Transient strain distributions during femtosecond laser-induced deformation inside LiF and MgO single crystals

Strain and stress dynamics inside MgO and LiF single crystals after photoexcitation by a focused femtosecond laser pulse were investigated by the observation of transient distributions of birefringence around the photoexcited region using a time-resolved polarization microscope. Both in MgO and LiF, propagation of two stress waves, which were attributed to quasi-longitudinal and quasi-transverse elastic waves, were observed, but crack propagation was observed only in LiF. Inside MgO, the observed strain distributions could be reproduced by elastic simulation, whereas inside LiF the strain distributions during crack propagation were largely different from the simulated ones; strain was widely distributed between cracks and the 〈110〉 regions in a stress wave, the strained region around the photoexcited region was smaller, and the strains in the 〈110〉 region and near the crack tips were larger than those by the simulation. The amplitudes of strain and stress in stress waves and temperature change in the photoexcited region were evaluated, and the origins of strain distribution change due to crack generation were discussed based on the differences between MgO and LiF