Dynamics of laser-induced phase transformation in bulk of monocrystalline sapphire

Abstract

Transient effects during and after the optical breakdown initiated by an intense sub-nanosecond laser pulse in bulk of monocrystalline sapphire were investigated using ultrafast time-resolved microscopy. The method provided direct quantitative information about temporal evolution of the laser-induced plasma channel as well as spatio-temporal characterization of the shock-waves. It was found that plasma channel evolved according to the classical model of moving breakdown, but in the initial phase there were signs of the laser-supported absorption wave (LSAW). The estimated maximum of the of absorption wave velocity in sapphire reached 340 km/s to decrease quickly to 53 km/s in the moment when the shock wave was emerged for the first time. At the final state of propagation, the front of the shock wave has converted to a spherical shape moving with a speed very close to the speed of acoustic wave in sapphire. The evolution of radii of the plasma channel and shockwave front were reasonably well fitted with the Sedov's scaling rule. The structural analysis of the post-irradiated site showed phase-transitions from single crystal to a polycrystalline or amorphous phase but only very close to the void walls.