Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO$_3$

TL;DR

This study demonstrates that ultrafast terahertz pulses can induce large, transient ionic displacements and polarization dynamics in BaTiO₃ ferroelectric thin films, revealing fundamental ultrafast polarization switching mechanisms.

Contribution

It provides the first direct experimental observation of ultrafast ionic and polarization responses in ferroelectrics driven by terahertz fields using femtosecond x-ray scattering.

Findings

Large amplitude Ti displacements along polarization axis

Transient polarization rotation on picosecond timescales

Long-lived vibrational heating effects observed

Abstract

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply sub-picosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO$_3$ ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond timescales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables direct comparison with first-principles-based molecular dynamics simulations, with good agreement obtained.