Study Of Short-pulse Laser Materials And The Failure Mechanism

Short pulse laser propagation and induced damage in dielectric materials, optical films and instantaneous thermal conduction in materials under short pulse laser irradiation are studied by theoretical modeling and numerical simulation.By calculating the electron density increase process based on Fokker-Planck equation, SiO2's damage thresholds under various laser wavelength and pulse duration are obtained which agree with related experiment results. The damage threshold decreases when laser pulse duration gets shorter, but it varies rather slightly for different wavelength due to the combination of sevcral effects. The role of impact ionization and photoionization to the damage threshold is discussed. When laser pulse duration is greater than lps, impact ionization dominates the material breakdown; when laser pulse duration is shorter than lps, the effect of photoionization become more prominent, but impact ionization is still of importance.The change over time of electron density and electric field of laser at different positions are calculated by solving the propagation equation and electron kinetic equation simultaneously. Reflection, transmission and energy deposition when propagating in SiO2 material of different energy density laser beams are discussed. Reflection and energy deposition increase with increasing laser energy density, transmission decrease on the contrary. Damage depth in material induced by laser beams reaches a maximum value and then to decline slowly as laser energy density increase. The longer the laser pulse duration, the deeper the damage depth will be.Based on standing-wave theory, a theoretical model with high energy absorption at the surface and interface is employed to calculate multilayer's temperature distribution and its evolution under repetition pulse laser irradiation. By comparing the results of repetition pulse laser and single pulse laser, it is found that when calculating the thermal effect of repetition pulse laser heating on multilayers, for some cases one can approximately simplify the repetition pulse laser to a single pulse one with the same radiation time and total energy density. For laser beams of same total energy density, because of thermal diffusion, the longer the pulse duration, the lower the temperature rise in the multilayers. The temperature distribution and the damage threshold for a high reflectance film under various wavelength show great difference.To analyze instantaneous thermal conduction in materials, temperature distribution and evolution in an aluminum sample and a stainless steel sample irradiated by 5ns and 5ps laser pulse is calculated based on the Fourier law and non-Fourier thermal wave theory respectively. When laser pulse duration is much larger than material relaxation time, the Fourier law and the non-Fourier thermal wave theory give nearly the same temperature distributions, which implies thermal wave effect can be neglected, whereas when the pulse duration is close to or shorter than material relaxation time, different temperature patterns are derived from the non-Fourier thermal conduction and Fourier thermal conduction models, with the former exhibiting remarkable thermal wave effect.