  We have studied the polarization dependence of coherent phonons in the
topological insulator Bi2Te3. Using polarization-dependent femtosecond
pump-probe spectroscopy, we measured coherent phonons as a function of angle
when the pump and probe polarizations were fixed, and the crystal orientation
was rotated. For isotropic detection, depending on the spot position,
oscillations either from only low- and high-frequency phonons of A1g symmetry,
or in addition from the mode at 3.6 THz were observed. All the modes were found
to be independent of the orientation of electric field vector with respect to
the crystal axes testifying to their full symmetry while no modes of lower
symmetry appeared in any polarization geometry. For anisotropic detection both
modes of Eg symmetry could be detected, but their amplitudes were considerably
smaller than those of A1g symmetry. To clarify the coherent phonon assignment
and the process of coherent phonon generation in Bi2Te3, the time-domain
measurements were complemented by spontaneous Raman scattering. The comparison
of frequency- and time-domain results and the polarization dependence suggest
that the 3.6 THz mode belongs to crystalline Te arising due to tellurium
segregation. A discrepancy between the time- and frequency domain data is
discussed.
