Lifetimes of confined acoustic phonons in ultrathin silicon membranes
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We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultrathin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from ∼4.7 ns to 5 ps with decreasing membrane thickness from ∼194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.
Cuffe, J., Ristow, O., Chávez, E., Shchepetov, A., Chapuis, P-O., Alzina, F., Hettich, M., Prunnila, M., Ahopelto, J., Dekorsy, T., & Sotomayor Torres, C. M. (2013). Lifetimes of confined acoustic phonons in ultrathin silicon membranes. Physical Review Letters, 110(9), . https://doi.org/10.1103/PhysRevLett.110.095503