X-ray free electron laser observation of ultrafast lattice behaviour under femtosecond laser-driven shock compression in iron*

Understanding the natures of shock compression of condensed matter has been an important subject over the past century. A femtosecond laser emerged as a new shock-driver approximately 20 years ago. Femtosecond laser-driven shock wave generates unique microstructures inside materials unlike conventional shock waves. Therefore, properties of this shock wave may differ from conventional ones, however, the lattice behaviours under the femtosecond laser-driven shock compression have never been clarified. Here we report the ultrafast lattice behaviours in iron shocked by a direct irradiation of a femtosecond laser pulse diagnosed using X-ray free electron laser diffraction. We found that the initial compression state caused by the femtosecond laser-driven shock wave is the same as that caused by conventional shock waves. We also found that the temporal deviation of peaks of stress and strain waves for the first time experimentally, which was predicted theoretically. Furthermore, the existence of plastic wave peak between the stress and strain wave peaks is a new finding that has not been predicted even theoretically. Our findings will open up new avenues for the development of novel materials that combine strength and toughness in a trade-off relationship.