Midinfrared-light-induced ferroelectricity in oxide paraelectrics via nonlinear phononics

I show that a nonequilibrium paraelectric to ferroelectric transition can be transiently induced using midinfrared pulses. This relies on a quartic lQ_lz²Q_hₓ² coupling between the lowest (Q_lz) and highest (Q_hₓ) frequency infrared-active phonon modes of a paraelectric material. Density functional calculations show that the coupling constant l is negative, which causes a softening of the Q_lz mode when the Q_hₓ mode is externally pumped. A rectification along the Q_lz coordinate that stabilizes the nonequilibrium ferroelectric state occurs only above a critical threshold for the electric field of the pump pulse, demonstrating that this is a nonperturbative phenomenon. A first-principles calculation of the coupling between light and the Q_hₓ mode shows that ferroelectricity can be induced in the representative case of strained KTaO₃ by a midinfrared pulse with a peak electric field of 170.16em0exMV0.16em0excm^-1 and duration of 2 ps. Furthermore, other odd-order nonlinear couplings make it possible to arbitrarily switch off the light-induced ferroelectric state, making this technique feasible for all-optic devices.