  We explore the phonon dynamics of thin films of the topological insulator
material Bi2Se2Te using ultrafast pump-probe spectroscopy. The time resolved
differential reflectivity of the films exhibit fast and slow oscillations. We
have given a careful analysis of variation of phonon frequency as a function of
film thickness attributing this to existence of standing acoustic modes.
However, no variation in the frequency of the optical phonon modes was found
with film thickness. This indicates that the optical phonons intrinsically
belong to the surface of the topological insulators. The fact that the acoustic
phonons can be tuned by changing the film thickness has tremendous potential
for room temperature low power spintronic devices and in topological quantum
computation.
