Ultrafast observation of multiple shock waves evolution and interaction processes in femtosecond laser processing

Revealing the expansion and interaction dynamics of multiple shock waves (SWs) induced by a femtosecond laser is important for controlling laser processing. However, the dynamics of SWs is a complex and ultrafast process, making it difficult to determine the specific laws that govern their evolution. In this study, we observed the various evolutionary and interaction processes of SWs generated by a femtosecond laser on fused quartz using pump-probe shadowgraphy. First, we discovered a time-invariant hemispherical plasma chamber (HPC) composed mainly of air plasma before the expansion of material SW, differing from other studies wherein the plasma chamber is usually not observed. Second, the coupling process between the plasma pressure waves (PPWs) and the material SW was studied. After the expansion of material SW, the front plasma of the HPC was ionized again, generating two PPWs in opposite directions and affecting the further expansion of material SW. Eventually, the material SW broke through the HPC and PPWs and evolved into a single SW that expanded continuously. Additionally, it was demonstrated that double pulses with different time delays could effectively control the SW coupling process and plasma evolution, thereby improving the laser-processing efficiency.