Measurement of thermal diffusivity of multilayer optical thin film systemsusing photoacoustic effect

In other to develop ultrahigh-power laser systems, laser mirrors must be hi ghly resistant to thermal shock and effectively cooled: therefore, the stud y of optical multilayer systems which have high thermal diffusivity has bec ome important. In this study, we designed and developed two-layer antirefle ction (AR) optical coating samples and five-layer high-reflection (HR) opti cal coating samples on glass substrate under different evaporation conditio ns of coating speed (10, 20 Angstrom/s) and substrate temperature (50, 100, 150, 200 degrees C), using two dielectric materials MgF2 and ZnS which hav e different refractive indices, and the through-plane thermal diffusivity w as measured using the photoacoustic effect. The thermal diffusivities of th e 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 Angstrom/s and substrate temperature is 150 degrees C, and is closely related to the morphology of the films.