Experimental And Theoretical Study On The Ablation Of Silicon By Femtosecond Laser Pulse Trains

The rapid development of femtosecond laser has revolutionized the processing of materials.However,under high laser fluences,the ablation quality of femtosecond laser deteriorates significantly because of the adverse effects such as coronal redeposition materials and contamination and damage of the substrate caused by high temperature dust.Meanwhile,because of the saturation and plasma shielding effects,the ablation efficiency decreases gradually with the increase of laser fluence.In recent years,femtosecond laser pulse trains with high repetition rate have been reported to be able to solve the above problems.Nevertheless,due to the lack of the laser systems or experimental methods which can adjust the repetition rate in a large range,there is still no systematic experimental study on the ablation effect of femtosecond laser pulse trains,and different results have been reported in different studies.In order to improve the ablation quality and ablation efficiency of high fluence femtosecond laser,as well as to verify the effectiveness of femtosecond laser pulse trains and to further explore the underlying physics,this paper carries out work from the following three aspects:(1)A device based on Fabry-Perot interferometer was designed and built to generate pulse trains whose pulse intervals can be adjusted continuously in a large range.At present,the adjustable range of the pulse interval of the experimental system is 1ps?3500 ps.(2)The ablation quality and efficiency of silicon by single femtosecond pulses and pulse trains with different pulse intervals were systematically investigated in the experiment.The main conclusions are as follows.In terms of ablation quality,pulse trains with intervals in the range of 50?100 ps achieved the most ideal ablation crater.The coronal redeposition materials at the edge of the crater were greatly reduced and the thickness reduced to 40%of that of single femtosecond pulses.In the meanwhile,there is almost no adhered dust or thermal damage on the surrounding substrate,and the quality factor increased from 0.52 to 0.89.As for the ablation efficiency,the ablation volume of pulse trains with pulse intervals of 300 ps,1000 ps and 2000 ps are larger than that of single femtosecond laser.Among them,the ablation efficiency of the 1000ps pulse train is 1.65 times that of single femtosecond laser.The maximum ablation efficiency obtained in our experiemtns is 8.7?m~3/?J.(3)The mechanism analysis and theoretical calculation were carried out from the perspectives of ablation quality and ablation efficiency respectively,and the main conclusions are as follows.The key to improve the ablation quality is the sufficient atomization of the ejected material by the subsequent pulses of the pulse trains.To fully atomize the ejected material,two conditions need to be met:(a)the ejected material detaches the substrate,(b)the ejected material has not spread further and still remains within the laser irradiation range.As for the ablation efficiency,the shielding effect and heat accumulation effect are the two main factors.According to the calculation,when the pulse interval is less than 200 ps,the plume almost completely absorbs the energy of the subsequent pulse,resulting in the decrease of ablation efficiency.With the diffusion of plume,the shielding effect is gradually weakened,and the ablation efficiency can be improved under the action of thermal accumulation effect.In conclusion,in this paper,the ablation effects and physics of femtosecond laser pulse trains under different pulse intervals were explored experimentally,which verify the effectiveness of high repetition rate femtosecond laser pulse trains,and provide the optimal parameter range.Meanwhile,by referring to the related literatures on the modeling of double-femtosecond-pulse ablation,the theoretical model was built and numerical calculation were carried out,which explain the experimental phenomenon,strengthen the understanding of pulse train ablation dynamics and establish the foundation for further theoretical research.