Modulation of Crack Generation inside a LiF Single Crystal by Interference of Laser Induced Stress Waves

When a femtosecond laser pulse is focused inside a LiF single crystal, four cracks are generated in the <100> directions from the photoexcited region. The time-resolved observation have elucidated that the tensile stress in the laser induced stress wave induces the generation and propagation of the cracks. In this study, to investigate the possibility of control of laser induced cracks inside single crystals, the interference of stress waves were generated by simultaneous fs laser irradiation at multiple spots inside a LiF crystal, and the effects to the crack formation were investigated. We found that the crack lengths depended on the photoexcited spots' distribution. The dependence was explained by different transient density distributions by different interference of stress waves. The simulated density distribution indicated that a compressive stress at a crack tip could prevent the crack from propagating further and an expansive stress along a crack could facilitate the crack propagation.