Light control of structural phase transitions in complex oxides*

The development of high-power laser sources in the THz frequency range has opened up opportunities for large-amplitude excitation of infrared(IR)-active phonon modes in materials. The nonlinear phononics mechanism relies on the anharmonic coupling between driven IR-active modes and other lattice modes to induce changes in crystal structure and functional properties on picosecond timescales. Most nonlinear phononics experiments to date have focused on the coupling between phonon modes at the center of the Brillouin zone. We show, using theory and first-principle calculations, that coupling between IR modes at the zone center and other modes at nonzero wavevector can be exploited to dynamically induce materials phases with translational symmetry that differs from that of the equilibrium crystal structure. We focus on KTaO₃, a cubic perovskite with no known structural phase transitions as a function of temperature, as a case study. We show that by exciting IR-active modes to large amplitude, an instability related to a tilt of the TaO₆ octahedra appears in the phonon dispersion curves at nonzero wavevector. Additionally, we find that the induced tilt patterns depend on the orientation of the light pulse. This perspective can also be used to provide additional insight into recent experiments of transiently induced ferroelectricity in SrTiO₃.