Modeling thermal emission in dental enamel induced by 9–11 μm laser light

Modeling the energy redistribution of laser radiation incident on biological tissue relies on the accurate determination of the optical properties of the tissue. Dental enamel has high absorption coefficients in the 9–10 μm range and therefore conventional transmission experiments cannot be used to determine these coefficients. One method that is useful for determining optical and thermal properties is time-resolved photothermal radiometry. Enamel was irradiated at fluences up to 2 J/cm2 with 9.3 μm, 9.6 μm, 10.3 μm, and 10.6 μm light from a CO2 laser with a pulse duration of approximately 100 μs. Estimations of the absorption coefficients of the materials were conducted at these wavelengths by introducing the absorption coefficient and emission coefficient of the material as a variable in a one dimensional heat conduction model. The model estimates the surface and subsurface temperature increase in a material due to laser heating based on the material's thermal and optical properties. Using this method, absorption coefficients for enamel are determined to be approximately 800 cm−1 for 10.6 μm, 1150 cm−1 for 10.3 μm light and 5000+ cm−1 for 9.3 and 9.6 μm light. The investigations considered the dependence upon the spectral properties of the region selected for detection which affect both the maximum value and the temporal evolution of the thermal emission. The real emissivity can lead to errors in the temperature measurements when the detected energy comes from a broad region of the spectrum where the emissivity is not constant. This study helps determine values for parameters used in models that predict subsurface temperature rise or ablation thresholds. It is a key step in determining the mechanism of ablation of dental enamel using lasers with wavelengths from 9–11 μm.