Crack formations inside a LiF single crystal by focusing a femtosecond laser pulse with controlled astigmatism

Abstract

After the photoexcitation inside a LiF single crystal by a focused femtosecond (fs) laser pulse, cracks are generated from the photoexcited regions [1]. Because the cleavage plane and direction in a LiF crystal are (100) and , respectively, four cracks are generated in the four directions when a fs laser pulse is incident normal to the (100) plane of the crystal. To apply fs laser induced crack formation to scribing of the crystal, unnecessary cracks must be suppressed, and formation of cracks only in a single direction is desirable. One possible method to suppress unnecessary cracks is to modulate the intensity distribution of fs laser pulse at the focus, because the distribution of thermal stress depends on the distribution of photoexcited volume. The simplest method to modulate the intensity distribution of laser beam at the focus is to make a line-shaped intensity distribution at the focus by inducing astigmatism in the beam. A line-shaped intensity distribution could generate larger stress in the specific regions and induce crack formation in the specific direction. In this study, we investigated the possibility that the fs laser induced crack formation inside a LiF single crystal could be controlled by a fs laser pulse with controlled astigmatism.