Figure 2: Spectra of photogenerated longitudinal and TA modes for different crystallographic orientations.

Transient optical reflectivity signals obtained for different grains of the polycrystalline BFO sample with pump (probe) wavelength of (a) 415 nm (590 nm) and (c,e) 400 nm (800 nm). The Brillouin oscillations (BO) are clearly visible. The Brillouin signal exhibits deep dependence on the grain while the electronic excitation (EE) is independent. (b,d,f) Plots of the corresponding fast fourier transforms of signals shown in (a,c,e). The grain 1–4 are inclined grains versus z axis of the unit cell of BFO and show clear shear TA modes. (g,h) Sketches of the mean macroscopic polarization P direction relative to the grain surface normal n. The surface orientation of grain 2 is indicated on the mosaic (inset of g) as an example. Grain 5 has its z axis (called optical axis) parallel to the grain surface and TA modes are nearly absent, while LA modes are visible with a splitting due to optical anisotropy of BFO (see Methods). The family of planes, to which belongs the surface of grain 5, is indicated with the black line on the mosaic (inset of h) (remark: the small signal at ~30 GHz for grain 3 is an artefact probably arising from a neighbouring grain partially illuminated by the laser beam).