Femtosecond laser-induced sub-50-nm period nanogratings with ultrahigh uniformity on graphite under water immersion

Laser-induced periodic surface structures (LIPSS) have gained increasing attention in the field of micro/nano fabrication, although achieving sub-100-nm period LIPSS with high uniformity remains a significant challenge. In this work, towards deep-subwavelength LIPSS on highly oriented pyrolytic graphite (HOPG), we demonstrate that ultra-uniform nanogratings of sub-50-nm periods and near-10-nm groove widths can be stably prepared via 800-nm femtosecond laser scanning irradiation with a high-NA objective lens under water immersion. The resulting nanogratings of strong polarization dependence, exhibiting exceptional surface flatness, period stability, and structural integrity, tend to appear at near-damage-threshold fluence regime with an appropriate effective pulse number. It turns out that the water immersion condition can significantly reduce the thermal effects of femtosecond laser ablation on HOPG, and thus via a mild, incubation-like scanning ablation process occurring in the nanogrooves with a continuous or jumping manner, this deep-subwavelength grating can achieve robust elongation growth, ensuring its long-range uniformity as well as minimal deposited debris and structural defects. Interestingly, the different incubation extension mechanisms for the mutually perpendicular and parallel settings between scanning direction and laser polarization bring not only distinct effective-pulse-number windows and somewhat different grating qualities, but also different extension stabilities in nanograting stitching via overlapping scanning lines and thus the optimal scanning strategy of parallel setting for large-area processing. In short, this study presents a convenient laser-processing approach for high precision fabrication of sub-50-nm gratings on HOPG, which would provide new insights into micro/nano-fabrication for optoelectronic metasurfaces and physics of the interaction between ultrafast laser and graphite.