表面亚波长周期结构的飞秒激光调控制备 
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1 引言
激光诱导周期表面结构(LIPSS)是光与物质相互作用产生的一种普遍物理现象,它起源于1965年Birnbaum等[1]采用纳秒脉冲激光在半导体锗表面诱导产生的一组周期性排列平行刻槽(而非烧蚀凹坑),后来被称为“条纹结构”。在早期的相关研究中,人们大都采用长脉冲激光作为光源,所形成的结构周期通常为波长量级[2-9]。随着飞秒激光器的普及化发展,人们开始重新关注这种表面周期结构的产生。不同于长脉冲激光作用结果,飞秒激光诱导的表面条纹结构周期通常为亚波长量级,即小于入射激光波长λ[10-25]。根据表面结构周期Λ和排列方向与入射激光波长和偏振方向的相对关系,可将其分为以下类型:1)低空间频率的垂直条纹结构(其中λ>Λ>λ/2)[12-20],其排列方向与入射光偏振相垂直;2)低空间频率的平行条纹结构(其中λ>Λ>λ/2)[16,20-21],其排列方向与入射光偏振相平行;3)高空间频率的垂直条纹结构(其中Λ<λ/2)[15-20,23],其排列方向与入射光偏振相垂直;4)高空间频率的平行条纹结构(其中Λ<λ/2)[16,24-25],其排列方向与入射光偏振相平行。实验表明,飞秒激光在金属和半导体等强吸收性固体表面容易产生第1类条纹结构,而在透明介质表面容易产生第2类条纹结构;当飞秒激光通量较低且脉冲累积数目较高时,介质和半导体表面容易产生第3类条纹结构,而金属表面容易产生第4类条纹结构。
材料表面亚波长条纹结构周期和排列方向可通过改变入射激光的波长、脉冲重叠数目、脉冲能量、偏振态和加工环境等参数获得改变[26-41]。前期研究证实,条纹结构周期随入射激光波长增大而增加[26-27],随脉冲累积数目增加而减小[28-31]。当激光通量增加时,亚波长条纹结构周期在铜、钨、镍钛合金、硅、石英等材料表面表现出了增加趋势[25,30,32-34],而在金属钛表面表现出了减小趋势[35]。另外,高折射率加工环境有助于减小条纹结构周期[36-38]。通常情况下,一维亚波长周期条纹结构排列方向与入射激光的线偏振方向垂直或平行[39-41]。
关于激光诱导周期条纹结构形成的物理机制是人们不断探索的一个重要科学问题。起初,Birnbaum等[1]认为它是由聚焦透镜衍射引起的激光选择性烧蚀而形成。随后Emmomy等[2]将其归因为入射激光和表面散射波相互干涉导致激光能量在材料表面的周期性沉积而形成,且结构周期依赖于激光波长和入射角度θ,即Λ=λ/(1±sinθ)。1982年,Keilmann等[3]提出了入射光与表面波干涉是这种结构形成的根本原因。Sipe等[4]认为表面散射光与折射光的干涉作用导致激光能量在材料表面不均匀分布,从而形成周期条纹结构。该模型定义一个“初始表面粗糙度”参数,并引入效率因子η来定量描述激光能量在材料表面的不均匀沉积,预测了材料表面可能形成条纹结构的倒格矢(k)方向,它在一定程度上能够合理解释长脉冲激光诱导产生的大周期条纹结构,但却无法深刻描述飞秒激光诱导亚波长周期条纹结构现象。后来,人们相继提出了多种理论来解释亚波长周期条纹结构的形成机理[42-53],其中,入射激光与等离激元波干涉被广泛采用[42-46]。
在激光垂直入射条件下,周期条纹结构的空间周期约等于表面等离激元波的波长,即
实验中,亚波长周期条纹结构的产生通常是基于多个飞秒激光脉冲累积照射结果,而上述模型未曾考虑多脉冲与材料作用的反馈机制[54]。在亚波长周期条纹结构产生的物理过程中,最初入射的飞秒激光与材料表面作用不仅产生随机分布的纳米结构,而且可能造成材料表面物化性质的变化,这将有助于后续飞秒激光激发表面等离激元波产生,从而导致激光能量在空间上的周期性分布并对材料表面烧蚀去除形成条纹结构雏形。当飞秒脉冲累积数目继续增加时,这些结构雏形将会提高入射激光与表面等离激元波的耦合效率,使得激光能量的空间周期性烧蚀效果增强。正是基于多脉冲作用的正反馈机制,共振激发的表面等离激元波与入射激光干涉最终导致亚波长周期条纹结构的产生。由于单束飞秒激光脉冲的时间间隔通常为1 μs~1 ms,脉冲作用之间的相互影响实际上是静态的,硬质的和不可调控的。
