Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields

TL;DR

This paper introduces an experimental method using intense THz-rate pump pulses to generate and study large amplitude coherent phonons in GeTe, revealing insights into phase transition dynamics and lattice behavior.

Contribution

It demonstrates a novel approach to driving and analyzing soft phonons in GeTe with THz-rate pulses, highlighting the ability to induce precursor states of phase transition.

Findings

Large red-shift of phonon frequency with THz-rate pulses

Saturation of phonon softening with single pulses

Near critical damping indicating phase transition precursor

Abstract

We propose an experimental technique to generate large amplitude coherent phonons with irradiation of THz-rate pump pulses and to study the dynamics of phase transition in GeTe ferroelectrics. When a single pump pulse irradiates the sample at various pump power densities, the frequency of the soft phonon decreases sub-linearly and saturates at higher pump powers. By contrast, when THz-rate pump pulse sequence irradiates the sample at matched time intervals to forcibly drive the oscillation, a large red-shift of the phonon frequency is observed without saturation effects. After excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped leading to a near critical damping condition. This condition indicates that the lattice is driven to a precursor state of the phase transition.