Evolution temporelle de la contrainte photoinduite dans les couches minces de BiFeO3 à l'échelle picoseconde

Bismuth Ferrite (BiFeO3) is a multiferroic material that combines ferroelectric, antiferromagnetic, and ferroelastic properties at room temperature, with notable coupling between its ferroic orders. This combination makes BiFeO3 a promising candidate for optoelectronics, high-frequency acoustic devices, and data storage technologies. Its interaction with light enables the generation of coherent acoustic phonons, providing an alternative to electrically-driven piezo-transducers, which are typically limited to gigahertz frequencies. Optical techniques extend the frequency range into the terahertz regime, opening possibilities for ultrafast devices and high-speed communication.This work investigates strain dynamics in BiFeO3 thin films, focusing on photoinduced acoustic phonons. Using time-resolved pump-probe spectroscopy, phonon dynamics are monitored in real time. While previous studies focused on low-fluence regimes, this study explores BiFeO3 behavior at fluences up to 6 mJ/cm2, reaching the material’s damage threshold. The high-fluence regime provides new knowledge into the interplay between electronic excitation, thermal effects, and phonon dynamics. A complementary approach involves time-resolved X-ray diffraction (TRXRD) measurements conducted at the FemtoMAX beamline at MAX IV (Lund, Sweden). For the first time, this beamline was used to study ultrafast photogenerated strain in BiFeO3 thin films, capturing unit cell deformations within the first few picoseconds after laser excitation. TRXRD results are compared with X-ray thermodiffraction measurements to disentangle thermal and photoinduced strain, enabling a detailed characterization of strain components along all three crystallographic directions.The role of ferroelectric polarization orientation and domain structure on phonon generation and propagation is also explored. In single-domain BiFeO3 thin films, phonon generation and propagation are straightforward, enabling clearer interpretation. In contrast, stripe-domain configurations introduce complexity due to variations in local polarization states, affecting phonon dynamics. The study extends to BiFeO3/LaFeO3 superlattices, focusing on their potential to generate and manipulate high-frequency (terahertz) phonons. By adjusting the superlattice structure, selective excitation of specific phonon modes is demonstrated, providing a novel approach to tailoring phonon frequencies for high-speed signal processing and optoelectronic devices. These findings contribute to a better understanding of strain dynamics and phonon propagation in complex oxides.