Controlling surface quality of sapphire via femtosecond laser ablation: synergistic effects of pulse overlap rates and phase-selective etching

Abstract Femtosecond laser micromachining technology has revolutionized the traditional single-process manufacturing paradigm, offering high-precision and high efficiency processing of hard and brittle material. This study investigates the role of pulse overlap rates in femtosecond laser ablation of sapphire crystals and develops a hybrid laser-etching technique for surface quality enhancement. Through experimentation with varying longitudinal ( δ a ) and transverse ( δ p ) overlap rates, three significant findings were demonstrated: firstly, quantitative determination of the ablation threshold (6.74 J cm −2 at 1030 nm/250 fs) accompanied by observed fluence-dependent groove morphology transitions; secondly, identification of δ a as the dominant factor controlling subsurface depth while δ p governs microgroove formation; thirdly, discovery of a phase-selective etching mechanism where laser-induced alumina phase transformation enables preferential material removal in KOH solution. The hybrid laser-etching strategy combining optimized laser parameters ( δ a = 74%, δ p = 83%) with subsequent chemical etching achieved a 57.5% reduction in surface roughness (374 nm versus the original 650 nm), demonstrating a strong synergistic effect between pulse overlap control and phase-selective etching. These results provide fundamental insights into overlap parameter effects in ultrafast laser processing and establish a practical methodology for precision surface engineering of transparent crystalline materials, with direct applications in optical component manufacturing and semiconductor substrate processing.