Study Of Filament-fringes Induced In Sapphire By Picosecond Laser

Sapphire is the natural crystal of aluminum oxide（Al₂O₃）,also known as corundum.Due to its excellent scratch resistance,good optical transparency,thermal stability,chemical inertness and biocompatibility,it is a material with broad application prospects.However,its hardness and brittleness pose a great challenge to its high quality processing.Conventional techniques such as diamond sawing,mechanical dicing and breaking can result in excessive tool wear and undesirable microcracks and damage.The rapid development of ultrafast laser processing technology has shown great potential for the precise machining of sapphire.The high intensity of ultrafast laser manifested in numerous nonlinear phenomena.One of the most interesting effects in the condensed transparent materials is the effect of laser filamentation,which has been actively studied for about a decade.In this dissertation,we present the experimental results of the investigation of picosecond laser filamentation in sapphire.The influence of picosecond laser pulse repetition rate,energy and pulse numbers on the filamentary tracks,its ionization dynamics based on Keldysh theory and resulted structural modifications are investigated systematically.In this dissertation,we first analyzed experimentally the effect of pulse repetition rate,energy and pulse numbers by comparing picosecond laser induced structural modifications in sapphire.The observed structural modifications are compared and discussed in terms of lateral size and length of the modified zones.The induced structural modifications consisted of a well-localized central core surrounded by smooth change in refractive index in the form of cracks followed by a submicron filamentary track.As the input pulse repetition rate increases,the starting position of the modified area shifts away from the laser source direction and length of the filamentary track decreases.The input laser energy has also effect on the starting position of the modified area and length of the filamentary track.As the input laser energy increases,the starting position of the modified area shifts towards the laser source direction.The filamentary track length increases with increasing input laser energy and a maximum filamentary track of length 1180μm is observed at 107μJ input pulse energy.Moreover,analysis shows that with increasing pulse numbers,the starting position of the modified area shifts towards the laser source direction.However,no significant difference was observed in the filamentary track length by changing pulse numbers.Furthermore,pulse numbers is observed to be the primary cause of cracks surrounding the focal volume,while the input pulse repetition rate and energy play minor roles in these cracks.Finally,in order to seek answers regarding the experimental results,the ionization dynamics in the process of photoionization and avalanche ionization during laser-induced filamentation is theoretically analyzed by using Keldysh theory for the experimental parameters.The electron density growth as a result of plasma generation by photoionization and avalanche ionization processes calculated from the theoretical model based on the experimental data in the case of our research is greater than the critical electron density required to cause sapphire breakdown.Numerical simulations uncovered that the density of electrons along the filament and around the geometrical focus increases with increasing laser pulse energy.These simulation results are consistent with the experimental results,showing that the filament length increases with the increase of laser pulse energy.The maximum electron density obtained by avalanche ionization is in the range 10⁴⁹-10⁶¹ cm^-3,while the electron density of photoionization is in the range of 10¹⁰-10¹¹ cm^-3.As compared to photoionization electron density,the increase rate of avalanche ionization is higher with increasing energy,reaching approximately 10⁴55 orders of magnitude greater than the photoionization.Therefore,the structural damage near the geometrical focus could be most probably induced by avalanche ionization.Furthermore,inspection of the modified zones uncover that the transverse diameter of the damage zone increases with increase in the laser energy due to electron plasma produced by avalanche ionizations,but no increase in the filamentary track diameter is observed.Therefore,we observe filamentary track length with uniform diameter in the experiment.This dissertation studies the picosecond laser induced filamentation and its related mechanism in sapphire,which can provide basic research guidance for highly precise laser processing of sapphire based on the nonlinear effect of filament formation.