Heat transport in solid target following relativistic laser–matter interaction

Recent measurements of energy transport along the surface of a planar Al target excited by obliquely-incident, 25 fs laser pulses focused to intensity up to 3 × 1018 W/cm2 in a ∼1 μm spot are analyzed using two-dimensional collisional particle-in-cell simulations. In the experiments, the target absorbed ∼50% of the pump energy, and surface heat, detected by imaging a delayed reflected probe pulse, propagated roughly isotropically outward from the pump spot as far as 12 ± 3 microns within 0.5 ps. The simulation without a pre-plasma predicts only 12% pump absorption and anisotropic heat transport. Moreover the simulated heated area increased as the square root of laser intensity, more slowly than observed in the experiment. The discrepancies are attributed to the presence of an intensity-dependent pre-plasma in the experiment that increased absorption and its intensity scaling. Anisotropic features are spoiled by strong magnetic fields that pervade the pre-plasma.