Quantized lattice vibrations called phonons are the primary heat carriers in semiconductors and certain metals. Mathematically, phonons are defined as the eigenmodesof an idealized crystal lattice with perfectly harmonic potential, and so, can propagate through the crystal without damping when a non-equilibrium distribution of phonons is established. Weak perturbative anharmonicities and defects/boundaries/imperfections in real crystals, however, cause damping of these phonon modes through phonon-phonon and phonon-defect scattering respectively. While first principles computational techniques have enabled accurate descriptions of some of the scattering processes that the phonon modes undergo [1-3], it has been challenging to extract any scattering-related information experimentally, due to the difficulties in exciting individual phonon modes in a non-equilibrium thermal experiment. In this talk, I will describe the theory and implementation of our recent work on the mode-resolved phonon scattering spectroscopy using the transient grating experiment, that enables the extraction of microscopic phonon scattering information, such as phonon mean free paths, directly from a thermal experiment. I will also show some results on the mode-resolved surface scattering rates of phonons in silicon nanofilms extracted using this technique .
 K. Chen*, B. Song*, N. Ravichandran* et al., Science 367(6477), 2020. [*: Equal contribution]
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 F. Tian, B. Song, X. Chen, N. Ravichandran et al., Science 361(6402), 2018
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