飞秒激光与材料相互作用中的超快动力学

Ultrafast Dynamics of Femtosecond Laser Interaction with Materials

飞秒激光以其超快超强特性可以实现三维复杂结构的微纳级别精密制造。超快光场与材料相互作用过程中存在大量新现象、新效应和新机制,亟待揭示。本文简要介绍飞秒激光与材料相互作用的基本过程以及飞秒激光泵浦探测技术的发展历程。首先重点阐述了高斯型飞秒激光与材料相互作用超快动力学的研究进展,根据不同的时间尺度分别对相互作用过程中的光子与电子相互作用、电子与晶格相互作用过程、相变,以及等离子体/冲击波喷发等过程的观测与机制分析进行总结;随后针对基于电子动态调控新方法的飞秒激光加工超快动力学进行综述,分析对比了其与传统飞秒激光和材料相互作用超快动力学的差异;最后针对该领域的发展方向进行了展望。

Significance Miniaturization of electronic and information devices has become the development trend in the age of science and technology. Owing to its benefits, including simple integration without contact, flexible and controllable fabrication, and low material loss, femtosecond laser micro/nano manufacturing has gradually become a crucial research direction in micro/nano manufacturing technology. The femtosecond laser can achieve ultra-precision, high-efficiency, and high-quality micro/nano fabrication of almost all materials. The interaction between femtosecond laser and materials is distinct from that of the traditional laser-material interaction process, owing to its ultra-short pulse characteristics, which is a nonlinear and unbalanced multi-temporal-scale ultrafast process. It involves various physical processes, like photon absorption and electron excitation, phase transition, plasma/shockwave radiation and eruption, material removal, and other ultrafast dynamics processes. These physical processes fundamentally influence the final structure of laser processed materials and can directly regulate the structure’s morphology and properties. Thus, to attain the constrained breakthrough and extensive application in femtosecond laser micro/nano fabrication, it is important to understand and regulate the ultrafast dynamic evolution of femtosecond laser interaction with materials. An in-depth study and understanding of the ultra-fast dynamic evolution mechanism in femtosecond laser processing will offer a theoretical fundamental and guidance for the realization of high-efficiency, high-precision, and high-quality femtosecond laser micro/nano fabrication. Therefore, facilitating the quick development of femtosecond laser micro/nano fabrication technology and its application.Process Femtosecond laser pump-probe technology is employed to investigate the ultrafast dynamics evolution in femtosecond laser processing. With the development of the time delay translation stage and optical imaging technology, temporal and spatial resolution has been improved. The imaging types include transmission type and reflective type, interferometric type, and holographic imaging. Numerous dimensions of ultrafast dynamics imaging in femtosecond laser fabrication have been achieved. Moreover, to probe the material response in femtosecond laser processing more completely, a multi-scale pump-probe system has been constructed to probe the whole process of femtosecond lasermaterial interaction.There are several investigations about the probe of photon-electron interaction, electron-lattice interaction, and plasma radiation and eruption in the process of femtosecond laser-material interaction, which indicates the physical mechanism of each stage. Moreover, the mechanism research for shaped femtosecond laser processing and femtosecond laser-excited chemical reaction synthesis of materials are also performed.The crucial physical parameters of the femtosecond laser-material interaction theoretical model can be evaluated by studying the photon-electron interaction process based on pump-probe technology that can guide the theoretical model’s enhancement and development. There is a lot of research to determine the crucial factors like electron decay time, electron-hole combination mechanism, and electron relaxation time, therefore regulating the electron density evolution, electron/lattice temperature, phase transition mechanism, and ablation findings. Furthermore, the probe of the photonelectron interaction process can show the nonlinear ionization mechanism in laser-material interaction, which guides the optimization of material processing conditions and parameters, and attains the effective and controllable manufacturing of structures.The evolution of transient optical properties during femtosecond laser materials interaction can be employed to investigate the electron-lattice interaction process, and further, show the phase transition and removal mechanism of materials in the picosecond-nanosecond time scale. Presently, femtosecond laser processing mechanisms on traditional materials like fused silica, silicon, and germanium under the picosecond-nanosecond time scale have been investigated. Recently, research on new materials, including two-dimensional materials has emerged, which shows the phase transition mechanism and ablation mechanism of emerging materials.The process of plasma eruption and shockwave propagation in the picosecond to nanosecond time scales after femtosecond laser processing plays a crucial role in the evolution of the final morphology and the investigation of material properties based on plasma. Current studies primarily focus on the influences of femtosecond laser parameters(including laser fluence, wavelength, and pulse width), material properties, and processing environment on the generation and propagation of plasma/shockwave. The findings show the impacts of air/material plasma excitation and shockwave expansion on the final morphology of laser-induced micro/nanostructures.New approaches based on electron dynamics control have been suggested in recent years. Therefore, in addition to investigating the traditional Gaussian pulse’s mechanism, some studies have also been reported about the interaction between spatial-temporal shaping femtosecond laser and materials. Currently, the studies focus on double-pulse femtosecond laser processing, Bessel laser processing, and simultaneous spatial and temporal focusing of femtosecond laser processing, which improves the development of spatial-temporal shaping of femtosecond laser processing. There are also investigations on the probe of the ultrafast dynamics of chemical reaction excited by femtosecond laser, showing the physicochemical mechanism in the process of material chemical reaction excited by femtosecond laser, expanding the application and development prospect of femtosecond laser micro/nano fabrication.Conclusions and Prospects Femtosecond laser manufacturing is forecasted to become the primary means of high-end manufacturing in the future, which will offer crucial manufacturing support to attain leapfrog development in new energy, aerospace, national defense, and other fields. It is the ideal technology to break through the manufacturing technology challenges of numerous core components, but there are still a lot of issues. To enhance the manufacturing accuracy, efficiency, quality, and controllability of femtosecond laser, it is crucial to have a deep understanding of the complex mechanism in femtosecond laser processing. In observing the temporal and spatial evolution of local transient electron dynamics in ultrafast laser manufacturing, the accuracy of temporal and spatial resolution, three-dimensional panoramic observation from various angles, and multi-scale ultrafast probe are the three major problems. In this study, the ultrafast dynamic in femtosecond laser micro-nano manufacturing is summarized, and the interaction between femtosecond laser and matter is summarized. The development history of femtosecond laser pump-probe technology is introduced, and the multi-scale ultrafast dynamic in various stages of femtosecond laser micro-nano manufacturing is summarized. Distinct material systems, processing environments, and mechanisms based on electrons dynamics control are summarized and compared, which offers a crucial observation fundamental and guidance for femtosecond laser micro-nano manufacturing.The current study about the ultrafast dynamics of femtosecond laser micro-nano manufacturing has broken through the drawbacks of traditional image sensors to attain higher frame rates and shutter speeds. To enhance the image acquisition’s speed, these studies more or less sacrifice one or more specific parameters. Additionally, traditional methods still have challenges, including challenges in light field reconstruction, single observation means, and difficulty in attaining precise coordination and coupling of numerous observation means. Combining the pump-probe technology, ultrafast continuous imaging technology with the four-dimensional scanning ultrafast electron microscopy technology, can develop a multi-scale quasi three-dimensional pump-probe system with substantial spatial-temporal resolution and dynamical continuous observation capability, which can be employed for observation of electron ionization and phase change during the evolution of the structures and properties in the femtosecond laser extreme manufacturing. High spatialtemporal resolution observation of multi-scale processes will revolutionize the research on ultrafast dynamics in femtosecond laser manufacturing.