High-quality diffractive optical elements on sapphire efficiently processed using shaped femtosecond laser

Sapphire, which possesses excellent optical and electronic properties, is widely used in optoelectronic components. However, femtosecond laser processing is often limited by high surface roughness, poor processing stability, and low efficiency. In this work, a pulse train (PT) with subpulse intervals ranging from 10 to 1000 ps was generated using a Fabry–Perot cavity. Using a single-pass direct writing followed by chemical etching in 15% HF solution for 1 h, extremely regular large-area gratings and 5×5 Dammann gratings were fabricated on sapphire. A low surface roughness of 3.2 nm and depth fluctuations ±15 nm were achieved. Under 680 nm illumination, the background noise in the diffraction pattern of the Dammann grating exhibited minimal, and the total diffraction efficiency and beam uniformity were as high as 53.2% and 93%, respectively, closely matching the design targets. The results of ultrafast dynamics, including time-resolved plasma emission, indicate that high-quality, efficient, and stable PT laser processing is attributed to the powerful ablation cooling effect, multiple ionization, vaporization of ejecta, and complete suppression of periodic ripples. This method can be further applied to engrave functional elements on three-dimensional hard and brittle crystal structures.