Etch rate and spectroscopic ablation studies of Er: YAG laser-irradiated dentine

The interaction of a 200-µs Er:YAG laser at 2.94 µm with human molar dentine has been studied by ablation depth rate measurements as well as time-resolved and optical multichannel analyzer emission spectroscopy. Ablation rates indicate a threshold fluence of ~5.2 J cm−2 for significant material removal with a low-fluence (<20 J cm−2) effective optical absorption coefficient of ~700 cm−1. Deviation from a Beer’s law dependence is significant in the range ~20 to ~60 J cm−2 and indicates a maximum effective plume absorption of ~1200 cm−1 at ~40 J cm−2, coinciding with the appearance of strong line and broadband optical emission in the visible region. Time-of-flight emission measurements yield maximum species-resolved ablation velocities of up to ~1.2 × 106 cm s−1, enabling calculation of plasma temperatures. The results suggest that etch-rate characteristics are driven by changes in plume absorption dynamics, which have a strong dependency on incident laser fluence.