 nm thickness on a quartz substrate. The results show a significant improvement in the constancy of the shape as well as the size of ablated feature, revealing an improved beam profile and beam energy distribution due to spatial filtering.     

Removal of nanoparticles on silicon wafer using a self-channeled plasma filament

The effective removal of nanoparticles from a silicon wafer surface was demonstrated using the self-channeled plasma filament excited by a femtosecond (130?fs) Ti:sapphire laser (?? _p=790?nm). The photoinduced self-channeled plasma filament in air reached a length of approximately 110?C130?mm from the first focal spot with diameters ranging from 40 to 50???m at input intensities of more than 1.0×10¹⁴?W/cm². By the scan of wafer using the X?CY?CZ stage during self-channeled plasma filament, the removal variation of nanoparticles on surface was observed in situ before and after the plasma filament occurred. The cleaning efficiency was strongly dependent on the gap distance between the plasma filament and the surface. The removal efficiency of nanoparticles reached 96?% with no damage to the surface when the gap was 150???m.