Ultrafast observation of shock wave formation in aluminum under direct femtosecond laser irradiation

Shock wave formation in aluminum after the direct irradiation of a femtosecond laser pulse with an intensity of 1014 W/cm2 onto the metal surface in air was observed using frequency-domain interferometry with picosecond temporal resolution. This high resolution allows us to accurately evaluate arrival time and rise time of the wave before and after shock wave formation. The temporal evolution of the rear surface velocity of the metal film had an ultrafast rise at the wavefront of less than 5 ps and a two-wave structure. As the incident pump laser energy decreased or the metal film thickness increased, the amplitude of the first wave decayed and the time separation between the two waves increased. The relationship between the particle velocity and shock velocity indicated that aluminum was elastically compressed in a longitudinal stress of 185 GPa, reaching a strain of approximately 30%. The estimated elastic strain rate was 6 × 1010 s−1 at 500 nm in depth. Through a thermal nonequilibrium state in the early stage, aluminum becomes a metastable elastic Hugoniot state under such high longitudinal stress in a region deeper than the diffusion length of laser-heated electrons.