  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.
