Measurement of Thermal Diffusivity of Multilayer Optical Thin Film Systems Using Photoacoustic Effect

In other to develop ultrahigh-power laser systems, laser mirrors must be highly resistant to thermal shock and effectively cooled; therefore, the study of optical multilayer systems which have high thermal diffusivity has become important. In this study, we designed and developed two-layer antireflection (AR) optical coating samples and five-layer high-reflection (HR) optical coating samples on glass substrate under different evaporation conditions of coating speed (10, 20 Å/s) and substrate temperature (50, 100, 150, 200°C), using two dielectric materials MgF 2 and ZnS which have different refractive indices, and the through-plane thermal diffusivity was measured using the photoacoustic effect. The thermal diffusivities of the samples were obtained from the measured photoacoustic signals by changing the modulation frequency of the Ar + laser beam. The results indicated that the thermal diffusivity is maximum when the coating speed is 10 Å/s and substrate temperature is 150°C, and is closely related to the morphology of the films.