Mechanism Research On Laser-induced Plasma Based On Porous Anodic Aluminum Oxide

Porous micro-nano material have a stronger light absorption coefficient than general materials,and it is ideal materials for improving the conversion efficiency of laser energy during the interaction between laser and matter.The surface of the anodized aluminum oxide(AAO)template have a periodic pore structure.AAO is simple to prepare and it has the characteristics of periodic micro-nano materials.In this paper,the laser absorption mechanism of AAO prepared by secondary anodization method is studied from the experimental and theoretical simulations,and comparative analysis is carried out.On this basis,the laser induced plasma characteristics were studied and analyzed in combination with the absorption mechanism.The research results are as follows:Three AAO templates with different pore sizes were prepared by secondary anodization.Three AAO templates were characterized by scanning electron microscopy(SEM)and atomic force microscopy(AFM).The results show that the hole spacing T of AAO is 450 nm,the thickness H of the oxide layer is 550 nm,and the pore diameter D is 200,300,and 400 nm,respectively.The pores of the porous layer are periodically distributed,and the pore sizes are uniform and evenly distributed.The light absorption properties of AAO templates were tested by near-infrared absorption spectroscopy.And the microstructure model of AAO film is analyzed,which can be studied from two levels: the pore structure of the surface of the material and AAO's own multilayer film structure that upper layer of the structure is porous alumina,the bottom layer(base)is aluminum material,and the barrier layer is alumina.The optical properties of AAO films are analyzed according to optical theory.And the mechanism of effective absorption of light waves caused by the microstructure of AAO film was analyzed in detail.The results show that high light absorption effect of AAO film is the result of multiple effects of multiple reflections at multiple interfaces on the surface of AAO film,thin film interference of multilayer structure and scattering and diffraction of surface periodic microstructure defects.The reflectivity of the AAO template was obtained by using the finite difference time domain(FDTD)method and compared with the experimental data of near-infrared absorption spectroscopy.The results show that the experimental results are in agreement with the simulation results.In the range of 200-400 nm aperture,as the aperture of AAO decreases,the AAO aperture becomes smaller,the number of light reflections in the sample increases,the reflectivity decreases,and the light absorption rate increases,which is a good explanation of the phenomenon that the AAO template plasma is enhanced in the experiment.When the pore size is small to a certain extent,the effect of the porous structure is weakened,and the reflectivity is improved,which is in line with the actual situation.The laser induced plasma characteristics of the AAO template were investigated by atomic emission spectroscopy and high speed photography.The result shows that in the range of 200-400 nm aperture,under the same laser energy,as the aperture of AAO decreases,the plasma duration becomes longer,the intensity of the emission spectrum becomes larger,and the plasma electron temperature and electron density are enhanced.Among them,the AAO plasma electron temperature is 2000~3500 K higher than the polished aluminum sheets and the AAO plasma electron density is twice as high as that of polished aluminum sheets.