相对于脉冲持续时间来说,飞秒激光在材料表面诱导形成亚波长周期条纹结构是一个极其漫长的动力学过程,其中包含诸多瞬态物理阶段,例如:电子吸收激光能量后的热化、热电子与冷晶格之间的能量传递、材料表面熔化的热动力学、材料表面冷却和凝固等[46]。这些均将影响激光能量在材料表面的周期性吸收和烧蚀去除,从而使结构形成过程变得异常复杂。最近,人们提出利用飞秒激光泵浦-探测实验来研究材料表面周期条纹结构形成的超快动力学过程[55-58]。例如,Hohm等[55]研究了SiO2表面亚波长周期条纹结构形成的动态过程,通过测量其中衍射光强随探测时间的变化曲线,揭示了材料表面在入射光照后Dt=0.3~100 ps时间范围内形成瞬态折射率光栅的物理过程。Cheng等[58]采用飞秒激光泵浦-探测显微成像技术研究了金膜表面亚波长周期条纹结构的形成过程,直接观测到了材料表面在入射激光照射后Dt=80~800 ps延迟时间范围内出现条纹结构现象。事实上,在传统的泵浦-探测技术实验中,由于延时入射激光仅被用作探测信号,因此其强度必须非常微弱才能保证对材料激发过程无影响。相反,如果探测光强足够大,则其入射会对泵浦激光导致的瞬态物理过程形成干扰,从而实现对表面周期结构形成的调制作用,调制效果与探测激光入射的时间阶段密切相关。为此,人们提出了双束飞秒激光调控制备技术[59-63],其中先入射激光引发的材料瞬态物理过程对滞后入射激光作用的影响是动态的,软质的和可灵活调控的。为此,Hohm等[59-60]利用该技术分析研究了熔融石英、硅、钛等材料表面亚波长周期条纹结构的形成情况。Jiang等[61-62]通过改变双束激光的延迟时间(Dt=0~1 ps时),实现了亚波长条纹结构从低空间频率(Λ=550 nm)向高空间频率(Λ=255 nm)的转化。
由于亚波长周期条纹结构具有改变材料表面物理和化学性能的本领,因此在各个领域都具有广泛应用潜能。例如:作为衍射光栅能够产生结构色,可应用于激光打标、光学数据存储、防伪、加密、显示等领域[64-66];能够改变材料表面的浸润性,可应用于自清洁、防水、防冰、防腐、微流体等领域[67-71];能够增强材料表面对光吸收、透射和热辐射效率,可广泛应用于太阳能电池、照明LED光源、集成电路、平板显示等领域[72-74];能够产生表面增强拉曼散射效应,可用来高灵敏探测微量分子、生物病原体以及病毒[75-77];能够使材料表现出双折射性能,可用于控制透射光的能量和偏振,制作波片、矢量光束转化器、光学涡旋产生器等微纳光学器件[78-82];能够增加材料表面生物相容性,可应用于整形外科钛移植等[83-86]。
然而,现阶段飞秒激光在制备表面周期结构方面仍然存在诸多问题。例如:表面结构形貌大多表现为一维光栅状分布,且容易出现空间弯曲、分叉和断裂等现象;其形成超快动力学过程和内在机理仍不十分清楚;采用光束聚焦方式导致激光作用区域较小,大面积制备耗时低效。另外,尽管二维周期结构可通过采用飞秒激光交叉扫描或者多束激光干涉法来获得[87-88],但其制备过程比较复杂,且形成的结构周期和单元尺寸多为微米量级。这些均严重制约了表面周期结构的快速发展和应用。因此,如何大面积快速制备高规整、高精度、多形状的表面结构是目前飞秒激光微纳加工领域面临的一个重要挑战。
2 提高亚波长周期条纹结构分布规整性的方法
由于飞秒激光诱导亚波长周期条纹结构与入射激光参数、材料性质以及加工环境密切相关,因此人们尝试从这三个方面来提高结构分布的均匀和规整性。
2.1 激光参数的优化
最近,Ruiz等[89]采用单束高重复频率线偏振飞秒激光(1030 nm, 500 fs, 1 MHz)并沿与激光偏振相垂直的方向移动扫描,在1 μm厚度的金属铬膜表面上实现了大面积高规整性一维周期条纹结构的制备,相应的扫描电子显微(SEM)结果如
2.2 材料性质的选择
最近,Gnilitskyi等[91]将单束高重复频率线偏振飞秒激光(1030 nm,213 fs,600 kHz)通过扫描阵镜系统聚焦照射在钼膜、钛膜、金膜、铜、铝和不锈钢等多种金属材料上,分析比较了其中亚波长周期条纹结构的产生效果。研究表明,高规整性条纹结构仅在钼膜、钛膜和不锈钢三种金属表面得以实现,结构周期分别为Λ=845,737,901 nm,排列方向均与入射激光偏振相垂直,结果如
