  We investigate the ultrafast optical response of PbTe to an intense
single-cycle terahertz pulse, resonant with the soft transverse optical (TO)
phonon mode of the crystal. We detect multifrequency oscillations of the
reflectance anisotropy, which we associate with nonlinear motion of the TO
phonon oscillator excited directly by the terahertz pulse. Our observation of
monotonically decaying optical anisotropy together with second harmonic
oscillations of the TO mode is an evidence of a transient non-centrosymmetric
state of the crystal lattice that can be accompanied by the ferroelectric
order. We suppose that this state is induced in the PbTe crystal by the intense
terahertz pulse via alignment of the local polar nanodomains. This hypothesis
is partially supported by the observation of coherent phonons near the
satellite phonon mode frequency that are impulsively generated by a femtosecond
laser pulse and are considerably enhanced by a synchronous terahertz pulse.
