Figure 1: Ultrafast pump-probe photoacoustic experiments on selected grains of polycrystalline ferroelectric BFO.

(a) Unit cell of the rhombohedral structure of BFO where the red, green and yellow spheres correspond to oxygen, bismuth and iron atoms. The coordinates (x,y,z) are those employed for tensorial calculations, and z direction corresponds to the polarization direction. (b) Schematic representation of the pump-probe experiment conducted onto a single grain of the polycrystalline sample with the white circle depicting the typical surface of the focused laser beam. The incident pump and probe beams are normal to the illuminated surface but slightly shifted off-normal in the figure, for clarity. The grain crystallographic orientations are revealed by the electron back scattering diffraction (EBSD) image obtained from Kikuchi patterns (each colour corresponds to a defined crystalline orientation in the rhombohedral (R) system, see Methods). (c) Principle of the ultrafast optical pump-probe experiments. The femtosecond pump light creates electron-hole pairs in the narrow band gap BFO that initiate the screening of the internal polarization P, which in turn leads to the emission of coherent longitudinal (LA) and transverse (TA) acoustic phonons through the inverse piezo-electric effect.