Investigation On Manipulating Solid Surface Microstructures By Femtosecond Laser Pulse Shaping

In recent years,the focusing of femtosecond laser has been used to induce the formation of submicron scale micro-nano structures directly on the surface of solid materials,which has become a promising new method for formation micro-nano structure on material surface.The surface microstructures can be optimized and controlled by changing the spatial field distributions or time-domain waveforms of femtosecond laser.Space shaping is a form of energy distribution that adjusts the Gaussian laser energy distribution to a specific one,making it more conducive to engineering application.The typical way of waveform shaping in time domain is to divide a laser pulse into two or more sub-pulses to form a pulse sequence,and to optimize the surface morphology by controlling the pulse delay.The essence of this method is to optimize and control the formation process of micro-nano structure by dividing the pulse arrangement in time domain and spatial distribution of laser energy.In this thesis,the effects of femtosecond laser energy distribution and double pulse delay on the formation of typical sub-micron structures are systematically studied.The main contents and main results are as follows:1.We study the effects of energy accumulation and spatial distribution of femtosecond lasers on the surface morphology of the material by using focused femtosecond laser to prepare submicron periodic structures on the surface of metallic titanium.It is found that in the process of controlling the period of the microstructure by changing the laser energy,an experimental phenomenon that the periodic structure disappears in the course of the variation of laser energy.Furthermore,we reveal the influence mechanism of the thermal impact depth of different femtosecond laser energies on the periodic structures.Based on the analysis of the influence of the femtosecond laser energy distribution on the periodic structure,the application of the flat-top femtosecond laser pulse in the preparation of the surface periodic structure is proposed and experimentally verified.It lays a foundation for the application of pulse shaping in the fabrication of surface microstructures.2.We use the typical double-pulse time-domain pulse shaping method to study the effect of regularity and physical mechanism of pulse delay on the sub-micron periodic structures,and realize the control of surface structure with a period from 350 nm to 560 nm.The reason why the period does not change with the delay when the energy of a single sub-pulse is higher than the formation threshold of the periodic structure is given.The physical mechanism of the disappearance of the structure period at large time delay is analyzed,which is related to the melting of the material surface caused by the previous sub-pulse.3.We use the polystyrene(PS)nanosphere to realize the laser energy redistribution in the laser focused spot.The near field enhancement of the beyond-diffraction scale is obtained on the surface of the substrate under the PS nanosphere,and the super-diffraction focusing effect is used for the research of nanohole rocessing.At the same time,combined with the double pulse time domain controlling,diameter of the prepared nanoholes controlled continuously(diameter range from 200 nm to 350nm)by optimizing the double pulse delay.