Delay-Selected Energy Coupling and Ablation Enhancement in Femtosecond Double-Pulse Irradiation of Aluminum

We investigate the effect of inter-pulse delay on femtosecond laser ablation of thin aluminum films using a dual-pulse setup at 800 nm with precisely controlled temporal separation. The ablation threshold, measured by an extrapolation method, first decreases and then increases with delays, reaching a minimum at ~1 ps. To reveal the underlying mechanism, time-resolved pump–probe reflection microscopy at 400 nm is employed. The transient reflectivity exhibits an ultrafast drop upon excitation, attributed to enhanced intraband absorption arising from Fermi smearing in the sp-band. At delays near 1 ps, a clear inflection in the reflectivity transient indicates the onset of surface thermal expansion and density reduction, which impedes efficient energy deposition into deeper material by the second pulse, thus lowering the ablation threshold. These results provide direct time-resolved evidence linking transient thermomechanical responses to delayed ablation threshold in femtosecond laser processing of aluminum.