Strain is a powerful tool for tuning physical properties of functional perovskite oxides like polar and/or magnetic orders in ferroics or multiferroics. In case of the model multiferroic BiFeO3 (BFO which displays many pressure instabilities [1], we show how epitaxial strain in BFO films is beneficial to tune the structure as well as the ferroelectric and magnetic properties [2-5]. We took advantage of this strain sensitivity as well as the recent renewed interest for photoinduced properties in ferroelectrics such as BFO [6] to explore the photostriction mechanism i.e. deformation under light illumination. By using an ultrafast pulsed laser which can be considered in somehow as a "uniaxial pressure" [7] in optical pump-probe measurements, we show that photogeneration/photodetection of coherent phonons in BFO leads 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 [8]. Such mechanism is also supported by using an ab-initioprocedure allowing the computation of the deformation of ferroelectric-based materials under light. This original numerical scheme allow to predict in BFO a photostriction effect of the same order of magnitude than the ones recently observed. A strong dependence of photostrictive response on both the reached conduction state and the crystallographic direction (along which this effect is determined) is also revealed [9]. In addition, we also reveal that the optical mode conversion process between ordinary and extraordinary light waves (and vice-versa) is efficient at tens of GHz frequency in BFO as well as ferroelectric materials like 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 GHz 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 [10]. [1] M. Guennou et al., Phys. Rev B 84, 174107 (2011)[2] Y. Yang et al., Compt. Rend. Phys. 16, 193 (2015)[3] I.C. Infante et al., Phys. Rev. Lett. 107, 2376011 (2011)[4] D. Sando et al., Nat. Mater. 12, 641 (2013)[5] D. Sando et al., Adv. Mater. submitted (2016)[6] C. Paillard et al., Adv. Mater. doi:10.1002/adma.201505215 (2016)[7] P. Ruello et al., Appl. Phys. Lett. 100, 212906 (2012)[8] M. Lejman et al., Nat. Comm. 5, 4301 (2014)[9] C. Paillard et al., Phys. Rev. Lett., in press (2016)[10] M. Lejman, et al., Nat. Comm., in press (2016)