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利用暗场条件下的泵浦探测成像技术,笔者所在课题组研究了激光损伤发生后在不同时间阶段的粒子喷射行为。如图2所示,是在完全相同参数下进行大量实验后获得的典型的图像。自左至右为泵浦激光辐照10 ns至150 ns后,由探测激光获得的瞬态图像,包含了不同时间延迟下粒子喷射初始并逐步发展的过程,以及在更长的时间尺度出现显著喷射现象。由此可知,激光损伤发生的前20 ns,并未发现明显的喷射粒子,主要原因是初始破坏时热力作用明显,将首先产生显著的材料蒸发和体材料破裂;随后在30~90 ns范围,随着体材料和团簇区喷出,喷射粒子因为速度的差异化而逐渐分离;在120~150 ns范围,喷射粒子特征非常明显,且这一过程将进一步持续,直至毫秒甚至更长时间尺度。
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在透射元件后面的不同位置放置粒子接收板,结合接收板上粒子的位置、与透射元件出射面的距离,可以计算出粒子的空间分布和喷射角度。近似认为粒子的喷射轨迹是以损伤破坏中心点为顶点的轴对称圆锥,根据公式(1)可以得出粒子接收板上每一个粒子的角度分布,其中d为粒子接收板到样品损伤点的垂直距离,即圆锥的高,d’为圆锥的底面半径,θ即为发散角。
$$ \theta ={\rm arctan}\left({d}'/d\right) $$ (1) 相同出射角度的粒子拥有类似的性质(因为每一个粒子质量极小,忽略重力因素的影响)。粒子的分布呈现对称圆形,且随着粒子接收板距离的不同,圆所覆盖的直径不同;粒子接收板离出射面越近,圆越小;粒子接收板离出射面越远,圆越大(粒子接收板最远距离为40 mm,当距离大于40 mm时,部分粒子已经无法飞跃到这个距离)。而比较不同损伤尺寸时,发现粒子分布的规律不随损伤凹坑尺寸的变化而变化,依旧呈现近似圆形分布。激光能量密度为300 J/cm2,接收板距离为25 mm,粒子分布示意图如图3(a)所示,大部分粒子主要集中在20°发散角内。
图 3 接收板上粒子的角度分布(a)和尺寸分布(b)特征
Figure 3. Angular (a) and size (b) distribution of particles on the receiving plate
图3(b)是粒子大小与喷射粒子个数之间的分布关系图。随着粒子尺寸的增大,粒子个数不断减小。大部分粒子主要集中在20 μm以内,存在极少数尺寸大于50 μm的粒子。
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图4为接收板设置于25 mm距离处,改变激光能量所得到的接收板上粒子分布情况。横坐标为距中心点之间的距离,每个数据点表示该点与前一个点之间的环形区域内的粒子个数。可以看到,随着能量的提高,接收到的粒子数更多,更多的粒子集中在距离中心9~10 mm的区域内,发散角大约20~22°范围内。
图 4 25 mm距离处不同激光能量辐照下粒子接收板上粒子的分布信息
Figure 4. Distribution of particles on the receiving plate at 25mm irradiated by different laser energies
图5是通过调节激光焦点位置,改变透射元件初始损伤的深度位置对接收板上的粒子分布的影响。图5(a)为正常的表层损伤,即激光焦点在元件与空气界面附近或在空气侧;图5(b)为深层损伤,将透镜的焦点前移至材料内部约30~50 μm位置,因焦深影响和损伤的不确定性,实际损伤的位置很可能在20~100 μm深度范围或更深。初始损伤发生在不同深度,其扩展过程和粒子出射特征存在差异。
图 5 当初始损伤位置在(a)样品表面(338 个粒子)和(b)内部时(424 个粒子)得到的接收粒子分布
Figure 5. Particle distribution when initial damage occurs (a) at the surface (338 particles) and (b) inside the material (424 particles)
对于正常的表层损伤,粒子喷射以前向为主,喷射方向相对集中,粒子分布在中心相对较多,整体在15°范围内;对于深层损伤,喷射方向相对发散,存在特定喷射方向,粒子分布在中心相对较少,但整体在20°内。
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图6为简易真空装置,结合机械泵可以实现低于百帕大气压强的真空环境模拟。如图7所示,1064 nm波长泵浦激光作用于透射元件出射面处会产生等离子体,等离子的强度会随真空度的改变而变化,其中图7(a)为正常大气压强。可以看到,喷发的等离子体分布于透射元件内外两侧,能量主要集中于空气一侧,随着真空度增加、大气气压降低,等离子体强度明显减弱。
图 7 等离子体喷发随真空度增加的变化(侧向喷射图)
Figure 7. Images of plasma eruption with increasing vacuum (lateral view)
图8给出了真空和大气环境下接收板上粒子的尺寸分布和喷射角度特征对比,能量密度300 J/cm2,接收板放置在透射元件出射面后方25 mm处。相比大气环境,在真空环境下接收板上收集到的喷射粒子数更多一些,而发散角分布基本一致,均在约20°以内。主要原因之一可能是,随着真空度降低,等离子体减弱,因此在喷射初期喷射粒子受到等离子体的影响减小。但是由于接收距离相对较短,且粒子的尺寸和分布主要由粒子喷射的初始状态决定,包括热力作用过程中冲击波和等离子体对材料的冲击、撕裂和蒸发等物理作用,喷射粒子具有的初始动能在喷射初期已获得,因此即使在真空环境下,喷射粒子的角度与尺寸分布并未有明显差异。
