Material Removal and Ejection Dynamics in Femtosecond Laser Machining of Microchannels in Transparent Materials

透明材料的高纵横比和高质量微通道在微光学、微电子学、微力学和生物医学等许多重要领域具有重要意义。然而，由于透明材料的脆性和低导热性，使用传统的加工技术很难制造它们。飞秒脉冲激光器提供了克服这些困难的潜力。然而，飞秒脉冲激光器产生的微通道的宽高比和质量是有限的。该奖项支持基础研究，以显著改善飞秒脉冲激光器产生的微通道的质量和纵横比。研究目标是建立(1)烧蚀机理（散裂、相位爆炸、碎裂等）与加工条件（激光强度、脉冲持续时间等）之间的关系；(2)烧蚀机理和喷射粒径/速度分布；(3)抛射粒子的大小/速度及其逃离长通道的能力。为了实现前两个目标，将开发一种基于物理的原子模型，包括分子动力学方法、蒙特卡罗方法和细胞内粒子方法，以激光参数和材料特性为输入。通过预测材料内部的温度、压力和电场分布，揭示不同加工条件下的主要烧蚀机理。该模型还将通过模拟激光与物质相互作用过程中的原子演化来预测抛射粒子的大小和速度。为了验证模拟结果，将采用时间分辨泵浦探针成像技术对相同条件下喷射粒子的大小/速度进行实验测量。为了实现第三个目标，原子模型的输出，如初始激光与物质相互作用后喷射粒子的温度、压力和尺寸/速度，将被用作随后开发的光滑粒子流体动力学模型的输入，以模拟大时间尺度下通道内喷射粒子的演化。对于具有给定尺寸和初始速度的颗粒，该模型将根据通道内的温度、压力和环境环境预测它们的逃逸或再沉积到通道侧壁上。利用时间分辨泵浦探针成像技术，现场观测抛射粒子的运动动态，如瞬态位置和速度，并与模型模拟结果进行比较。

High aspect ratio and high quality microchannels in transparent materials are critical in many important areas, such as micro-optics, microelectronics, micromechanics, and biomedicine. However, it is difficult to fabricate them using traditional machining techniques, due to the brittle nature and low thermal conductivity often found in transparent materials. Femtosecond pulsed lasers offer the potential to overcome these difficulties. However, the aspect ratio and quality of microchannels produced by femtosecond pulsed lasers are limited. This award supports fundamental research to enable significant improvement in the quality and aspect ratio of microchannels produced by femtosecond pulsed lasers.The research objectives are to establish the relationships between (1) ablation mechanisms (spallation, phase explosion, fragmentation, etc.) and machining conditions (laser intensity, pulse duration, etc.); (2) ablation mechanisms and ejected particle size/velocity distributions; and (3) the size/velocity of an ejected particle and its capability of escaping a long channel. To achieve the first two objectives, a physics-based atomistic model, consisting of a molecular dynamics method, a Monte Carlo method, and a particle-in-cell method, will be developed, with laser parameters and material properties as the inputs. By predicting the distributions of temperature, pressure, and electric field within the materials, dominating ablation mechanisms will be revealed under different machining conditions. This model will also predict the sizes and velocities of the ejected particles by simulating the atom evolution during the laser-matter interaction. To verify the simulation outputs, the sizes/velocities of the ejected particles under the same conditions will be experimentally measured by the time-resolved pump-probe imaging technique. To achieve the third objective, outputs of the atomistic model, such as the temperature, pressure, and the sizes/velocities of the ejected particles after the initial laser-matter interaction, will be used as inputs into a subsequently developed smooth particle hydrodynamics model, to simulate the ejected particle evolution within the channel in a large time scale. For particles with given sizes and initial velocities, the model will predict their escape or redeposition onto the channel side walls, based on the temperature, pressure, and ambient environment inside the channel. Ejected particle moving dynamics, such as their transient locations and velocities, will also be observed in-situ using the time-resolved pump-probe imaging technique, and compared with model simulation results.