Observation of Laser Ablation and Shock Generation in Silicon as a Function of Pulse Length at Constant Fluence*

We discuss the impact of laser-pulse length on ablation and shock generation in silicon while maintaining a constant fluence. The experiments were conducted on the OMEGA EP laser at the Laboratory for Laser Energetics. The targets consisted of three-layer planar structures comprised of Si, Cu, and SiO2 layers. The Si layer was irradiated by a frequency-tripled laser with varying pulse lengths (250 ps to 10 ns) at a constant fluence. The plasma conditions of the ablated plasma were determined by analyzing the time-resolved x-ray spectroscopy data. The analyses revealed that increasing the pulse length at a constant fluence decreases electron temperatures and densities. Furthermore, longer pulses with lower intensities lead to deeper ablation regions but a lower ionization balance in the silicon layer. Analyses of time-resolved interferometric and pyrometric data showed that the shock temperature, velocity, and pressure at the Cu–SiO2 interface also decrease as the pulse length is prolonged. Simulated profiles generated using radiation-hydrodynamics simulations FLASH agreed with experimental measurements. These findings stress the importance of laser-pulse length in plasma ablation and shock generation for laser impulse studies. This work is supported by the Department of Defense and Defense Threat Reduction Agency under award number HDTRA1-20-2-0001 and Johns Hopkins University.