Ultrafast light-induced coherent optical and acoustic phonons in few quintuple layers of the topological insulator Bi2Te3

Ultrafast lattice dynamics of a few quintuple layers (QLs) of the topological insulator (TI) Bi2Te3 is studied with time-resolved optical pump-probe spectroscopy. Both optical and acoustic phonons are photogenerated and detected. Here, in order to get new insights into out-of-equilibrium electron-phonon coupling and phonons dynamics in confined TI, different nanostructures are investigated (single or polycrystalline QL assemblies and nanocrystallized islands). Contrary to previous literature claims, we show that even for nanostructures containing only 10 QLs, the symmetric A1g(I) coherent optical phonon is efficiently photogenerated and no restriction due to structural confinement appears. We also observe that whatever the arrangement of the nanostructures, the A1g(I) optical phonon features are similar (lifetime). We also report the observation of confined coherent acoustic phonons propagating from QLs to QLs whose spectrum is, this time, very sensitive to the atomic arrangement. In the case of the single-crystalline ultrathin film, time-of-flight analysis of these acoustic phonons provides direct estimates of the elastic properties of these nanostructures as well as some estimates of van der Waals interactions between QLs.