Light-Assisted Creation and Control of Novel Ferroelectric Phases

The intense electric field of the ultrashort mid-infrared laser pulse normally drives phonon excitations into nonlinear regimes – nonlinear phononics. As a result, it can induce structural phase transitions and hidden phases of matter with exotic properties. In ferroelectric materials, understanding these nonlinear phononic couplings and how light activates/adjusts them can unveil routes to selectively induce or control some ferroelectric phase transitions, e.g., polarization reorientation and even the creation of topological electric defects. In this talk, I will reveal and explain a biquadratic competitive coupling between a high-frequency phonon and the main polarization (soft) mode that could result in a so-called squeezing effect of ferroelectric polarization – which we found to be responsible for the observed reversal of electrical polarization under terahertz laser pulses. Moreover, I will also show the possibility of utilizing the squeezing effect to control the orientation of ferroelectric polarization and even induce topological nontrivial electric textures. These results show the important role of nonlinear couplings in light-induced phenomena and demonstrate promising applications of terahertz all-optical control of ferroelectric materials.