图 1. 单束线偏振飞秒激光在不同金属表面诱导产生的一维高规整性周期条纹结构。(a) 1 μm厚铬膜[89];(b) 50 nm厚钛膜[90];(c) 300 nm厚钼膜[91]
Fig. 1. High-regular one-dimensional periodic ripple structures obtained on several metal surfaces using linearly polarized single-beam femtosecond laser incidence. (a) 1 μm chromium film[89]; (b) 50 nm titanium film[90]; (c) 300 nm molybdenum film[91]
2.3 加工环境的改变
最近,本课题组的Wang等[92]在高真空环境下利用单束线偏振飞秒激光(800 nm,50 fs,1 kHz)经透镜聚焦在25 nm厚度的金属铬膜上,来提高表面亚波长周期条纹结构形成的规整性。研究结果表明,在给定光照和扫描条件下,当真空腔中的空气压强降低至P=1.0×10-4 Pa时,铬膜表面出现高规整性的平行沟槽结构,其中排列方向与入射激光偏振相垂直,结构周期为Λ=360 nm,刻槽宽度为w=150 nm,深度为h=120 nm,如
图 2. 单束飞秒激光在1.0×10-4 Pa真空环境下在25 nm厚度铬膜表面产生的一维高规整性周期条纹结构[92]。 (a)采用透镜聚焦;(b)采用柱透镜聚焦
Fig. 2. Highly regular one-dimensional periodic ripple structures on 25 nm Cr film by linearly polarized single-beam femtosecond laser beam under vacuum condition of 1.0×10-4 Pa[92]. (a) Focusing with a convex lens; (b) focusing with a cylindrical lens
2.4 时间延迟双光束作用
最近,本课题组的郑昕等基于延迟时间可调的双束飞秒激光,提出利用调控材料表面瞬间非平衡态物理特性来提高亚波长周期条纹结构的规整性。具体实验过程为:首先将从钛宝石激光器输出的单束线偏振飞秒激光(800 nm,50 fs,1 kHz)经过钒酸钇(YVO4)双折射晶体后,获得空间上共线传输、偏振方向相互垂直、并具有特定延迟时间(Dt=1.2 ps)的双束飞秒激光,然后将其通过柱透镜聚焦在金属钨表面,制备获得了高规整性分布的一维亚波长周期条纹结构。研究发现,当双束飞秒激光的能量比值接近R=1∶3或3∶1时,则在光照区域内容易形成大面积的高规整性条纹结构,如
图 3. 双束延时飞秒激光在金属钨表面产生的大面积高规整性周期条纹结构。(a)结构形貌;(b)结构方向角色散量δθ
Fig. 3. Large-area high-regular one-dimensional periodic ripple structures formed on tungsten surface using temporally delayed double femtosecond laser beams with orthogonal polarizations. (a) Structural morphology; (b) calcualted dispersion δθ in the structure orientation angle
3 共线延时飞激光对亚波长周期结构空间周期的动态调控
