Speckled Laser Pump–Thermoreflectance Microscopy Probe to Measure and Study Micro/Nanoscale Thermal Transport: Numerical Simulation

Nondiffusive thermal transport in solids and their micro/nanostructures is a key subject in the research of micro/nanoscale heat conduction. A number of laser and optical techniques to measure or capture the nondiffusive behaviors of heat carriers have been developed, such as transient thermoreflectance, time-domain thermoreflectance (TDTR), transient thermal grating (TTG), and so on. Here, we propose a novel method to study micro/nanoscale heat transport, namely, speckled laser pump–thermoreflectance microscopy probe. In this technique, micrometer to few hundred nanometer size random heat spots are generated by a speckled laser pump pulse, and the time–space evolution of heat spots are recorded by thermoreflectance microscopy images of the probe pulses arriving at different delay times. Fourier transform is applied to analyze the thermoreflectance images and extract the thermal decaying time for different spatial frequencies and along different in-plane directions. Thermal conductivity at different spatial frequencies, which includes the nondiffusive transport information, is obtained in this way. By numerically performing simulation of anisotropic Brownian motion and solving phonon Boltzmann transport equations under the initial condition of random heat spots, we retrieve the preset anisotropic thermal conductivity and the nondiffusive behavior of reduced thermal conductivity with increasing spatial frequencies, proving the validity of this technique. The innovative method can also be applied to study electron and spin transports, and holds the potential to facilitate the experimental research and understanding of nanoscale energy transport.