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飞秒激光烧蚀石英玻璃等透明电介质材料是一种非线性电离机制[26-27],主要表现为雪崩电离[28-29]和多光子电离[30-31],电介质材料中的自由电子主要通过上述两种机制来吸收入射激光能量,当材料导带中的自由电子密度超出其临界自由电子密度时,便会造成电介质材料的损伤, 从而达到去除的目的。
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玻璃材料内种子自由电子的数量极少,但这种种子自由电子可以连续地吸收入射激光光子能量,从而具备特定的动能,这属于自由载流子吸收,如图 1a所示。当种子自由电子的动能超出电介质材料的禁带宽度时,材料中的束缚电子会与种子自由电子产生碰撞而被电离到导带,这属于碰撞电离,如图 1b所示。在激光能量较高的情况下,通过碰撞电离产生的新种子自由电子将会持续碰撞材料内部的其它束缚电子,这会激发出数量更多的自由电子,并使其数目表现出指数式的增长规律,这属于雪崩电离[32-34]。
为准确描述雪崩电离机制,一般采用Thornber雪崩电离系数[35-36]α(E)来描述碰撞电离速率与电场强度之间的变化规律:
$ \alpha (E) = \frac{{eE}}{\mathit{\Delta }}\exp \left[ { - \frac{{{E_{\rm{i}}}}}{{E\left( {1 + E/{E_{\rm{p}}}} \right) + {E_{\rm{t}}}}}} \right] $
(1) 式中, E为电场强度,Δ为禁带宽度,e为电子电荷,Et为载流子克服热散射所需的场强,Ep为克服光学声子散射所需的场强,Ei为克服电离散射所需的场强。
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石英玻璃的禁带宽度一般比超短脉冲激光的单光子能量大,使得石英玻璃等电介质材料对入射激光能量的吸收效率较低,进而导致这种材料较难吸收单个光子能量。在超短脉冲激光的能量密度以及光子简并度较高的条件下,材料中的束缚电子同时吸收多个光子能量的效率将显著增加。当束缚电子在吸收激光能量后所具有的动能超出电介质的禁带宽度时,其会被激发到导带成为自由电子,这属于多光子电离[32-34],如图 2所示。其中Ephoton表示入射光子能量。
为准确表征多光子电离机制,一般采用Keldysh光致电离系数[37]WPI(E)来描述其与电场强度之间的变化规律:
$ \begin{array}{c} {W_{{\rm{PI}}}}(E) = \frac{2}{{9\pi }}\omega {\left( {\frac{{m\omega }}{\hbar }} \right)^{3/2}} {\mathit{\Phi }} \left( {{{\left( {2N - 2\frac{\mathit{\Delta }}{{\hbar \omega }}} \right)}^{3/2}}} \right) \times \\ \exp \left( {2N\left| {1 - \frac{{{e^2}{E^2}}}{{4m{\omega ^2}U}}} \right|} \right){\left( {\frac{{{e^2}{E^2}}}{{16m{\omega ^2}U}}} \right)^N} \end{array} $
(2) 式中, ω为激光频率,ħ为普朗克常数,m为空穴对有效质量,N为一个束缚电子被电离时所吸收的光子数目,Φ(·)为Dawson积分,U=Δ-e2E2/(4mω2)。
多光子电离机制和雪崩电离机制有相似之处,两种机制都是一种非线性电离机制。在激光能量密度较高时,多光子电离机制占主导地位;在激光能量密度较低时,雪崩电离过程也可发生。因此,雪崩电离机制的激发条件范围更广,而多光子电离机制的激发条件相对更严格,但两种电离机制的主导地位在飞秒脉冲的脉宽区间内是能够进行过渡转换的。
飞秒激光烧蚀减薄石英玻璃研究进展
Research progress on thinning quartz glass by femtosecond laser ablation
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摘要: 飞秒激光烧蚀加工与传统皮秒或纳秒激光加工相比,具有热作用区域小、激光分辨率高、能够抑制等离子体的物理屏蔽效应等优势,因而被广泛应用。基于雪崩电离和多光子电离效应,阐述了飞秒激光烧蚀透明电介质材料机理,介绍了飞秒激光烧蚀制备不同微结构件现状,综述了近年来国内外飞秒激光微纳加工石英玻璃烧蚀点、烧蚀线和烧蚀面微特征的研究方法和研究进展,对微功能结构件的实际应用情况进行了总结,分析了现阶段飞秒激光在加工透明电介质领域存在的不足,最后对该技术的发展进行了展望。Abstract: Femtosecond laser ablation processing has small thermal zone, high laser resolution, and can weaken the physical plasma shielding effect when compared with traditional picosecond or nanosecond laser ablation. So, it is widely used in many areas. The mechanism of femtosecond laser ablation of transparent dielectric materials was expounded based on avalanche ionization and multiphoton ionization effects. In addition, the current status of femtosecond laser ablation of different microstructures was introduced. Particularly, the research methods and progress of ablation points, ablation lines and ablation surfaces of femtosecond laser micro-nano processing were reviewed at home and abroad in recent years. Also, the practical application of micro-functional structural components was summarized. Moreover, the current shortcomings of femtosecond lasers in the field of processing transparent dielectrics are analyzed, and the development of this technology is prospected.
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Key words:
- laser technique /
- femtosecond laser /
- quartz glass /
- ablation thinning /
- micro-features
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