Disentangling thermal birefringence and strain in the all-optical switching of ferroelectric polarization

TL;DR

This paper investigates how infrared pulses induce birefringence and strain in ferroelectric barium titanate, revealing non-thermal effects' significant role in all-optical switching of ferroelectric order.

Contribution

It develops a model to distinguish thermal and strain effects in optical switching, providing experimental evidence of non-thermal strain contributions.

Findings

Strain-induced patterns are much larger than heating effects predict.

Distinct spectral signatures differentiate heat- and strain-induced birefringence.

Non-thermal effects significantly influence optical switching mechanisms.

Abstract

Recent works have demonstrated that the optical excitation of crystalline materials with intense narrow-band infrared pulses, tailored to match the frequencies at which the crystal's permittivity approaches close to zero, can drive a permanent reversal of magnetic and ferroelectric ordering. However, the physical mechanism that microscopically underpins this effect remains unclear, as well as the precise role of laser-induced heating and macroscopic strains. Here, we explore how infrared pulses can simultaneously give rise to strong temperature-dependent birefringence and strain in ferroelectric barium titanate. We develop a model of these two coexisting effects, allowing us to use polarization microscopy to disentangle them through their spatial distributions, temporal evolutions and spectral dependencies. We experimentally observe strain-induced patterns that are an order of magnitude larger than that which can be accounted for by laser-induced heating alone, suggesting that non-thermal effects must also play a role. Our results reveal the distinct fingerprints of heat- and strain-induced birefringence, shedding new light on the process of all-optical switching of order parameters in the epsilon-near-zero regime.