Direct measurement of lattice behavior during femtosecond laser-driven shock front formation in copper

Abstract

Femtosecond laser-driven shock waves exhibit characteristic features that form distinctive microstructures not formed by plate impacts or nanosecond laser-driven shock waves. A key to understanding this phenomenon is understanding the lattice behavior inside the shock front, which is the boundary between the ambient and shock compression states. However, direct measurements of the lattice spacing inside a femtosecond laser-driven shock front have not yet been performed. Here, we report in situ measurements of lattice spacing using x-ray free electron laser diffraction with a pulse width of <10 fs during the shock rise in single-crystal copper irradiated directly in air with a femtosecond laser pulse on the order of 1014 W/cm2 at a pulse width of 101 fs. The lattice spacing of the femtosecond laser-irradiated single-crystal Cu (002) plane starts to compress 6.3 ps after femtosecond laser irradiation. It takes 15.7 ps for the plane to reach peak compression, at which point the compressive elastic strain is 24.3%. Therefore, the shock front was found to form at an elastic compressive strain rate of 1.55 × 1010/s in this shock-driving situation. It is suggested that the initiation of plasticity under such ultrafast deformation at the most elastic compression is based on both dislocation multiplication and dislocation generation mechanisms.