Optical Pump-Probe Thermoreflectance Imaging for Anisotropic Heat Diffusion

Thermal diffusion in anisotropic and inhomogeneous materials are investigated by full-field thermal imaging using optical pump-probe thermoreflectance microscopy. Target sample material is a composite of nickel clusters, approximately 10 microns in size, distributed inhomogeneously within polydimethylsiloxane (silicone) resin. Such a material presents special challenges when characterizing thermal parameters. For example, the nickel clusters are similar in size to the optical excitation spot size when performing ultra-fast time domain thermoreflectance measurement. This introduces uncertainty as to whether one is measuring the material average response over areas larger than the embedded metal clusters, or the local response of individual clusters. The proposed optical pump probe imaging method can separate material average (macroscopic) and local (microscopic) properties by characterizing sample surface temperature change over both large and small excitation and probe regions. Experimental measurements are compared to both an analytic heat spreading model and finite element numerical simulation. Material anisotropy in the analytic solution is modeled using a well-known effective thickness method, which shows good agreement with the numerical simulation. Results suggest the proposed characterization method and associated analytical solution are applicable for thin or film-like materials with inhomogenous internal composite distribution and anisotropic thermal conductivity.