Tracking the ultrafast light-induced shear strain in multiferroic BiFeO unit cell3 Understanding structural distortions induced such as h01 and h01. In the pseudo-cubic approximation and by ultrashort laser pulses is a necessary step in the absence of strain, the corresponding families of lattice planes are equivalent and exhibit equal interplanar distances towards advanced applications in GHz- with d= d. That symmetry, preserved by the longitudinal h01 h01 THz photostriction. Although little studied strain, is broken when the light induces a shear strain in the to date, transverse distortions (shear) have unit cell with d ≠ d (Fig. 1).h01 h01 the particularity of being associated with FIGURE 1 a structural angular momentum and could constitute a channel for energy exchange between, for example, the spin and phonon subsystems in materials with magnetic orders. This work presents a quantitative and simultaneous measurement of photo-induced EXCITING WITH VISIBLE PHOTONS longitudinal and transverse structural AND SEEING WITH X-RAYS distortions in the prototypical BiFeO3 To achieve for the first time this complete measurement, a multiferroic material. (001)BiFeO single crystal was photoexcited by a 400 nm3 femtosecond laser pulse and probed by X-rays. In order to It is well established that a laser pulse impinging on the surface match the photoexcited volume and the volume probed by of a sample yields a depth-dependent elastic stress and, in X-rays, we have adopted the grazing incidence geometry for turn, a strain wave that propagates at the speed of sound the time-resolved X-ray diffraction (Fig. 2). The experiments were from the surface to the interior of the sample [1]. Knowing conducted at CRISTAL beamline in the low-α mode. As shown how the unit cell distorts at the picosecond time scale under in Fig. 2, our measurements first give evidence for different the action of a short light-pulse provides a new insight on the time-evolutions of the d201 and d interplanar distances after201 carrier-acoustic phonon coupling mechanisms in general [2] photoexcitation, which is a direct consequence of photoinduced and, in the particular case of multifunctional oxides such as shear strain as illustrated in Fig. 1. With a model, it has been the BiFeO material, it opens perspectives for a light-control of possible to extract both the out-of-plane and in-plane strain 3 components that are plotted in Fig. 3. It is the first time that coupling between charge and magnetic orders with the lattice. the full time-dependence of the light-induced strain of BiFeO The time-resolution required to follow the strain development unit cell is reconstructed. 3 in this BiFeO material approximates 10 ps, which is dictated3 by both the laser penetration depth (few tens of nm) and the FIGURE 2 speed of sound (few nm/ps). Such time resolution is attainable in the low-α mode of operation of SOLEIL synchrotron. To date, most diffraction experiments have focused on the determination of the strain component perpendicular to the sample surfaces (longitudinal strain, i.e. out-of plane lattice parameter) [3, 4]. However, in anisotropic lattices, ultrafast laser action can lead also to shear strain (in-plane lattice distortion), associated, in the case of BiFeO, to an intrinsic ferroelectricity-3 based in-plane symmetry breaking [5]. To extract the shear strain, it is necessary to register two asymmetric Bragg peaks 68