Sub-micrometer pillar formation in fused silica using double-pulse back-surface laser ablation

This work presents an investigation into the characteristics of double-pulse, back-surface ablation of fused silica via the use of a picosecond pulse laser with a wavelength of 532 nm. The processing depth of back-surface ablation was found to be greater than that of front-surface ablation for a given laser power. The processing depth of back-surface ablation was further increased via the use of double-pulse ablation using an appropriate laser fluence combination for the first and second pulse relative to the single-pulse ablation threshold, Fth. Setting the laser fluence of pulses in ascending order (where the first pulse is below Fth and the second pulse is greater than Fth) is found to increase the processing depth. Furthermore, non-trivial features of the processing trace were found; an axisymmetric circular trench with a sub-micrometer scale protrusion at its center was observed in the case of double-pulse, back-surface ablation using laser pulses in an ascending order of laser fluence under certain conditions. The protrusion has a pillar-like shape with a diameter of several hundred nanometers; the pillar-like structure has a height of about 1.3 mu m from the trench bottom, and protruded 0.7 mu m from the back-surface plane. When the first pulse had a laser fluence of greater than Fth, the protrusion was not present and the shape was similar to that obtained when using a single-pulse ablation. The results of this work show the superiority of double-pulse, back-surface ablation of fused silica compared with the equivalent single-pulse technique in terms of enhanced throughput and the capability to create unique sub-micrometer structures.