Nonlocal nonlinear phononics

TL;DR

This paper extends nonlinear phononics by demonstrating the excitation of propagating phonon-polaritons in ferroelectric LiNbO3, enabling bulk control of polarization over large depths and potential solitonic domain wall formation.

Contribution

It introduces a novel approach to nonlinear phononics using propagating polaritons, separating optical drive from functional response spatially and enabling bulk material control.

Findings

Resonant mid-infrared pulses excite an 18 THz phonon at the surface.

Ferroelectric polarization is reduced throughout 50 microns depth.

High excitation amplitudes induce polarization reversal and potential solitonic domain walls.

Abstract

Nonlinear phononics relies on the resonant optical excitation of infrared-active lattice vibrations to coherently induce targeted structural deformations in solids. This form of dynamical crystal-structure design has been applied to control the functional properties of many interesting systems, including magneto-resistive manganites, magnetic materials, superconductors, and ferroelectrics. However, phononics has so far been restricted to protocols in which structural deformations occur locally within the optically excited volume, sometimes resulting in unwanted heating. Here, we extend nonlinear phononics to propagating polaritons, effectively separating in space the optical drive from the functional response. Mid-infrared optical pulses are used to resonantly drive an 18 THz phonon at the surface of ferroelectric LiNbO3. A time-resolved stimulated Raman scattering probe reveals that the ferroelectric polarization is reduced over the entire 50 micron depth of the sample, far beyond the ~ micron depth of the evanescent phonon field. We attribute the bulk response of the ferroelectric polarization to the excitation of a propagating 2.5 THz soft-mode phonon-polariton. For the highest excitation amplitudes, we reach a regime in which the polarization is reversed. In this this non-perturbative regime, we expect that the polariton model evolves into that of a solitonic domain wall that propagates from the surface into the materials at near the speed of light.