Investigation Of Femtosecond Laser Surface Treatment With Functional Structures

Plasma,nanoparticles and periodic surface micro-and nanostructures will be produced during the interaction between femtosecond laser and matter.Among them,the characterization of femtosecond laser-induced low temperature plasma could provide a theoretical basis for the plasma sputter coating;the method of femtosecond laser-produced nanoparticles holds the stable,rapid and controllable features;particularly the laser-induced periodic surface structures(LIPSS)could enhance the absorption of light waves and be equipped with the super-hydrophobic property,which have broad applicative prospects in manufacturing biomimetic self-cleaning materials,artificial biomedical materials,new energy materials and large marine shells.In this thesis,the purpose is to investigate the mechanisms and the methods of the femtosecond laser surface treatment,adequate and reliable results are provided,and the theoretical and experimental works establish the foundation for industrial applications.Now the works of this thesis are summarized as follows:1.A Langmuir probe system is employed to diagnose the characteristics of the low temperature plasma induced by femtosecond laser in vacuum,and to obtain the electron temperature of the plasma.Because the evolution of the electron temperature is an important parameter of the theoretical model discussed in this thesis,i.e.two-temperature model,the detected electron temperature in the experiment is used to verify the rationality of using two-temperature model in the particular experimental conditions.In addition,automatic adjustment of the bias voltage loaded on the Langmuir probe is performed by a self-made bias voltage module,and the data could be stored automatically by the PC-controlled oscilloscope.Therefore,numerous data can be obtained in a short time,and the efficiency and the reliability of the obtained characteristics of the plasma could be improved.2.High repetition rate femtosecond laser is used for the ablation of the copper films both in air and water.In air,copper films with different thicknesses are employed as targets,and receive the chromatic annuli composed nanoparticles around the ablation crater with a radius of a hundred microns.Through scanning electron microscopy and micro-Raman spectroscopy analyses,the nanostructures and material composition could be obtained to clarify its mechanisms.In water,the copper films surfaces are decorated with copper oxide and cuprous oxide nanoparticles after femtosecond laser radiation,and cuprous oxide nanocubes could also be obtained,the mechanism of their generation is proposed.In addition to the experimental work,the two-temperature model is used to calculate the temperature of the copper films with different thicknesses induced by femtosecond laser radiation in air,the electron temperature can be detected experimentally by the Langmuir probe system and provide a comparison to the theoretical results for interpreting the experimental phenomena mentioned above.3.Surface treatment experiments on silicon surfaces are finished with the help of femtosecond laser,LIPSS have been obtained and this thesis focuses on employing a modified two-temperature model and Sipe-Drude model for simulation.Two kinds of experimental phenomena which are neglected before have been elaborated: first,ripples bending and splitting;second,grooves production.Through simulation,for the first time the two experimental phenomena are explained,which helps to control and design the surface micro-and nanostructures.The work provides the theoretical basis for the transformation of the experiments into the industrial applications.4.In order to combine the theoretical results in the single-pulse incident condition with the experimental results in the multi-pulse incident condition,the incubation effects of the thresholds of ripples and grooves are investigated.Incubation effect is commonly used to indicate the reduction of the laser ablation threshold with the increasing of the laser pulses,and it is found in this work that the incubation effect could also apply to the thresholds of ripples and grooves.Through analyses of the incubation effect,establish the correspondence between the individual surface structure threshold and the number of pulses.5.For improving the femtosecond laser machining system,a q-plate is placed in the system to generate optical vortex(OV)laser which is used for the surface treatment.By changing the parameters of the OV laser in the experiments,geometric morphology of LIPSS is successfully controlled.Moreover,the method to analyze the femtosecond OV laser machining is proposed for the first time,which associates the experimental results with the OV laser features and lays the foundation for manufacturing complex surface micro-and nanostructures.