Study on the material removal mechanism in two-step ultrashort pulse laser polishing of single crystal diamond

Single crystal diamond (SCD) is widely used in high-end optics, semiconductors, and nanostructured devices owing to its exceptional physical and chemical properties. However, its extreme hardness and chemical inertness pose significant challenges for achieving high-quality surface polishing. In this study, a two-step ultrashort pulse laser polishing strategy is proposed, employing a high-energy, low-frequency laser for efficient material removal, followed by a low-energy, high-frequency laser for surface refinement. A key feature of this approach is the correlation between laser ablation depth and the maximum profile height (Rt) inherited from the preceding processing step, which enables the establishment of a physically guided range of process parameters for controlled polishing. The effects of laser parameters on material removal depth, surface roughness, morphology, and chemical composition of SCD were systematically investigated. The results show that the surface roughness Sa is reduced from 165 nm to 33 nm, corresponding to an approximately 80% reduction. Surface chemical composition analyses indicate that laser-induced oxidation is promoted during the first polishing step and subsequently suppressed during the second step, accompanied by the effective removal of particles and debris. This surface evolution leads to enhanced hydrophilicity, which is beneficial for improving uniformity and efficiency in subsequent chemical mechanical polishing. Raman spectroscopy confirms that no graphitization or residual stress is introduced under optimized conditions, demonstrating the advantage of ultrashort pulse laser processing for SCD. This study clarifies the material removal mechanism in two-step laser polishing of SCD and provides a robust and scalable approach for fabricating high-quality diamond devices.