Interaction studies of multimode pulsed HF laser with enamel tissue using photothermal deflection and spectroscopy

The interaction of a multi-line (2.6-3 micro m) HF laser with enamel tissue is reported. The etch rate experiment showed an ablation threshold fluence of 47 Jcm(-2), but using the dispersion theory for optical coefficients of n and k for enamel at dominant laser lines, this value reduced to congruent with 25 Jcm(-2), making it more comparable with other investigations. The main mechanism of enamel ablation at 2.78 microm where its absorption is very high is thought to be due to microexplosive evaporation of water within the enamel matrix, i.e., thermomechanical decomposition. Modelling showed that with 46% reflectivity from the enamel surface, the maximum temperature rise will be about 500 degrees C, which is much less than the melting point of hydroxyapatite. Also, using a photothermal deflection technique showed that, at fluences below threshold a considerable amount of heat was emitted from the surface, which was detected as a single compressive wave. The spectroscopic studies indicated that calcium was the main chemical element observed in the plasma with emission spanning from 420 to 620 nm.