Orthogonal lattice distortions inside crystalline Si upon sub-threshold femtosecond laser-induced excitation

Abstract

Material processing with femtosecond lasers has attracted enormous attention because of its potential for technology and industrial applications. In parallel, time-resolved x-ray diffraction has been successfully used to study ultrafast structural distortion dynamics in semiconductor thin films or surface layers. However, real-world processing applications mostly are concerned with bulk materials, which prevents the use of x-ray surface based techniques. For processing applications, a fast and depth-sensitive probe is needed. To address this, we present a novel technique based on ultrafast x-ray dynamical diffraction (UDD) capable of imaging transient strain distributions inside bulk crystals upon laser excitation. This pump-probe technique provides a complete picture of thetemporal evolution of ultrafast distorted lattice depth profiles. We demonstrate the potential of UDD by studying a thin Si single crystal upon single pulse femtosecond optical excitation. Our study reveals that below the melting threshold strong lattice distortions not only longitudinal, but also transversal to the propagation of the strain wave appear on picosecond time scales along the single crystal. The observation of this transversal deformation after laser excitation contradicts previous work that were not able to observed it, what could be related to the high sensitivity of dynamical diffraction with respect to the lattice distortions. The speed of propagation of this ultrafast transversal strain deformation is observed to be slower to the longitudinal sound speed for Si as described in the bibliography.