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

超短脉冲激光辐射（t = 500 fs）聚焦到蓝宝石体积中。通过非线性吸收过程，如激光辐射能量的焦体积中的隧穿和多光子吸收，被沉积到固体中。由于电子和热过程，导致材料结构的变化，从而导致材料密度和折射率的变化。由飞秒激光辐射引起的改性是选择性湿法蚀刻，而未改性的材料几乎不受影响。通过这种写入技术和随后的蚀刻工艺，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蚀刻（ISLE）工艺进行基本了解。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当是可实现的。一方面，工艺参数对晶体结构的影响（例如非晶化），另一方面，研究了激光诱导改性对蓝宝石可刻蚀性的影响。应通过实验确定所用工艺参数与可蚀刻性之间的可量化相关性，例如聚焦过程中因球面像差引起的焦点体积变形、周期性纳米结构的形成以及热积聚效应的发生。此外，研究已经改性的材料以及由此引起的折射率修改对随后的激光脉冲的影响，以描述已经改性的材料对激光辐射的焦点几何形状和吸收行为的影响。在这个项目中，选择了一个整体的方法，使后续的精确工艺开发的三维结构的体积蓝宝石激光诱导蚀刻。

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.