The study of frequency-scan photothermal reflectance technique for thermal diffusivity measurement

A frequency scan photothermal reflectance technique to measure thermal diffusivity of bulk samples is studied in this manuscript. Similar to general photothermal reflectance methods, an intensity-modulated heating laser and a constant intensity probe laser are used to determine the surface temperature response under sinusoidal heating. The approach involves fixing the distance between the heating and probe laser spots, recording the phase lag of reflected probe laser intensity with respect to the heating laser frequency modulation, and extracting thermal diffusivity using the phase lag-(frequency)(1/2) relation. The experimental validation is performed on three samples (SiO2, CaF2, and Ge), which have a wide range of thermal diffusivities. The measured thermal diffusivity values agree closely with the literature values. Compared to the commonly used spatial scan method, the experimental setup and operation of the frequency scan method are simplified, and the uncertainty level is equal to or smaller than that of the spatial scan method.