Transparency measurement of lithium fluoride under laser-driven accelerating shock loading

摘要

Transmissibility limitation is one of the most important barriers in the use of windows for shock experiments if optical probes are used. In this article, we focus on the widely used window material lithium fluoride (LiF) and investigate its optical response under laser-induced shock-compression conditions. A long-pulse laser is shaped to create a continuous accelerating shock wave propagating through the LiF window in the range of 100-400 GPa. The variation of measured optical transmission with shock pressure shows that the LiF window stays transparent even when the shock stress is higher than 350 GPa and transforms to a total opaque state at about 400 GPa. The present experiment exhibits an obviously higher shock stress for LiF windows to lose transparency compared with previously reported results. The discrepancy in experimental results is considered to be due to the difference in shock thickness for a typical time scale at different experimental platforms. Meanwhile, the possible reasons for the discrepancy between our experimental data and the previous ab initio calculations can be suggested by the effect of thermal relaxation between electrons and ions or the overestimation of the absorption coefficient in theoretical calculations. This finding of higher transmissibility limitation may be of importance to researchers who use LiF as a window material in future shock-compression experiments, especially at the laser platform.