Ultrafast reversal of the ferroelectric polarization by a midinfrared pulse

Abstract

We 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 with the ferroelectric mode or to the excitation of the ferroelectric mode itself depending on the pulse frequency. To begin with, we write the LiNbO3 crystal symmetry invariant thermodynamic potential including electric field and nonlinear phonon coupling terms. We solve the equations of motion determined by this potential for phonon coordinates numerically in classical approximation. We explain the transient polarization reversal observed in the experiment by 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. We argue that the polarization could be ultimately reversed when this field is screened.

Highly Confined Phonon Polaritons in Monolayers of Perovskite Oxides

“…Specifically, we use density functional theory to compute the 2D phononpolariton dispersions and polaritonic figures of merit, including propagation-quality, confinement, and deceleration factors, of the highly dispersive high-frequency phonons in three ABO 3 -type perovskites: SrTiO 3 , KTaO 3 , and LiNbO 3 . We chose these candidates, because they are well studied in literature and at the same time continuously new phenomena and applications are being discovered and predicted, such as multiferroic quantum criticality [45], engineering of superconductivity [46] and ferroelectricity [47][48][49][50][51], nonlinear optical and electro-optical applications [52,53], novel Hall effects [54], and even the generation of Moiré lattices [55]. We find that while the propagation quality of 2D phonon polaritons in the oxide perovskites is slightly lower than that found in hBN due to the complexity of the perovskite structure, their confinement and deceleration factors yield comparable numbers.…”

Ferroelectric Polarization Reversal vs Pump Spot Shape

“…Thus, the depolarization factor turns out to be about 𝑁 = 0.1, which is twice as high as was estimated above from geometric considerations, but nevertheless is in reasonable agreement with all the approximations made. The values of the other parameters used in the calculation were determined in [32] in accordance with [31] The polarization dynamics for a half-smaller depolarizing field 𝐸 𝑑 = 1.4 MV/cm, which should be attained for a squeezed pump spot with a ratio 𝑎/𝑐 = 0.25, as discussed above, shows polarization reversal (see solid lines in Fig. 3).…”

“…However, in this way, only a transient switching of the polarization was achieved in a monodomain LiNbO 3 sample, followed by the polarization recovery to its initial value on a timescale of about 0.2 ps [31]. After that, it was suggested that the polarization reversal was prevented by the depolarizing field created by charges at the domain boundaries and it was proposed to use a metal wire around the pump spot to screen these charges [32].…”

“…To trace the dependence of the polarization dynamics on the depolarizing field, I solve a system of two equations of motion for the pumped optical phonon coordinate 𝑄 IR and the ferroelectric mode 𝑄 𝑃 of the form [32]:…”

“…The first term in Eq. ( 5) does not contribute much in the polarization reversal and was neglected here, while the values of the other coupling constants were taken 𝑐 2 = 34 meV/u 2 Å4 and 𝑐 3 = 17 meV/u 2 Å4 as in [32]. Together with the value of the depolarizing field 𝐸 𝑑 = 2.8 MV/cm [32], this coupling makes it possible to reproduce well the dynamics of polarization observed in [31] (see dotted lines in Fig.…”