Nanoscale thermal transport and phonon dynamics in ultra-thin Si based nanostructures

We study the dynamics of acoustic phonons in ultra-thin free-standing silicon membranes both experimentally and theoretically. We discuss the impact of the lifetimes of the acoustic phonons on the thermal transport properties of the membranes with thicknesses ranging from 8 nm to 1.5 µm. The phonon lifetimes are determined by measuring the dynamic variation of the reflectivity using ultra-fast pump-probe spectroscopy. This is achieved by asynchronous optical sampling (ASOPS) of two actively coupled femto-second laser oscillators. The coherent acoustic phonon lifetime is obtained from the dynamical variations of the reflectivity with a sensitivity of 10⁻⁵ and a time resolution of about 50 fs. The experimental results are compared to theoretical calculations considering both intrinsic and extrinsic relaxation scattering processes.