Electronic origin of ultrafast laser-induced ferroelectricity in SrTiO3

Abstract Following an ultrafast laser excitation, paraelectric SrTiO3 can be driven non-thermally into a metastable ferroelectricity state. Despite achieving ultrafast control of ferroelectricity, the fundamental mechanism and dynamics of the photoinduced phase transition remain ambiguous. Here, the determinant formation mechanism of ultrafast ferroelectricity in SrTiO3 is traced by non-adiabatic dynamics simulations. That is, the selective excitation of multiple phonons, induced by photoexcited electrons through the strong correlation between electronic excitation and lattice distortion, result in the breaking of the crystal central symmetry and the onset of ferroelectricity on the ultrafast time scale. Laser illumination leads to a uniaxially stretched lattice deformation along the laser polarization direction, generated by nonequilibrium forces from optically excited electrons. The accompanying population transition between 3dz2 and 3d(x2-y2) orbitals excites multiple phonon branches, including the two high-energy longitudinal optical modes, so as to drive titanium ion away from the center of oxygen octahedron and generate a metastable ferroelectric phase. These findings provide new insights into the understanding and manipulation of laser-induced ferroelectric phase transition, and suggests new schemes for the optical control of electronic and structural quantum states in complex materials.