Pump-probe microscopy reveals the mechanisms of double-pulse processing of metals

Recent advances in ultrafast laser technology have enabled the delivery of high pulse energies at MHz repetition rates, but often at peak fluences well above the optimal range for efficient ablation. Temporal pulse shaping can mitigate this issue by distributing energy across multiple sub-pulses separated by defined inter-pulse delays. However, when delays are too short, interaction of preceding pulses with the ablation plume severely reduces ablation efficiency. This study investigates the physical origins of such efficiency losses in double-pulse ablation of copper and steel. Ablation depth and efficiency were measured for inter-pulse delays between 12.2 ns and 231.8 ns. These results were compared with time-resolved pump–probe microscopy measurements of the transient reflectance to quantify plume shielding. A strong correlation is found between second-pulse efficiency and plume transmittance. Plume shielding is identified as the dominant mechanism limiting ablation efficiency in both materials. For copper, redeposition further reduces efficiency, leading to strongly reduced net ablation at inter-pulse delays shorter than 50 ns. Maximum losses of up to 63% in Cu and 21% in steel are observed at inter-pulse delays of 12.2 ns. Full recovery of second-pulse efficiency is achieved at inter-pulse delays exceeding 150 ns for Cu and 30 ns for steel, corresponding to maximum feasible repetition rates of 6.7 MHz and 33 MHz, respectively. These findings highlight the importance of optimizing inter-pulse delay in double-pulse laser processing and establish time resolved microscopy as a powerful tool for predicting inter-pulse delay dependent ablation efficiency in double-pulse laser processing.