An insight into the triple-laser processing mechanism for producing nanometer-scale smooth surfaces on single-crystal diamond

Diamond is a promising wide-bandgap semiconductor for high-power and extreme-environment applications, but femtosecond laser processing of single-crystal diamond often suffers from irregular surface morphology and residual stress concentration due to crystal anisotropy. This study proposes a triple-laser processing strategy to suppress parameter sensitivity and crystal anisotropy-induced defects in diamond processing. First, continuous-wave CO₂ laser irradiation is used as a uniform thermal pretreatment to generate sp²-hybridized carbon structures on the diamond surface, thereby enhancing laser absorptivity. Subsequently, picosecond YAG laser processing reduces the effective ablation threshold through multi-pulse energy deposition and promotes stress relaxation within the modified surface layer. Finally, femtosecond laser irradiation enables nanoscale surface finishing via high-peak-power-induced multiphoton absorption. High-resolution transmission electron microscopy and complementary characterization reveal that CO₂ and YAG laser treatments induce a pre-graphitized surface layer, which reduces the energy input and thermal effects during subsequent femtosecond laser processing. This layer shifts the material removal pathway from phase transformation followed by material removal to direct removal of graphitized carbon during femtosecond laser processing, thereby reducing parameter sensitivity, phase transformation-induced stress, and crystal-orientation dependence. Thermal stress calculations demonstrate that the proposed strategy effectively mitigates anisotropic effects in single-crystal diamond. A nanoscale flat surface with a roughness of 3 nm Ra was obtained, while subsurface defects, including dislocations, modified layers, and microcracks, were markedly suppressed. These findings reveal the atomic-scale response mechanisms of diamond during triple-laser processing and provide a theoretical basis for the ultra-precision micro/nano manufacturing of diamond.