Shift-current-induced strain waves in LiNbO3 mapped by femtosecond x-ray diffraction

The response of the crystal lattice to an electric shift current induced via the two-photon bulk-photovoltaic effect in a lithium niobate (LiNbO3) crystal is directly mapped by femtosecond x-ray diffraction. Acoustic strain waves of large amplitude are generated by piezoelectric coupling to the current-related polarization while other mechanisms such as anharmonic phonon-phonon couplings and electron-phonon coupling through deformation potentials play a minor role. A striking variation of the strain wave speed occurs as a function of the relative orientation between the crystal's c-axis, the direction of the current flow, and the polarization of the incident pump pulse. The observed behavior is relevant for a large class of ferroelectrics.