Heat diffusion and debris screening in Er:YAG laser ablation of hard biological tissues

The influence of heat diffusion on mid-infrared laser ablation of hard organic tissues was investigated by applying Er:YAG laser pulses with pulse lengths from 100 mu s to 1 ms and of varying energy to dentin and enamel slices without water-spray cooling. At short pulse durations, the onset of ablation is abrupt in both tissues under test. and the crater depths increase quasi-logarithmically with applied laser fluence as a result of debris screening of the crater bottom. In a quantitative analysis of this effect, the observed variation of crater diameters with the applied laser fluence must be taken into account. The influence of the pulse duration on the debris screening effect is demonstrated by an improved analytical model of the process. At longer pulse durations, the ablation efficiency is significantly reduced at fluences close to the ablation threshold, which results in a double-threshold character of the ablation process. This effect is attributed to diffusion of heat from the interaction region during the laser pulse. A simple analytical model of heat diffusion and ablation front dynamics is presented that predicts the fluence interval where this effect can be expected, depending on pulse duration and properties of the treated tissue.