Transient Observation Of Electronic Dynamics During Femtosecond Laser Micromachining

Femtosecond laser processing has become a powerful tool for micro and nano fabrication due to its extreme field ionization mechanism with high precision,low damage and no material selection.The physical mechanisms utilized in the process are multi-timescale and ultrafast in nature,and the different mechanisms are both coupled and competing with each other.In such a complex and extreme non-equilibrium process,the dynamics of the initial electrons dominate the subsequent processing results,but the fundamental characteristics of the electrons at the early stage of material removal(before 1 ps)are still not fully characterized.In this thesis work,the transient imaging and theoretical calculations of the initial process of ultrafast laser-material interaction are carried out from both experimental observations and theoretical models,starting from the nature of the femtosecond laser and the fundamental laws of its propagation ionization mechanism.The transient reproduction and theoretical analysis of the process problem of processing transparent materials in atmospheric environment are also carried out.The evolution of the initial electronic state of the strong field interaction material is explored and studied comprehensively,and high time-resolved photography of the light field transport and remodeling effects in the actual processing of multi-pulse irradiated materials is conducted.The formation of process problems such as "induced streaks" and "microcracks" in transparent materials by pulse strings is explained.Also,two targeted measurement schemes are proposed in order to explore the transient response and electronic states of materials under subperiods of femtosecond pulses.The core research work of this paper is as follows:For processing scenarios in atmospheric environments,the time-space process of transport ionization of femtosecond laser pulses in air is photographed by building a femtosecond time-resolved pump-probe shadow imaging experiment.The experimental results show that the ionized air plasma is shuttle-shaped.With the increase of time delay,the transient electron number density increases and then decreases,but its dissociation wavefront velocity is decreasing.The variation of the free electron number density is solved and normalized to the intensity distribution of the pulse,and it is found that the saturation of the electron number density is reached within one femtosecond pulse time.The transition moments of multiphoton ionization and tunneling ionization were obtained,and the complete process of ion diffusion and complexation under different focusing conditions was also obtained.It provides a basis for the next study of the propagation and ionization of femtosecond pulses in transparent media.A femtosecond time-resolved pump-probe shadow imaging movable observation module was built to study the single-pulse propagation filament formation evolution and multi-pulse induced microstructure-assisted filament formation process in quartz for the multi-pulse ablation scenario in practical processing and process problems in transparent materials.The single-pulse study shows that high laser flux corresponds to higher transient electron number density,while the long-focus objective forms longer plasma filaments.Under 20-fold focusing conditions,the maximum electron number density monotonically increases at 1 ps time;under 40-fold focusing conditions,the maximum electron number density increases and then decreases at 1 ps time.Multi-pulse studies show that the propagation of the laser field is determined by the combined effect of self-focusing,plasma scattering,and refraction of the microstructure under the action of high heavy frequency multi-pulses.The microstructure reshapes the subsequent laser field.As seen by high time-resolved images,different plasma filaments are formed due to the difference in morphology between the sidewalls at the edges of the microstructure and at the center.As the focus feeds into the interior of the material,the sidewall slope changes,while the electron number density of the formed plasma is changing accordingly.Finally,the microhole formation process of multi-pulse-acting single-crystal diamond was photographed by the observation module,and the relationship between the microhole depth and width and the pulse number time was obtained by nonlinear fitting.Based on the exploration and study of the evolution of the electronic state at the early stage of the strong field interaction,and to address the problem that the exposure time of the conventional pump detection "light shutter" is too long to detect the subperiodic physical process of the pump pulse,a subperiodic measurement scheme of the nonlinear compression detection light pulse width and the subperiodic measurement scheme of the spreading pump light pulse width of the block material are designed.The subperiodic measurement scheme is designed.A specific optical measurement idea is provided for the transient response of the material at the early stage of light field action.