Transient reflectivity traces measured for nanometer-sized films of the
topological insulator Bi2Se3 revealed GHz-range oscillations driven within the
relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV
photon energy. These oscillations have been suggested to result from acoustic
phonon dynamics, including coherent longitudinal acoustic phonons in the form
of standing acoustic waves. An increase of oscillation frequency from ~35 to
~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the
interplay between two different regimes employing traveling-acoustic-waves for
films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR)
modes for films thinner than 40 nm. The amplitude of oscillations decays
rapidly for films below 15 nm thick when the indirect intersurface coupling in
Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The
frequency range of coherent longitudinal acoustic phonons is in good agreement
with elastic properties of Bi2Se3
