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与纳秒或者更长脉冲的激光加工过程相比,飞秒激光加工通常被称为非热加工过程。这使飞秒激光在材料加工中展现出了更加优异的性能,能够实现高精度、高质量的微纳制造。对于长脉冲激光(例如大于1 ns),材料被加热后有足够的时间在聚焦体积内熔化,并扩散到周围材料中。熔融材料的一部分蒸发并以高速微滴的形式喷射,其余熔融材料重新凝固,这导致材料的加工质量较差,形成热影响区、浮渣和微裂纹等缺陷。相比之下,对于飞秒激光来说,由于其脉冲时间极短,热扩散的长度通常比激光穿透长度小得多。因此,飞秒激光可以诱导材料快速电离,并将辐照体积区域的材料直接转化为等离子体、蒸汽和纳米液滴的混合物,随后进行喷射。而在加工过程中产生的材料融化和热扩散可忽略不计,从而可以实现高清洁度和高精密的加工质量[9]。图1表明了长脉冲激光和飞秒激光加工之间的明显差异[10]。图1(a)和1(b)为利用飞秒激光(200 fs)和纳秒激光(3.3 ns)在100 μm厚的钢箔上钻孔的扫描电子显微镜(SEM)照片。从图中可以看出,飞秒激光烧蚀产生的是一个具有锋利边缘、陡壁和较小热影响区的烧蚀孔。而纳秒激光烧蚀则在烧蚀孔周围产生了严重的溶胀。虽然飞秒激光可以减少加工区域热影响区的形成,但重复频率高于几百赫兹的辐照也会产生热积累现象,从而形成一个比激光光斑尺寸大得多的显著热影响区[11-12]。因此,较高重复频率的飞秒激光辐照有时会导致烧蚀质量恶化。
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当飞秒激光照射到介质和宽带隙晶体等透明材料上时,由于激光峰值强度极高,会产生非线性吸收过程,从而引发强烈的光吸收。图2展示了电子激发过程的单光子和多光子吸收[13]。如图2 (a)所示,当单个光子能量大于材料带隙时会被吸收,并使一个电子被激发到导带;相反,当光子能量小于带隙时则不能激发电子,所以没有吸收。然而,当极高密度的光子入射到材料上时,即使能量小于材料带隙,一个电子也可以被同时吸收的多个光子激发(图2(b)),这种现象被称为多光子吸收。多光子吸收通常只能发生在激光焦点附近,在这种情况下,飞秒激光脉冲与透明材料之间的相互作用只能发生在焦点附近,此处的峰值强度最高。基于这一特点,飞秒激光可以实现透明材料内部的加工和改性,这是传统激光难以做到的。由此看来,飞秒激光与材料的非线性吸收是其能够满足三维成型、材料内部加工等特殊制造手段的根本原因。
由于极高峰值强度和非线性多光子吸收的特点,飞秒激光能够在透明材料内部以空间选择的方式进行三维加工,这也是飞秒激光另一显著特点之一[14-15]。非线性吸收发生的过程与激光强度密切相关,只有当激光强度超过特定的临界值时,才能有效地诱导非线性吸收发生,而这个临界值取决于加工材料和脉冲宽度。当具有足够脉冲能量的飞秒激光束聚焦在透明材料内部时,非线性吸收局限于材料内部焦点附近的区域,此时激光强度超过临界值。利用飞秒激光这一独特特性,再结合适当的刻蚀、退火工艺,就可以制备各类微光学部件、微流控器件和信息存储器件。目前,该技术已被广泛应用于光子器件、三维光波导、生物芯片制造及多维光存储等领域。
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热影响区抑制和非线性多光子吸收的结合使得飞秒激光加工具有远超过衍射极限的空间分辨率。如1.1节所述,飞秒激光可以通过减少加工区域的热扩散来抑制热影响区的形成,由于热扩散可忽略不计,加工区域中的每一个像素几乎都与聚焦激光束的光斑大小相对应。光斑大小ω0由衍射极限决定(ω0=0.16λ/NA,其中λ是激光波长,NA为透镜的数值孔径)。因此,飞秒激光理论上可以进行不到半波长的空间分辨率的纳米制造。
利用非线性多光子吸收可以进一步提高空间分辨率。对于单光子吸收,其吸收激光能量的空间分布呈高斯分布。而多光子吸收的吸收能量分布要比单光子的分布更窄[16]。n光子吸收的有效光束尺寸ω可表示为:
$$ \omega = {\omega _0} / \sqrt n $$ 式中:ω0为聚焦激光束的实际光斑大小。由此可以看出多光子吸收可以克服激光束的衍射极限ω0,并达到亚衍射极限的分辨率。
此外,激光的阈值效应也可提升激光加工的分辨率。在包括烧蚀在内的很多加工过程中,激光强度存在一个阈值,只有超过此阈值,吸收光子能量才会发生反应。因此可以通过调节激光脉冲能量,使脉冲激光的能量尽量逼近阈值强度窗口,从而将加工尺寸减小到小于激光光斑尺寸,进一步提高加工精度。理论上,阈值效应对加工分辨率没有限制,通过对激光功率的稳定和控制,可以利用780 nm的飞秒激光获得比20 nm更好的分辨率[17]。但在实际应用中,飞秒激光输出功率的波动使得高分辨率的三维制造变得很困难,通常能够实现的空间分辨率为100~200 nm[18]。
Progress and application of nonlinear laser manufacturing (Invited)
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摘要: 超快激光是指脉冲宽度极窄的激光,其瞬时功率极高,与物质之间的相互作用呈现出非线性、非平衡、多尺度的状态。超快激光具有超快(脉冲持续时间短)、超强(瞬时功率高)、超精细(加工结构精细)等特点,由此实现的非线性激光制造技术可以打破传统微纳制造的局限,实现各类难加工材料和复杂微纳结构的超精细制造,精度可达亚微米至纳米量级,在微光学、生物医学、智能电子器件等前沿领域体现出了独特的应用价值。文中主要聚焦飞秒激光微纳加工技术前沿,简要概括了飞秒激光加工的特点;介绍了飞秒激光加工的主要技术手段,包括飞秒激光直写和飞秒激光并行加工;讨论了飞秒激光加工技术的前沿应用领域,如微纳光学器件、微流体器件、多功能结构化表面、生物医学工程等;最后,对飞秒激光加工制造技术未来的发展趋势和研究方向进行展望。Abstract: Ultrafast lasers refer to lasers with very narrow widths. Ultrafast laser has extremely high instantaneous power, and its interaction with matter presents a non-linear, nonequilibrium and multi-scale state. With the unique characteristics of ultrafast(short pulse duration), ultrahigh (high instantaneous power) and ultrafine (fine processing structure), the realized nonlinear laser manufacturing technology can break the limitations of traditional micro-nano manufacturing and realize the ultrafine processing of various difficult-to-machine materials and complex micronano structures. It demonstrates unique application value in frontier fields such as microoptics, biomedicine, and intelligent electronic devices with precision ranging from sub-micron to nanoscale. The frontiers of femtosecond laser micro/nano machining technology was focused on. Firstly, the processing characteristics of femtosecond laser technology and the main processing methods were introduced, including femtosecond laser direct writing and femtosecond laser parallel processing. Then, the frontier application fields of femtosecond laser processing technology were discussed, such as micro-nano optical devices, microfluidic devices, multifunctional structured surfaces and biomedical engineering. Finally, the future development trend and research direction of femtosecond laser processing technology were prospected.
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Key words:
- ultrafast laser /
- femtosecond laser /
- nonlinear interaction /
- micro-nano manufacturing /
- 3D fabrication
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