Investigation of selective laser-induced etching in the volume of sapphire

超短脉冲激光(t=500fs)被聚焦到蓝宝石的体积中。通过像隧穿和多光子吸收这样的非线性吸收过程，激光在焦体积中的辐射能量被沉积到固体中。由于电子和热过程，引起材料结构的变化，从而引起材料密度和折射率的变化。由飞秒激光辐射引起的修饰可以选择性地湿法蚀刻，而未修饰的材料几乎不受影响。利用这种写入技术和随后的刻蚀工艺，甚至可以在透明材料的体积中产生复杂的中空结构。在本项目中，通过系统地改变工艺参数，将对蓝宝石的体积选择性激光诱导刻蚀(ISLE)工艺有一个基本的理解。因此，基于所识别的工艺窗口和蚀刻工艺的可量化预测，三维结构的后续工艺开发应该是可实现的。一方面，工艺参数对晶体结构(如非晶化)的影响。另一方面，研究了激光诱导修饰对蓝宝石刻蚀性能的影响。所使用的工艺参数与蚀刻能力之间的定量相关性应通过实验确定，例如聚焦过程中球差引起的焦区变形、周期性纳米结构的形成以及热聚集效应的发生。此外，还研究了已改性材料以及由此引起的折射率变化对后续激光脉冲的影响，以描述已改性材料对焦面几何和激光辐射吸收行为的影响。在这个项目中，选择了一种整体的方法，以实现随后通过激光诱导蚀刻对蓝宝石体积中的三维结构进行精确工艺开发。

Ultrashort pulsed laser radiation (t = 500 fs) is focused into the volume of sapphire. By nonlinear absorption processes like tunneling and multi-photon absorption in the focal volume of the laser radiation energy is deposited into the solid. Due to electronic and thermal processes a change in the material structure and thus a change of the material density and the refractive index are caused. Modifications induced by femtosecond laser radiation are selectively wet etchable, whereas the unmodified material remains almost unaffected. With this writing technique and the subsequent etching process it is thinkable that even complex hollow structures are produced in the volume of transparent materials.In this project the fundamental understanding of the process will be developed for in-volume selective laser-induced etching (ISLE) of sapphire by systematically varying the process parameters. Therefore a subsequent process development of three-dimensional structures shall be realizable based on the identified process window and a quantifiable prediction of the etching process. On the one hand, the effect of process parameters on the crystalline structure (amorphization e.g.) and on the other hand, the influence on the etchability of sapphire by laser-induced modifications are studied. A quantifiable correlation between the used process parameters and the etchability, such as an induced deformation of the focal volume due to spherical aberrations during focusing, the formation of periodic nanostructures and the occurrence of heat accumulation effects shall be determined experimentally. Furthermore, the influence of the already modified material and thus the induced refractive index modifications on the subsequent laser pulses is investigated in order to describe the influence of the already modified material on the focal geometry and the absorption behavior of the laser radiation. In this project a holistic approach is selected to enable a subsequent precise process development for three-dimensional structures in the volume of sapphire by laser-induced etching.