  Fully symmetric A1g phonons are expected to play a dominant role in electron
scattering in strong topological insulators (TIs), thus limiting the ballistic
transport of future electronic devices. Here, we report on femtosecond
time-resolved observation of a pair of A1g coherent phonons and their optical
control in two strong 3D TIs, Bi2Te3 and Bi2Se3, by using a second pump pulse
in ultrafast spectroscopy measurements. Along with well-defined phonon
properties such as frequency and lifetime, an obvious phonon chirp has been
observed, implying a strong coupling between photo-carriers and lattices. The
coherent phonon manipulation, on the other hand, allows us to change the phonon
amplitude selectively but does not affect either the frequency or coherence
lifetime of the chosen mode.
