Relation between subsurface formation mechanism and ablation efficiency during femtosecond laser processing of polycrystalline alumina

Femtosecond laser processing is highly effective for the ultra-precision machining of ceramics. However, in multi-layer processing of polycrystalline alumina, a significant challenge arises: the material removal efficiency sharply decreases and saturates as the number of pulses increases. To clarify this mechanism from a microscopic perspective, this study analyzed the subsurface structure of polycrystalline alumina processed near the ablation threshold (2.6 J/cm 2 ) using transmission electron microscopy (TEM). The observations revealed "selective amorphization," where only specific crystalline grains were amorphized immediately after the first layer was processed, along with the formation of nanovoid chains at grain boundaries. These modified layer morphologies remained almost identical regardless of the number of layers (N = 1 to 10). This suggests that energy reflection and stagnation, caused by the anisotropy of the four-photon absorption process at a wavelength of 515 nm and the orientation mismatch with adjacent grains, occur during the initial layer irradiation. These results suggest a mechanism in which the first layer of irradiation instantaneously transforms the pristine bulk into a modified layer with scattering and shielding properties, thereby inhibiting effective energy propagation into the underlying bulk material.