Modulation and mechanism of shockwaves induced on metals by femtosecond laser double-pulse

An ultrafast pump-probe shadowgraph imaging system and a laser induced breakdown spectroscopy (LIBS) system were used to investigate the modulation and mechanism of shockwaves of nickel (Ni) and aluminum (Al) induced by femtosecond laser double-pulse. With a suitable pulse delay, the bulge in the axial direction of the shockwave can be effectively eliminated and the shockwave expanded evenly in a hemispheric structure. The radial expansion distance of shockwaves of Ni and Al increased by 30% and 50% respectively with double-pulse excitation. The strong coupling of laser with liquid layer and plasma shielding effect were responsible for the enhancement of shockwaves within different regimes of pulse delay. Different enhancement trends were observed for the shockwaves of Ni and Al in the initial regime of pulse delay. Further study indicated that the reduced ablation volume of Al counteracted the enhancement effect of double-pulse on plasma and shockwave and it was the dominant mechanism for the stagnant of Al shockwave. The LIBS results of Ni and Al showed good agreement with the increasing tendency of shockwaves and confirmed the non-negligible effect of the removal volume on the intensity of plasma and shockwave. The results and mechanism exhibited the potential of femtosecond laser double-pulse in plasma and shockwave modulation and application.