Generation of strain using light is a key issue for future development of ultrasonic devices. Up to now, photo-induced GHz-THz acoustic phonons have been mainly explored in metals and semiconductors, and in artificial nanostructures to enhance their phononic emission. However, despite their inherent strong polarization (providing natural asymmetry) and superior piezoelectric properties, ferroelectric oxides have been only poorly regarded. Here, by using ultrafast optical pump-probe measurements, we show that photogeneration/photodetection of coherent phonons in BiFeO3 ferroelectric leads, at room temperature, to the largest intensity ratio ever reported of GHz transverse acoustic wave versus the longitudinal one. It is found that the major mechanism involved corresponds to screening of the internal electric fields by light-induced charges, which in turn induces stress by inverse piezoelectric effect. This giant opto-acoustic response opens new perspectives for the use of ferroelectric oxides in ultrahigh frequency acoustic devices and the development of new GHz-THz acoustic sources.

The ability to generate efficient giga–terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse, we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics.

Ultrafast lattice dynamics of few quintuple layers of topological insulator (TI) Bi2Te3 is studied with time-resolved optical pump-probe spectroscopy. Both optical and acoustic phonons are photogenerated and detected. Here, in order to get new insights on the out-of-equilibrium electron-phonon coupling and phonons dynamics in confined TI, different nanostructures have been investigated (single or polycrystalline QLs assemblies and nano-crystallized islands). Contrary to previous literature claims, we show that even for nanostructures containing only 10 quintuple layers (QLs), the symmetric A1g(I) coherent optical phonon is efficiently photogenerated and no restriction due to the structural confinement appears. We also observe that whatever the arrangement of the nanostructures, the A1g(I) optical phonon features are similar (lifetime). We also report the observation of confined coherent acoustic phonons propagating from QLs to QLs whose spectrum is, this time, very sensitive to the atomic arrangement. In the case of the single crystalline ultrathin film, the time of flight analysis of these acoustic phonons provides direct estimate of the elastic properties of these nanostructures as well as some estimates of Van der Waals interactions between QLs.