Anharmonic coherent dynamics of the soft phonon mode of a PbTe crystal

TL;DR

This study explores the ultrafast optical response of PbTe to terahertz pulses, revealing nonlinear phonon dynamics and transient non-centrosymmetric states that suggest potential ferroelectric behavior induced by intense terahertz excitation.

Contribution

It demonstrates the direct excitation of nonlinear soft phonon modes in PbTe and identifies transient non-centrosymmetric states possibly linked to ferroelectric order induced by terahertz pulses.

Findings

Detection of multifrequency oscillations in reflectance anisotropy.

Observation of a transient non-centrosymmetric state in PbTe.

Enhanced coherent phonons near satellite mode frequency.

Abstract

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.