Pulsed laser damage in thin film coatings: Fluorides and oxides

Abstract The absorbing-inclusion model of pulsed-laser-induced damage in thin films is reviewed and generalized. A solution is derived which can accommodate arbitrary absorption functions and pulse shapes. It is shown that this model applies to regions which were previously excluded. This occurs when the wavelength dependence of Mie absorption by a small inclusion in a thin film is taken into account. When this is done an additional wavelength dependence of the imaginary index of refraction is observed. That is, the localized absorbing region exhibits an intrinsic absorption wavelength dependence. The model reveals that fluoride film damage is thermally dominated and dependent on the thermal properties of the film materials with absorption that appears generally to be material independent but wavelength dependent. Furthermore, it also shows that thermal diffusion plays an important part in oxide film damage; however, there is apparently some material dependence not accounted for in the present model. It is postulated that this material dependence enters through the absorption process. As might be expected, absorption in oxide films is also shown to be wavelength dependent.