Mechanism of surface quality enhancement in MHz burst femtosecond laser machining of single-crystal diamond revealed by in-situ observation

Diamonds are expected to have widespread applicability across various fields; however, owing to their extreme hardness, precise processing using conventional methods remains challenging. Among the different approaches, laser processing has attracted considerable attention owing to its non-contact nature, which enables machining without inducing subsurface damage, and its flexibility in creating complex, non-planar geometries. Burst pulse laser irradiation has been reported to improve surface quality; however, the underlying physical mechanisms remain unclear. In this study, the machining process of single-crystal diamond {100} surfaces was investigated through in-situ high-speed imaging combined with post-processing microscopic evaluation, to elucidate these mechanisms. The experimental results revealed that increasing the number of sub-pulses per burst generally enhanced the surface quality, particularly under low-energy irradiation, which may be associated with reduced air ionization as the energy delivered by each sub-pulse decreases. In the near-threshold regime, however, the surface quality exhibited a non-monotonic dependence, indicating an optimal processing window; an excessively large sub-pulse count per burst can destabilize material removal. In addition, the amount of optically visible scattered particles decreased and the deposited debris became finer with increasing number of sub-pulses per burst. This tendency may be associated with burst-induced plume interaction, including possible gas-phase cluster nucleation driven by subsequent sub-pulses within each burst. These tendencies are particularly pronounced under low-energy irradiation. The insights obtained from this study significantly advance the understanding of the material removal mechanisms involved in the ultrashort pulse laser machining of single-crystal diamonds. Ultimately, these findings can contribute to establishing robust and precise laser-based processing methods for highly brittle and difficult-to-machine materials.