本课题组的王瑞平[93]最近通过实验研究发现,单束蓝色飞秒激光(400 nm,50 fs,1kHz)在金属钼表面制备的一维条纹结构容易产生空间分裂和断裂现象,从而严重影响了结构的整体规整性。为了解决该问题,作者采用共线传输、偏振方向平行、延迟时间可调和具有不同中心波长的双束飞秒激光(400 nm、800 nm,50 fs,1 kHz)聚焦照射在金属钼表面,其中强度低于材料烧蚀阈值的近红外(λ=800 nm)飞秒激光先入射在样品表面。实验结果发现,在双束飞秒激光延迟时间为Dt=10 ps时,金属钼表面可以形成规整的条纹结构分布,相应的结构周期为Λ=280 nm,排列方向与激光偏振相垂直,如
图 4. 双色延时飞秒激光在金属钼表面诱导产生的亚波长周期条纹结构[93]。(a) Dt=10 ps和(b) Dt=100 ps时的结构形貌;(c)结构周期随双束激光延迟时间的变化关系
Fig. 4. One-dimensional subwavelength periodic ripple structures formed on Molybdenum surface using temporally delayed double femtosecond laser beams with different wavelengths[93]. (a)(b) Structural morphology at the time delay of Dt=10 ps andDt=100 ps, respectively; (c) variation of the structure period as function of the time delay between double laser beams
4 共线延时飞秒激光对亚波长周期结构方向的动态调控
在单束飞秒激光作用的实验中,虽然亚波长周期结构的排列方向可以通过入射激光偏振方向的改变获得调协,但这种方法实际上是基于材料“稳态”情况而操作,因此根本没有涉及其中的超快动力学过程,从而无法实现对结构形成形貌及其他特征的有效改善。接下来,将重点介绍利用共线延时双束和三束飞秒激光在材料“超快非平衡态”情况下,对亚波长周期条纹结构排列方向进行灵活调控的研究进展,并且揭示其中的新现象,新机理。
4.1 对金属表面周期结构方向的动态调控
Bonse等[54,94]实验研究了偏振方向垂直的双束飞秒激光在同时照射(Dt=0 ps)情况下,在金属钛材料表面诱导亚波长周期条纹结构的情况,结果表明,此时形成结构方向实际上是由双束飞秒激光的相干叠加作用来决定,并随双束飞秒激光能量比发生改变。最近,本课题组的赵波[95]采用能量相同、共线传输和不同线偏振的双束飞秒激光(800 nm, 50 fs, 1 kHz)经物镜聚焦照射在单晶铜表面,分析研究了周期条纹结构排列方向在双束激光延迟时间Dt=0~60 ps范围的动态演化过程[95]。在双束激光线偏振方向夹角为θ=45°的情况下,实验观测到了Dt=0 ps时表面周期条纹结构的形成情况,如
实验中通过逐步增加双束激光的延迟时间,获得了表面周期结构方向倾斜角随延迟时间的演变过程,结果如
此外,作者还研究了双束激光的线偏振方向夹角和脉冲时间宽度对亚波长周期条纹结构方向倾斜随延迟时间演变过程的影响。当双束激光偏振夹角从θ=70°减至θ=30°时,结构方向倾斜角均出现类似的周期振荡和单调衰减行为,但在延迟时间Dt<12 ps范围内,结构方向倾斜角的振荡幅度从Dα=14°减至Dα=7°,振荡频率在f=0.48~0.6 THz范围内变化;在延迟时间Dt>12 ps范围内,结构方向倾斜角的衰减速率随偏振夹角增大而变快;当延迟时间大于Dt=40 ps时,结构方向倾斜角全部趋近于零。在双束激光偏振夹角为θ=45°情况下,当脉冲时间宽度增加至τ=1 ps时,结构方向倾斜角仍然出现了周期振荡和单调衰减行为,但此时其振荡频率和幅值分别减至Dα=5°和f=0.3 THz;若脉冲时间宽度继续增加至τ≥10 ps,则结构方向倾斜角无振荡行为出现,仅发现其随延迟时间增加的单调衰减行为。相关的理论分析认为,这种结构方向倾斜的振荡行为是由先入射飞秒激光在金属表面激发非平衡物理过程,其中包括瞬态折射率光栅、相干声学声子和晶格硬化等效应,然后对滞后入射飞秒激光非共线激发的表面等离激元波进行调控而产生。该研究不仅提供了一种诊断记录飞秒激光作用超快动力学过程的新方法,而且为有效调控微纳米结构制备提供了新思路[95]。
