Anomalous Absorption in Optical Coatings

The observation of isolated damage sites in optical thin films subjected to pulsed laser irradiation has lead to a model based upon localized absorbing regions. This “inclusion initiated” model does not adequately treat several details of the radiation absorption process, but relies principally on thermal conduction as the sole energy transport process. Despite these rather severe shortcomings, qualitative predictions of this theory agree remarkably well with much of the quality experimental data. On the other hand, quantitative agreement can only be achieved if absorption coefficients several orders of magnitude higher than those measured for todays thin films are employed, which themselves are already very high compared to the materials value in bulk form.

In order to explain this apparent discrepancy a detailed theory of the spatial and temporal evolution of the highly absorbing regions must be conceived. In this paper we present our initial steps in this direction. The absorption/thermal diffusion process that leads to damage is modeled by two equations; the evolution equation for the free carrier density and the thermal diffusion equation. In practice these equations are strongly coupled. We decouple the free carrier equation by assuming that all rate constants are independent of the lattice temperature. The solutions of these simplified equations are presented along with their sensitivity to the magnitude of several of the unknown rate constants. Those regions of parameter space in which the original thermal diffusion model is valid are pointed out, as well as the potential implications for more appropriate modeling of the absorption process as functions of experimental and material variables.