Ultrafast reversal of the ferroelectric polarization by a midinfrared pulse

I calculate the ferroelectric polarization dynamics induced by a femtosecond midinfrared pulse as measured in the recent experiment by R. Mankowsky et al., Phys. Rev. Lett. 118, 197601 (2017). It is due to the nonlinear coupling of the excited infrared-active phonon mode with the ferroelectric mode or to the excitation of the ferroelectric mode itself depending on the pulse frequency. To begin with, I write the ${\mathrm{LiNbO}}_3$ crystal symmetry invariant thermodynamic potential including electric field and nonlinear phonon coupling terms. I solve the equations of motion determined by this potential for phonon coordinates numerically. I explain the transient polarization reversal observed in the experiment by the action of the depolarizing electric field, which is due to bound charges at the polarization domain boundaries, and give a reasonable estimate for its value. I argue that the polarization can be reversed when this field is screened.

Figure 1

Sketch of the bulk and domain polarization ${\mathbf{P}}_{b,d}$ and the electric field $\mathbf{E}$ due to the bound charges in the sample when irradiated by a terahertz pulse; free charges on the surface are drawn with circles (a). Time dependence of the terahertz radiation electric field (thick, red) and the corresponding infrared mode amplitude $Q_{A_1{\text{(TO)}}_4}$ (blue) nonlinearly coupled to the ferroelectric mode at a fluence of 95 mJ/${\mathrm{cm}}^2$ (b).

Figure 2

Ferroelectric mode dynamics for the pump pulse fluences of 30, 60, and 95 mJ/${\mathrm{cm}}^2$ and frequency $\omega =19$ THz when $c_{1,3}=0$ and $E_d=0$ (dashed) and $c_1=10, c_3=17$ meV/${\mathrm{u}}^2$ Å${}^4$ for $A_1\left({\text{TO}}_4\right)$ and $E_d=2.8$ MV/cm (solid). Red dots are at minimums of $Q_P$.

Figure 3

Minimum of $Q_P$ (normalized by $Q_P^e$) as a function of fluence for different frequencies of the pump pulse. At ${\nu }_{\text{pump}}=7$ THz two lines correspond to opposite signs of the pump pulse electric field. Experimental data points are from Ref. [19].

Figure 4

Susceptibility as a function of the pump frequency normalized to unity at 19 THz. Experimental data points are from Ref. [19].

Figure 5

Sketch of the screen (in golden) on the top of the crystal around the pump laser spot and the electric charges distribution (free charges are in circles).