图 5. 双束飞秒激光在单晶铜表面诱导亚波长周期条纹结构随延迟时间的演变过程[95],其中两束激光偏振方向的夹角为θ=45°。(a)在零延时情况的结构形貌;(b)结构方向倾斜角随延迟时间的变化曲线
Fig. 5. Temporal evolution of the subwavelength periodic ripple structures formed on copper surface using temporally delayed double femtosecond laser beams with the polarization intersection angle of θ=45°[95]. (a) Structural morphology at zero time delay incidence; (b) measured time-delay dependent slantwise orientation angle of the structures
4.2 对半导体表面周期结构方向的动态调控
类似于单晶铜情况,本课题组的He等[96-97]采用不同线偏振方向的双束飞秒激光(800 nm,50 fs,1 kHz)聚焦在半导体4H-SiC表面,分析研究了亚波长周期条纹结构方向随延迟时间的演变过程。
图 6. 偏振夹角为θ=30°的双束延时飞秒激光在4H-SiC表面诱导亚波长周期条纹结构随延迟时间的演化过程[96-97]。(a)在零延迟照射时结构形貌;(b)条纹倾斜角随延迟时间的变化曲线
Fig. 6. Temporal evolution of the subwavelength periodic ripple structures formed on 4H-SiC surface using temporally delayed double femtosecond laser beams with the polarization intersection angle of θ=30°[96-97]. (a) Structural morphology at zero time delay incidence; (b) measured time-delay dependent slantwise orientation angle of the structures
随后,作者分析认为上述表面结构方向的改变行为实质上是来源于先入射飞秒激光在材料表面激发的瞬态折射率光栅,对滞后入射飞秒激光非共线激发表面等离激元波的调控作用。但由于金属与半导体材料在性质上存在区别,因此飞秒激光作用过程中的超快物理现象也不尽相同。对于4H-SiC材料而言,当延迟时间为Dt= 0~20 ps时,材料中Auger复合效应的存在使得先入射飞秒激光激发的载流子浓度快速减小,从而导致其引发的瞬态折射率光栅效应急剧减弱,并对滞后入射飞秒激光非共线激发表面等离激元波的调控作用也快速单调变化,最终使得结构方向的倾斜角出现快速单调减小行为。在延迟时间Dt>20 ps情况下,材料中Auger复合效应消失和4H-SiC材料较小热导率将会使得瞬态折射率光栅的热弛豫衰减过程变缓,并对滞后入射激光非共线激发表面等离激元波的调控行为变得微弱,从而导致结构方向倾斜角维持在一个非零值附近。
不仅如此,作者还采用偏振方向不同的三束延时飞秒激光,分析研究了4H-SiC材料表面周期结构产生情况[98]。其中三束飞秒激光线偏振方向互不相同,它们之间的夹角分别为θ1=θ2=30°,每两束激光之间的延迟时间分别为Δt1=10 ps和Δt2=42 ps。典型实验结果如
图 7. 偏振夹角为
Fig. 7. Temporal evolution of the subwavelength periodic ripple structures formed on 4H-SiC surface using temporally delayed three femtosecond laser beams with the polarization intersection angles of θ1=θ2=30°[98]. (a) Structural morphology at Δt1=10 ps and Δt2=42 ps; (b)(c) measured time-delay dependent slantwise orientation angle of the structures at Δt1=10 ps and
