Time-resolved imaging of CaF2 poly-crystal response following 355 nm nanosecond laser irradiation

Abstract

The evolution of 355 nm nanosecond laser-induced damage to ${{\rm CaF}_2}$ poly-crystals is investigated by using the time-resolved pump-probe shadowgraph technique. The damage morphologies of the front surface, rear surface, and interior of the ${{\rm CaF}_2}$ crystal are imaged by optical microscopy. When the rear surface is focused by one laser pulse throughout the front surface, three shock waves (SWs) and one SW are observed in the air beside the front and rear surfaces, respectively. When the laser energies are 40 mJ and 60 mJ, at delay time of 1000 ns the radii of SW fronts beside the front surface are 2569.8 µm and 2831.7 µm, while those beside the rear surface are 1012.9 µm and 1078.1 µm, respectively. The filamentary channels inside the ${{\rm CaF}_2}$ crystal are established before the end of a laser pulse at energies of 25, 40, or 60 mJ. The average propagation velocities of SWs along the filamentary channel are approximately 8.2 µm/ns. The maximum diameters of channels can reach approximately 53 µm and 128 µm for 25 mJ energy and 40 mJ energy, respectively. The experimental results help to explore the mechanism of laser-induced ${{\rm CaF}_2}$ poly-crystal damage in nanosecond regime.