Atmosphere Regulation And Mechanism Of Filamentation Induced By Picosecond Laser In Sapphire

The propagation of ultrafast laser in optical medium will cause Kerr self-focusing effect when the peak power exceeds the threshold.At the same time,the medium is ionized to produce plasma defocusing effect.The dynamic balance of Kerr self-focusing and plasma defocusing produces a typical nonlinear phenomenon,which is called filamentation.The ultra-long and non-diffractive transmission of filamentation is expected to solve the defects of short focal depth,limited focus scale and limited degree of freedom Z axis in conventional laser(including ultra-fast laser)processing technology.The regulation of the filamentation(plasma,length,position,etc.)is the foundation of its application in high-precision material processing.However,it is very difficult to control the filamentation due to the complex physical phenomena and mechanism.In particular,because the properties of solid materials cannot be changed,the regulation of filamentation in solids only dependent on optimization of laser parameters will be limited by laser itself.In this paper,it is presented to achieve the regulation of filamentation by the ambient gas during laser processing.The influence rule and mechanism of different gas types and gas velocities on the filamentation induced by picosecond laser in sapphire was studied by coaxial nozzle which is the common way in laser processing.The evolution of the plasma on a nanosecond time scale was obtained by the self-designed ultra-high speed image acquisition system.Based on the analysis on the morphology of the modified area and the physical model of the surface plasma dynamics,the influence of gas type and gas velocity on the filamentation in sapphire was revealed.In this paper,355 nm picosecond laser was used to induce filamentation in sapphire.Ar,N₂ and O₂,which were usually used in laser processing,were selected and provided by coaxial nozzle.The main laser experimental parameters were optimized firstly according to the demand of plasma acquisition and morphology analysis.Synchronization schemes of ultra-high speed image acquisition system were designed and built to solve the synchronization of ultra-high speed camera and picosecond laser pulse.The nanosecond evolution images of the plasma under three gases were obtained.There is the obvious expansion process of the surface plasma in Ar,meanwhile,the lifetime of the filamentation plasma in sapphire is the shortest.No obvious surface plasma expansion can be observed under N₂ and O₂ conditions.The lifetime of the filamentation plasma is longer in sapphire under N₂and O₂.This indicates that different gas types result in the difference of the laser pulse energy to induce filamentation in sapphire.The following analysis results of the morphology around the laser incident point and the filamentary tracks are consistent with those from the plasma transient images.The filamentary track in sapphire is the shortest under Ar but the longest under O₂.It is proved that the ambient gas can effectively regulate the filamentation induced by picosecond laser in sapphire.High density surface plasma reducing the laser energy entering sapphire is found under Ar.However,there is no obvious surface plasma expansion process in O₂,so there is no negative effect on the subsequent laser energy to disturb the excitation and maintenance of the filamentation in sapphire.The mechanism of atmosphere regulation on the filamentation in sapphire was revealed by finite element simulation analysis on the dynamics of surface plasma in three ambient gases.It is found that the thermal conductivity of the ambient gas is the main factor affecting the behavior of the surface plasma.The heat exchange efficiency between surface plasma and Ar is the lowest due to the lowest thermal conductivity of Ar,so the lifetime of surface plasma is the longest.The thermal conductivities of N₂ and O₂ are higher,resulting better heat dissipation effect on the surface plasma.The longer filamentary tracks in solids can be obtained in the ambient gas with high thermal conductivity.The influence of gas velocity on the filamentation in sapphire was studied by changing the nozzle diameter and the gas pressure.It is found that the influence of gas velocity on the filamentation depends on the gas flow state according to the plasma analysis from the obtained ultra-high speed images.The length of the filamentary track in sapphire increases linearly with the increase of the gas pressure when the diameter of the nozzle is 4 mm.This supports the following investigation on the regulation of filamentation based on the stable filamentary phenomena.The surface morphology of the laser incident points and the filamentary tracks formed with 4 mm nozzle and under different gas velocities were analyzed.It is found that high speed gas flow from 5 bar nozzle pressure can significantly reduce the melting area around the laser incident point,meanwhile,induce the longest filamentary track in sapphire.The dynamics of the surface plasma under different gas velocities were analyzed by finite element simulation.It is found that the convective heat dissipation effect of high-speed airflow can make the surface plasma cool down rapidly and suppress the expansion of the surface plasma.This can reduce the absorption effect of the surface plasma on the subsequent laser energy,so as to obtain longer filamentary track in sapphire.Based on the experimental analysis and theoretical simulation,in this paper,the regulation of atmosphere on the filamentation induced by picosecond laser in sapphire was systematically studied.It is presented and verified that the ambient gas can provide a reliable way to regulate the filamentation in the transparent solid medium,which expands the regulation of ultrafast laser filamentation.The presented mechanism provides a theoretical basis for improving the accuracy of atmosphere regulation,further promotes the development of ultrafast laser processing based on non-linear effect.