5 共线延时飞秒激光制备多类型二维亚波长周期阵列结构
目前,金属表面二维周期微结构可利用飞秒激光交叉直写和多束干涉等方法来获得[87-88],但通常情况下这些结构周期和单元尺寸为微米量级,难以满足纳米光子器件的制备和应用需求。本小节将重点介绍本课题组在利用延时飞秒激光调控制备多类型二维亚波长周期阵列结构方面取得的研究进展。
5.1 在金属表面调控制备二维亚波长圆包、三角形和菱形周期阵列结构
实验中,将从钛宝石激光器输出的单束飞秒激光(1 kHz, 800 nm, 50 fs)通过钒酸钇双折射晶体后,产生偏振方向相互垂直且有特定延迟时间(Dt=1.2 ps)的双束共线传输飞秒激光,它们经柱透镜聚焦后垂直照射在块体金属钨表面。当入射激光总能量为E=0.21 mJ,能量比为R=1∶3,扫描速度为v=0.03 mm/s时,制备获得了高规整分布的二维圆包状周期阵列结构[99-100],如
另外,实验中如果将样品位置移至光束焦点前0.2 mm处,并在延迟时间Dt=1.2 ps、总能量E=0.18 mJ、能量比R=1∶1的条件下,可制备获得二维分布的高规整三角形周期阵列结构[99,101],如
图 8. 偏振方向垂直的双束延时飞秒激光在金属钨表面形成的二维周期阵列结构。(a)圆包状[100];(b)三角形[101];(c)菱形[99]
Fig. 8. Various types of two-dimensional periodic structure arrays on tungsten surface using temporally delayed double femtosecond laser beams with orthogonal polarizations. (a) Spherical cap[100]; (b) triangular[101]; (c) rhombus[99]
通过总结分析,可以确定双束飞秒激光的能量密度和能量比是造成材料表面阵列结构形貌不同的主要因素。不同构型结构的形成均与双束飞秒激光激发材料超快动力学过程之间的关联耦合作用密切相关。二维圆包状周期阵列结构是由双束飞秒激光在材料表面各自激发瞬态折射率(温度)光栅并随后发生关联而形成。而在二维三角形周期阵列结构形成过程中,在先入射飞秒激光激发的瞬态折射率光栅调制作用下,滞后入射的飞秒激光通过非共线激发方式产生两组新的表面等离激元波,从而形成三角形周期阵列结构。
5.2 在金属表面调控制备二维亚波长椭圆状周期阵列结构
最近,本课题组的Cong等[102]在实验上采用两束偏振方向垂直和中心波长不相同的飞秒激光(800 nm,400 nm,50 fs,1 kHz)共线延时聚焦照射,在块体金属钼表面获得了高规整性二维椭圆状周期阵列结构[102],如
图 9. 偏振垂直的双色延时飞秒激光在金属钼表面诱导形成的二维椭圆状周期阵列结构[102]。(a)结构形貌;结构周期在(b)水平和(c)垂直方向上随延迟时间的变化曲线
Fig. 9. Two-dimensional elliptical-shaped periodic structure arrays formed on molybdenum surface using temporally delayed two-color femtosecond laser beams with orthogonal polarizations[102]. (a) Surface morphology; (b)(c) measured time-delay dependent structure period in the vertical and horizontal directions, respectively
5.3 在金属表面调控制备二维亚波长条纹-颗粒复合结构
最近,本课题组的秦婉婉等人采用单束线偏振的蓝色飞秒激光(400 nm,50 fs,1 kHz)经物镜聚焦垂直照射在单晶铜表面,观测到了二维亚波长的条纹-颗粒复合结构[103],如
