Mechanisms Of Micro/nano Processing With Temporally Shaped Femtosecond Irradiation Via Electron Dynamics Control

Femtosecond laser micro-nano fabrication technology has already become one of the hot a research field of micro-nano manufacturing. With the rapid development of opt ical devices, femtosecond laser pulse shaping technology came into being and was developed, which was soon applied to the applied of laser based micro-nano fabrication technology. Due to the possibility of electron dynamics control(control of electronic excitation, ionization and recombination, etc) with temporally shaped femtosecond laser, the transient localized optical and thermodynamical properties, which is governed by electron dynamics, can be better controlled. This also offers the possibility of regulations of phase change process and the subsequent proceesses for high quality, high efficiency, and high precision of material processing. The introduction of temporal pulse shaping technology into material processing increases demand for a better understanding of fundamental physics involved. Much remains to be clarified and/or uncovered. It is of both scientific and technological significance to to explore a better and better micro-nano material fabrication with the enlightening of fundamental research. This research was carried out under this background, and the main contents of this thesis include the following three aspects:(1) A new model is proposed by improvements of modeling in electronic excitation and optical absorption processes for fused silica. The emprovement is supported by the published experimental data, as well the experimental work demonstrated in this thesis which offers clues for its further generalization. The improved modeling is capable of grasping the key physics involved in the interaction of temporally shaped pulse with the material. Consequently, the prediction accuracy for material ablation threshold is greatly improved, which helps with the problem quenching for traditional modeling.(2) The key mechsnisms for temporally shaped pulse ablation threshold was revealed with the proposed model. It is found that the evolution of temporally shaped pulse ablation threshold is governed by the interplay of three competing sources of seed electrons initiating an electronic avalanche: residual conduction-band electrons left by the previous pulse, photoionization of atoms in dense media and photoionization of STEs by subsequent pulses. The third source leads to many pulse-separation independent phenomena(e.g. surface damage/ablation size) for pulse-trainprocessing when it becomes dominant, and can contribute to the repeatable processing with smaller size.(3) Contrary to the common belief that a split of one ultrashort laser pulse into two or more temporally delayed sub-pulses leads to a reduction in ablation spot size, a new and unexpected phenomenon was reported that a significant enhancement of ablation size(depth and surface entrance size) can be achieved by that split for nickel. The physical origins for the appearance of ablation enhancement on nickel, as well the absence of that for many other metals such as copper, were revealed with the numerical modeling based on the improved Two-Temperarure Model. The suppression of electronic heat diffusion and enhancement of electron-phonon coupling rate, benefitting from the reduction in electron temperature due to the pulse split, are mainly responsible for the observed enhancement on nickel, the opposite effect on electronic heat diffusion and electron-phonon coupling rate leads to the suppression of ablation on copper. This study suggests a technique to the selective femtosecond laser material processing.