Femtosecond Laser Micro-machining On Multiple Materials And Its Application

There are three distinct characteristics of femtosecond(fs)laser:ex-tremely short pulse width,wide spectral range and extremely high instan-taneous peak power.Compared with long pulse laser(nanosecond and picosecond lasers),femtosecond laser micromachining has obvious thresh-old effect,weak thermal effect,fewer sputters and high processing accuracy.Therefore,the application of femtosecond pulses in microfabrication has a bright future.The common processing materials of femtosecond laser are mainly metals and dielectrics.The difference between them is that metals have many free electrons,while the electrons of dielectric materials are mostly in valence bands.Low power femtosecond lasers are generally used to change the optical properties of crystal materials,such as refractive index or polarization direction.After the high power laser acts on the material,the valence band electrons absorb the energy of many photons and then transit to the conduction band to form seed electrons.Then,a large number of free electrons are formed rapidly through the avalanche ionization process,and plasma will be formed when the electron number density reaches a certain value.At the same time,heat is transfered from electrons to the lattice,causing the lattice to melt or coulomb explosion,and removing the material.Based on the mechanism of femtosecond laser and material,this paper mainly studies the microfabrication and application of various materials based on femtosecond laser,which mainly includes three parts:Femtosecond laser shock peening(LSP)was used to investigate the strengthening effect of magnesium alloys.The shock wave induced by high temperature and high pressure plasma produced by femtosecond laser acting on the material surface reacts on the material surface.When the pressure of shock wave exceeds the elastic limit of material,plastic deformation will be induced,residual compressive stress will be introduced,and nano-grain layers will appear after the surface grains are subjected to stress,thus improving the hardness and other properties of the material surface.The direct ablation group and confined ablation groups were set up.Results show that the effect of direct LSP in air is optimal,and the hardness can be increased by 70%on the original basis.Then the hardening effect of femtosecond laser and nanosecond(ns)laser shock peening on Mg-3Gd alloy was compared.The fluence of femtosecond laser with 430?J and nanosecond laser with 9J are 34.2J/cm~2and 71.7J/cm~2,respectively.The hardness of the sample treated by ns-LSP was increased by 45.1%,which is lower than that of fs-LSP.For femtosecond laser,weaker single pulse energy can obtain higher intensity and a better hardness strengthening effect,so fs-LSP has great advantages over ns-LSP.In addition,the effects of single pulse energy and overlap on hardness are investigated.The results show that both single pulse energy and overlap can effectively enhance the shock wave effect.Finally,the evolution of the sample surface after LSP was investigated by various micro-characterization techniques,such as SEM,XRD,AFM,OM and so on.The results show that fs-LSP can significantly increase the surface roughness.The metallographic images show that grain refinement occurs on the surface of the sample subjected to shock wave,which can well explain the enhancement of microhardness on the surface of the material.The effect of femtosecond laser drilling on hard and brittle materials of alumina and aluminium nitride ceramics was investigated.Firstly,based on the linear regression method and experimental data,the ablation threshold of alumina and aluminium nitride ceramics can be calculated as 6.1 J/cm~2and 2.7 J/cm~2,respectively.Then,the changes of each index of the hole drilled under different laser powers are studied,and the relatively optimized processing power is finally summarized.For alumina ceramics,200 mW laser power produces the highest aspect ratio.The circularities of the hole entrance and exit are higher and closer,and the debris around the hole is relatively less.For aluminium nitride ceramics,190 mW femtosecond laser pulse can produce the highest quality of small hole,which has the highest aspect ratio,the smallest heat affected zone and the smallest taper angle.The only weakness of the hole is a lower circularity.Type ? waveguide with a height of 16?m and a width of 10?m is written directly in lithium niobate crystal by femtosecond laser.The minimum loss of waveguide is 3.17dB/cm.Considering the potential ability of femtosecond laser to directly write photonic crystals and microcavities,a novel all-optical logic gate based on W1 defect waveguide and L2 defect microcavity of lithium niobate is designed.Firstly,some basic concepts of photonic crystals and defect structures are introduced.Then,based on the lithium niobate dielectric cylinder,the effects of two defect structures,W1 defect waveguide and L2 defect microcavity,on the internal beam of photonic crystals are investigated.Then all-optical logic gates of AND,XOR as well as NOT based on the above two defect structures are designed.The extinction ratio of XOR gate can reach 23dB.Finally,based on the AND and XOR gates designed above,an all-optical half-adder is designed.The simulation results show that the expected half-adder function can be achieved.