激光滞后入射。在时间延迟为Dt=28 ps、激光通量F800=0.04 J/cm2和F400 = 0.1 J/cm2条件下,获得了如
图 10. 单束和双色延时飞秒激光在金属铜表面诱导形成的二维亚波长条纹-纳米颗粒复合结构[103]。(a)单束蓝色飞秒激光作用结果;(b)双色延时飞秒激光作用结果;(c)~(f)沟槽宽度w、颗粒直径d、颗粒周期Λp、颗粒周期与直径比值Λp等参数随先入射激光通量的变化曲线。
Fig. 10. Two-dimensional subwavelength ripple-particle hybrid structures formed on the copper surfaces using different femtosecond laser
6 结束语
本文重点论述了飞秒激光诱导材料表面亚波长周期表面结构过程中形貌规整性差、结构单一、调控手段不灵活、制备效率低等问题的解决方法。在单束飞秒激光照射情况下,通过设置扫描方向、限制光斑尺寸、选择特性材料和引入高真空加工环境等方法显著提高了一维亚波长周期条纹结构在金属薄膜表面形成的规整性;在偏振垂直的双束延时飞秒激光照射情况下,通过严格控制激光脉冲能量比在块体金属钨表面制备获得前所未有的高规整一维亚波长周期条纹结构;在偏振平行的双色飞秒激光照射下,通过改变延迟时间实现了金属钼表面一维亚波长周期条纹结构周期在高和低空间频率内的转化;利用偏振垂直的双束延时飞秒激光在金属钨表面制备获得了具有不同形貌特征(圆包状、三角形、菱形)二维亚波长周期阵列结构,并通过改变激光能量密度和能量配比实现了结构形貌的互相转化;同时利用双色延时飞秒激光分别在金属钼和铜表面制备形成了二维亚波长椭圆周期阵列和条纹-纳米颗粒复合结构,并实现了对其分布周期、单元尺寸等结构参数的灵活调控;另外,在利用共线延时飞秒激光束调控制备周期表面结构的同时,还发现了材料中的晶格硬化、非热声学声子激发、瞬态折射率形成、表面等离激元波非共线激发、Plateau-Rayleigh不稳定性等一系列超快物理现象。目前采用柱透镜聚焦方式将飞秒激光诱导周期表面结构的制备效率提高到了接近工业化生产水平。
总之,尽管说飞秒激光诱导亚波长周期表面结构在形貌特征、排列分布、空间周期和制备效率等方面已经获得了一定程度的控制产生,但相应的超快动力学过程和物理机制仍缺乏统一认识。特别是,当采用皮秒时间延迟多束飞秒激光照射时,如何全面和深刻理解其中多个光-物质作用动态过程之间的关联与耦合,包括表面等离激元在金属非平衡状态下的激发与调控,及其后续能量弛豫对表面微纳结构产生的影响等,均是未来需要深入探索和解决的科学问题。另一方面,如何利用飞秒激光在金属表面实现微纳结构的多维度、多类型、高效率和高质量构建,仍然是这一研究领域有待解决的关键技术问题。我们相信针对上述问题研究呈现出的新现象和新规律不仅将会极大丰富激光与物质作用的研究体系,而且也将有助于解决当前普遍存在的加工效率和加工精度之间的固有矛盾,从而引发人们对飞秒激光微纳制造手段之精彩、发展潜力之深远的惊叹!
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Article Outline
赵波, 郑昕, 邹婷婷, 谢洪波, 辛巍, 杨建军, 郭春雷. 表面亚波长周期结构的飞秒激光调控制备[J]. 激光与光电子学进展, 2020, 57(11): 111404. Bo Zhao, Xin Zheng, Tingting Zou, Hongbo Xie, Wei Xin, Jianjun Yang, Chunlei Guo. Control of Subwavelength Periodic Surface Structure Formation with Femtosecond Laser Pulses[J]. Laser & Optoelectronics Progress, 2020, 57(11): 111404.
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