Thermal Transport by Individual Energy Carriers in Solid State Material

Knowledge of transport processes plays a critical role in the development and application of materials in many technologies. As manufacturing technologies continue to push the geometries of materials to smaller scales, traditional means of predicting and measuring transport properties begin to fail. Micro and nanoscopic effects tend to alter transport phenomena in materials, leading to new physics and different properties from the bulk state. In particular, the dynamics of thermal transport of a material varies greatly in both spatial and temporal senses. Different energy carriers have intrinsically different mechanisms of thermal transport; depending on the time and lengths scales in question, the contribution to the overall thermal transport by one carrier may be vastly different than others. To characterize and understand the dynamics of thermal transport at these small scales, novel ultrafast experimental techniques and theories are crucially needed. This work will discuss the efforts made to develop a framework to measure and differentiate the dynamics of transport processes of a material due to different energy carriers using ultrafast optical techniques. This dissertation is organized as follows.

Chapter 1 gives a background in the theory of thermal transport. This will serve as the foundation for the physical models that are used to extract thermal properties from experimental works. A brief review of the advances in ultrafast experimental and theoretical works will also be given. This will assist in placing this work in the context of ongoing work in the thermal transport community. Chapter 2 illustrates the experimental setups and physical models used to measure the effective thermal transport properties of thin film materials. Steady-state optical measurements are used to quantify the effective, in-plane, anisotropic, thermal conductivity of a 2D material. Time resolved, ultrafast optical measurements are used to quantify the effective, out-of-plane, thermal conductivity of a material. Chapters 3 and 4 demonstrate the capabilities of an ultrafast spatiotemporal scanning pump-probe system, where the high temporal and nanometric resolution measurements directly probe the electron contribution to thermal transport in metals as well as the ambipolar diffusion of carriers in semiconductors. Lastly, Chapter 5 summarizes this dissertation and provides a discussion on the use of the developed experimental capabilities to probe transport of emerging materials.