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前面讨论的都是正入射、输出面的透射元件损伤,除此之外,课题组还研究了45°入射时粒子的喷射特征。入射激光在材料内部因折射率的差异并不是正好45°入射到输出面,但会在SiO2-空气界面折射后以45°出射。图9(a)为出现丝状损伤时的粒子喷射图像。丝状的方向与激光入射方向一致,是表面损伤后形成的等离子体沿激光入射方向拓展而成。图9(b)给出了类似条件下的等离子体喷发图像。尽管等离子分布是长时间的积分强度,但其形貌和粒子喷发特征类似丝状沿线都残留了等离子体痕迹,在空气中等离子体分布同样沿法线对称,且出现多个小峰值,这与强烈的粒子喷射特征相对应,但等离子体强度由中心向周围衰减,而粒子喷射由中心向空间中发散。
Distribution characteristic of ejected particles from transmissive element induced by nanosecond laser
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摘要: 光学元件在高能量激光辐照下将发生激光损伤,损伤过程伴随着材料破裂、结构崩塌和粒子喷射,而向外喷出的粒子将影响周围光学元件的激光损伤性能。搭建了双光束泵浦探测成像系统,获得了激光诱导透射元件粒子喷射的瞬态图像,并根据粒子接收板的统计数据,获得了喷射粒子的空间分布、粒子尺寸和出射角度特征。同时,针对不同激光能量、不同的粒子接收距离以及真空度,对喷射粒子分布的影响因素进行了研究;此外,还结合能谱仪对激光诱导金属膜喷发后的分布规律、不同金属元素在大气与真空环境中的喷射行为进行了对比分析。实验结果表明:透射元件在激光作用下喷发出微米尺寸的粒子形态,金属膜的喷发主要以原子态或熔融液滴为主;粒子在最终接收板上的分布特征主要受初始条件的影响,而真空环境影响相对较小;不同薄膜制备工艺对金属膜的喷发特征影响较为明显。Abstract: Optical instrument will be damaged when irradiated by high-energy laser. The damage process is accompanied by material fracture, structural collapse and particle ejecting, and the particles emitted will affect the performance of surrounding optical instruments. In this paper, a dual-beam pump-probe imaging system was built to obtain a transient image of the particles ejected form transmissive element induced by nanosecond laser. A particle receiving plate was placed behind the sample, so that the spatial distribution of the particles as well as the size and angular characteristics were analyzed. Then, the effects of different laser energies, different particle receiving distances and vacuum environment on the particle distribution were studied. In addition, combined with EDS spectrum detection technique, the characteristic of particles induced in the metal film was investigated. Also, the differences between different kinds of metal film and the influence of the vacuum environment were studied. The results show that the transmissive substrate emits micron-sized particles under laser induction, while atomic state particles or molten droplets from metal film. The distribution characteristics of the particles on the final receiving plate are mainly affected by the initial conditions. There is little change in the vacuum within a few pascals, and different metal films are also significantly affected by their own properties.....
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