Title: Multiscale measurement of electron dynamics in femtosecond laser- material interactions
A multiscale measurement system is proposed and developed to detect the spatiotemporal electron dynamics of the nonlinear, non-equilibrium multiscale process of femtosecond laser-material interactions. The system includes 1) femtosecond-to-nanosecond transversal pump-probe shadowgraphy, 2) nanosecond-to-millisecond time-resolved plasma photography, 3) nanosecond-to-millisecond laser-induced breakdown spectroscopy, and 4) millisecond-to-second continuous industrial imaging. By virtue of the multiscale ability, the electron dynamics in femtosecond laser-material interactions can be revealed at different scales. For instance, from femtosecond to picosecond timescale, the free electron generation, electron relaxation and decay time, and the electron spatiotemporal distribution in materials were investigated. Especially, the inversely dependency of the free electron relaxation time on the electron density of fused silica was investigated, which can be explained by electron-ion scattering regime. From picosecond to nanosecond timescale, the early plasma expansion and following shockwave evolution were observed. The time-resolved observation results revealed that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave was enhanced in the longitudinal direction due to crater-assisted laser-induced air breakdown. Furthermore, we have observed the electron density distribution optimized by spatially-shaped femtosecond pulses in plasma at the focus point, the manipulated plasma/shockwave expansion, and phase change, which were further compared with the drilling dynamics induced by traditional Gaussian beam. This study revealed the cylindrical shockwave-induced compression mechanism in the formation of microholes with a diameter of 1.6 μm and an aspect ratio of 500:1.
Keywords: Femosecond laser-material interactions, electron dynamics, multiscale measurement
Lan Jiang , professor and doctoral supervisor of Beijing Institute of Technology, the first batch of leading talents in scientific and technological innovation of "ten thousand talents plan" - national high level talents special support plan, professor of Cheung Kong Scholar, distinguished young scholar from National Natural Science Foundation, and the chief scientist of National Key Basic Research Program of China (973 Program), subject matter expert of National High Technology Research and Development Program of China(863 Program), leader of innovation team leader of Ministry of Education, commission member of Science & Technology Commission Ministry of Education, and he is the head of the overall expert group of the National Key R&D Program of “Additive Manufacturing and Laser Manufacturing”. He is mainly engaged in in the field of nano/micro laser fabrication.