Ultrafast spectroscopy of thermal transport and energy conversion

Abstract

We use ultrafast spectroscopy to investigate a number of ultrafast thermal transport processes, and this paper is intended to provide a review of two applications of the method: one on direct probing of the atomic vibrations in thermoelectric materials; the other on the thermal transport across metaldielectric interface. Atomic vibrational behaviors in thermoelectric materials are important for thermoelectric energy conversion. In misch-metal (Mm) filled (p-type) and single (La, Ba, or Yb) filled (n-type) antimony skutterudites, a high temperature thermoelectric materials for waste heat recovery, different filling species cause coupled vibrational modes between the guest atoms and the host lattice at different frequencies, which scatter phonons in different spectral spans and help to lower the lattice thermal conductivity and improve the thermoelectric figure of merit. The scattering processes of the different filling species are probed using ultrafast spectroscopy. Furthermore, using the Debye model for the measured lattice thermal conductivity together with the measured vibration frequencies and scattering rates, it is shown that the scattering due to the coupled vibrational modes has a considerable contribution to the suppression of lattice thermal conduction. Heat transfer processes at an interface between two materials are also of importance in many electronic devices. The electron-phonon coupling at a metal-dielectric interface is studied using the ultrafast spectroscopy, on the samples of gold thin films on silicon substrates. The two-temperature model is used to obtain the electron-phonon interface resistance or conductance at the gold-silicon interface, which quantifies the direct metal electron to dielectric phonon coupling strength. The effects of film thickness and probing wavelength are investigated in detail along with a Drude-Lorentz model to obtain a good estimate of interface thermal resistance/conductance. This work demonstrates the ultrafast spectroscopy as a powerful technique for thermal transport research.