Giant optical enhancement of strain gradient in ferroelectric thin films and its physics origin

TL;DR

This paper demonstrates a giant, optically induced enhancement of strain gradients in ferroelectric thin films, enabling dynamic control of flexoelectric effects for potential optical memory applications.

Contribution

It reveals a new method to dynamically enhance strain gradients in ferroelectric films using light, surpassing mechanical deformation limits, and elucidates the underlying piezoelectric mechanism.

Findings

Achieved strain gradients of 10^5-10^6 m^{-1} via optical excitation.

Linked the enhancement to a transient screening electric field.

Enabled potential for optical switching of ferroelectric polarization.

Abstract

The coupling between strain gradients and polarization, known as flexoelectricity, offers a new mechanism to control the functionality of dielectric materials. However, for the effect to be practically attractive, dynamic control of the strain gradient with magnitudes far exceeding those achievable via mechanical deformation (~10 $m^{-1}$) is needed. Strain-engineered thin films exhibit extraordinary strain gradients of $10^5-10^6 m{-1}$ arising from structural relaxation within a short space range that greatly enhances the steady-state flexoelectric effect. Here we report a giant, optically initiated dynamic enhancement of the strain gradient, also on the order of $10^5 -10^6 m^{-1}$, in ferroelectric BiFeO3 epitaxial thin films via time-dependent coherence analysis of X-ray diffractions. The finding opens the door for dynamic coupling of the flexoelectric effect with light, making optical switching of polarization, and thus application such as direct optical writing of non-volatile ferroelectric memory, possible. A combination of time-resolved X-ray scattering and optical spectroscopy shows that the enhancement of the strain gradient is due to a piezoelectric effect driven by a transient screening electric field, opening the opportunity for new ways of studying flexoelectric effect in strain engineered ferroelectric thin films.