| Femtosecond laser(fs-laser) has the follow advantages:(1) Utrashort pulse duration,(2)high instantaneous power density,(3) high machining accuracy(accurate threshold),(4) good focusability. Therefore, the interaction between fs-laser and material has shown great application potentials and attracted many researchers to study. Many results have been obtained. This physical process has also been applied more and more widely in many fields,such as micro-processing and biological treatments of various materials. Titanium, as a kind of transition metal with various applications, harbors a vast range of qualities, such as(1) high temperature resistant,(2) small density,(3) high strength,(4) strong corrosion resistance,(5)good alloy performance, as well as(6) good biological compatibility. What is more, titanium is extensively distributed in the crust. Titanium has wide applications in aerospace,shipbuilding, biological and medical treatments, and environmental science et. al. When fs-laser pulses that are powerful enough are focused on the titanium surface, they will cause damage and ablation on the titanium surface and produce target plasma with high temperature and pressure. The investigations on the process of fs-laser ablation on the surface of the titanium and surface morphologies have a guiding significance on micro-processing of fs-laser on titanium and its alloy. Simultaneously, the researches on the characteristics of the fs-laser induced titanium plasma will benefit the deeper understanding in excitation,ionization and dynamic process after femtosecond laser treatment. At the same time, the research should also have a referential significance to the applications of fs-laser induced breakdown spectroscopy(FLIBS).A systematical investigation on the ablation process of titanium induced by fs-laser pulses and the characteristics of titanium plasma has been done. The ablation process and morphologies are investigated in experiments and a theory model is established to simulate the ablation process and morphologies, getting the main mechanisms of the interactions between fs-laser and titanium. Simultaneously, the fs-laser induced periodical surface structures are comprehensively studied, discussing the formation mechanisms of the structures. The works above have a guiding significance for the micro-processing technology on titanium. The spectrum diagnostic of fs-laser induced titanium plasma is done as well. The characteristics of the plasma spectroscopy and dynamical processes are studied. The electron temperature and electron density as well as their evolutions are investigated, too. To sum up,the main works of the investigations are as follows.1. The process of laser induced breakdown of air is investigated. The breakdown processes and thresholds of air under fs-laser and nanosecond laser(ns-laser) are investigated in experiment, respectively. A two-step ionization model to describe the process of ns-laser induced air breakdown is established, according to the non-linear interaction between laser and air. The ns-laser breakdown threshold of air and time node between the two breakdown steps are determined based on the model. The breakdown thresholds at different pressures are measured experimentally. The theoretical simulations fit experimental results very well. The fs-laser induced breakdown of air is described by photoionisation, electron heating and impact ionization processes. The breakdown process and threshold are investigated theoretically and compared to experimental values. The breakdown processes and thresholds under ns-laser and fs-laser are discussed and compared.2. The fs-laser ablated morphology is investigated in experiment. It is concluded that the heat-effected zone besides the ablated crater is observed on the titanium surface after fs-laser treatment. The radius and depth of the ablated crater and their evolutions with laser fluence and pulse number are studied.3. A three dimensional two temperature model is established in the cylindrical coordinate system to simulate the ablation process and surface morphology of titanium induced by fs-laser. The thermophysical parameters of titanium are corrected by the theory of electron density of state. The process of fs-laser ablation is described in detail by the simulation. The radius and depth of the ablated crater are determined. The two-dimensional and three-dimensional ablation morphologies are drawn out. The simulation results are in good agreement with the experimental data. The behavior of the radius and depth of the crater with the variations of laser parameters are discussed. It is concluded that the three-dimensional two temperature model after parameter corrections has the capacity to describe the fs-laser ablation of titanium.4. The fs-laser induced periodical surface structures are investigated systematically in experiment. Three types of the periodical surface structures are observed on the titanium surface, including ripples, microgrooves and rings. The behaviors of the three types of structures with the changes of laser fluence and pulse number are discussed in detail according to the experimental results. It is concluded that the formation mechanisms of the three structures compete with each other. Each type of structure evolves with the laser parameters as well. The processing window of each type of structure is given based on quantitative experimental data. The formation mechanisms of the structures are investigated in theory. The theory of surface plasma wave is applied to interprete the formation of characteristicsã€evolution and distribution of the ripples. The microgrooves are explained by the interaction between the scattered laser and surface wave induced by ripples. An optical interaction model is established to interpret the formation and evolution of the rings structure.5. The time-resolved spectroscopy is acquired in experiment to study the evolutions of the spectrums of different components in plasma. The evolutions of ionic spectral lines are faster than that of atomic ones. The expanding velocity on two directions of plasma plume is determined by means of time-of-flight spectroscopy. It is found that spectrum decays the plume expands slower as the distance from ablated centre increases. Simultaneously, the dependence of plasma spectrum and expansion on the air pressure is discussed. It is concluded that the ionic lines are enhanced while the atomic lines are suppressed when the air pressure increases. But the evolutions of both ionic and atomic lines with time get slower and the expansion of plasma is suppressed at higher pressure.6. The electron temperature and density of the fs-laser induced titanium plasma are determined by means of plasma diagnostics. The two parameters are as high as 104 K and 1017cm-3 when the power density reaches 2.17×1016 W/cm2. The two parameters decreases with time going but appears a first quick back slow trend. The influences of pressure on the two parameters are discussed. It is concluded that both the electron temperature and electron density increase as the pressure grows. But decreasing speeds of attenuations of these two parameters with time are obtained at higher pressures. |
