Thermal Diffusivity Measurement of Two-Layer Optical Thin-Film Systems Using Photoacoustic Effect¹

Abstract

In this study, we designed and developed two-layer antireflection (AR) optical coating samples on glass substrates, using different evaporation conditions of coating rates and substrate temperatures for two dielectric materials, MgF₂ and ZnS, with different refractive indices. The through-plane thermal diffusivity of these systems was measured using the photoacoustic effect. The optical thicknesses of MgF₂ and ZnS layers were fixed at 5λ/4 (λ=514.5 nm) and λ, respectively, and the thermal diffusivities of the samples were obtained from the measured amplitude of the photoacoustic signals by changing the chopping frequency of the Ar⁺ laser beam. The results demonstrated that the thermal diffusivity of the sample fabricated under the conditions of 10μÅ·s⁻¹ and 150°C had the maximum value and that the results were directly related to the microstructure of the film system.