Extension of a crack at a specific location has been observed inside a LiF single crystal after parallel photoexcitation at multiple spots by focused femtosecond (fs) laser pulses. In this study, to elucidate the mechanism of crack extension at a specific location, we observed the dynamics of crack propa-gation in a LiF single crystal after parallel fs laser irradiation by a pump-probe microscope. The ob-servation showed that there are fast and slow propagation dynamics; the velocity of the faster prop-agation is comparable to the sound velocity, whereas that of the slow propagation was more than ten times slower than the sound velocity. The slower propagation was observed only in extended cracks. In addition, we observed the transient birefringence distribution during slower propagation (10-24 ns) and found that the birefringence was distributed around normal cracks while there was little strain distribution around extended cracks. Therefore, we interpreted that residual strain, which is the origin of the observed birefringence, in tens nanoseconds could suppress the extension of normal cracks and the slower propagation appeared as a result of the disappearance of the residual strain.
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