Formation of an extended nanostructured metal surface by ultra-short laser pulses: single-pulse ablation in the high-fluence limit

The threshold character of ultra-short-pulse laser ablation allows the formation of sub-diffraction-limited structures. In order to achieve nanostructuring of an extended area in reasonable production times, parallel production is highly desirable. In this paper we analyze the results obtained by nanostructuring using a self-assembled microlens array formed by deposition of quartz spheres directly on a noble-metal surface or on a quartz spacer layer. The quartz spheres are removed by a single laser pulse, so the structures formed are the result of single-shot ablation. The size of the holes formed depends on the laser fluence and the thickness of the transparent spacer layer. The hole depths are significantly larger than the optical penetration depth, indicating that heat diffusion plays an important role. The results are analyzed by solving the two-temperature diffusion model numerically in one dimension. The results from the numerical simulation lead to the formulation of a simple analytical model for the ablation at high fluence, which reproduces the results of the simulation quite well and is in qualitative agreement with the experimental data.