Spatially resolved thermoreflectance techniques for thermal conductivity measurements from the nanoscale to the mesoscale

Thermoreflectance techniques, namely, time- and frequency-domain thermoreflectance (TDTR and FDTR, respectively), are ubiquitously used for the thermophysical characterization of thin films and bulk materials. In this perspective, we discuss several recent advancements in thermoreflectance techniques to measure the thermal conductivity of solids, with emphasis on the governing length scales and future directions in expanding these advances to different length scales and material structures. Specifically, the lateral resolution of these techniques, typically on the order of several micrometers, allows for an understanding of the spatially varying properties for various materials. Similarly, limitations of TDTR and FDTR with respect to their volumetric probing regions are discussed. With a recently developed steady-state thermoreflectance technique, these limitations are overcome as probing volumes approach spot sizes. Finally, recent pushes toward the implementation of these techniques without the use of a thin metal transducer are presented, with guidelines for future avenues in the implementation under these specimen configurations.