Submicrometer surface shaping and surface repair of metals via laser ablation

Metal foils with precise thicknesses are required for use as laser targets in high energy density physics experiments. To achieve the micrometer scale tolerances accompanying these applications, the authors have developed a laser ablation technique utilizing burst pulses from a femtosecond laser operated in concert with a high-speed galvo scanning system. For authors’ application, an optimum laser processing format consisted of 250 fs laser pulses arranged in pairs, with the pulse separation in each pair equal to 400 ps and high galvo scanning speeds (∼2000 mm/s). These parameters enable the authors to thin and shape metals (depleted uranium, beryllium, tantalum, and iron) and remove unwanted variations in foil thickness over meaningful foil sizes. Double femtosecond pulses enable ablation that mitigates some undesirable pre-existing surface structures such as residual scalloping and scratches and lead to a surface roughness near 300 nm. Precise ablation rates were correlated to laser processing area and foil thickness. Finally, ablation rates were correlated to bulk surface temperature by tracking workpiece heating during ablation. These techniques yield the ability to apply highly controlled thickness maps with micrometer accuracy and tens of nanometer precisions. This control has demonstrated the ability to level foils with micrometer scale thickness deformities and create various programmed patterns that are difficult or out of reach of traditional machining methods.