Ultrafast Reversal of the Ferroelectric Polarization

Abstract

The ability to manipulate ferroelectrics at ultrafast speeds has long been an elusive target for materials research. Coherently exciting the ferroelectric mode with ultrashort optical pulses holds the promise to switch the ferroelectric polarization on femtosecond timescale, two orders of magnitude faster compared to what is possible today with pulsed electric fields.Here, we report on the demonstration of ultrafast optical reversal of the ferroelectric polarization in LiNbO 3 . Rather than driving the ferroelectric mode directly, we couple to it indirectly by resonant excitation of an auxiliary high-frequency phonon mode with femtosecond mid-infrared pulses. Due to strong anharmonic coupling between these modes, the atoms are directionally displaced along the ferroelectric mode and the polarization is transiently reversed, as revealed by time-resolved, phase-sensitive second-harmonic generation. This reversal can be induced in both directions, a key pre-requisite for practical applications. 2The ferroelectric polarization is typically controlled with static or pulsed electric fields.Switching is in this case an incoherent process, with speed limited to hundreds of picoseconds by the nucleation and growth of oppositely polarized domains [1,2,3]. To overcome these limitations, attempts have been made to drive the ferroelectric mode coherently with light pulses. These strategies, which have been based either on impulsive Raman scattering [4,5,6,7] or direct excitation of the ferroelectric mode [8,9,10] using THz radiation, have not yet been completely successful.Recent theoretical work [11] has analyzed an alternative route to manipulate the ferroelectric polarization on ultrafast timescales. It has been proposed that a coherent displacement of the ferroelectric mode could be achieved indirectly, by exciting a second, anharmonicallycoupled vibrational mode at higher frequency. The underlying mechanism is captured by the following minimal model, which is illustrated in Fig. 1. Consider a double well energy potential along the ferroelectric mode coordinate Q P as shown in Fig. 1Here, ω p is the frequency of the ferroelectric mode for a fixed temperature T below the Curie temperature T C . Take then a second mode Q IR with frequency ω IR , described by the Fig. 1(a), which is anharmonically coupled to the ferroelectric mode with a quadratic-linear dependence of the interaction energy aQ IR 2 Q P . The total lattice potential can then be written asIn equilibrium, Q P takes the value of one of the two minima of the double well potential Fig. 1(b). For finite displacements Q IR , this minimum is first displaced and then destabilized as Q IR exceeds a threshold value (colored lines in Fig. 1(b)).Importantly, the direction of this displacement is always pointed toward the opposite potential well and independent on the sign of Q IR [11].The corresponding dynamics of the two modes for resonant periodic driving of Q IR by a midinfrared light pulse is obtained by solving the equations of motionHere, f t) is the driving pulse ...

Ultrafast reversal of the ferroelectric polarization by a midinfrared pulse

“…The thus obtained potential would be appropriate for a very fast dynamics of the ferroelectric mode. At the same time, in the experiment [11] the characteristic time change of is not smaller than its inverse frequency. Moreover, the signal of the second harmonics vanishes at some point.…”

“…2. In the experiment [11] for flu- Figure 3. Minimum of (normalized by ) as a function of fluence for different frequencies of the pump pulse.…”

“…At pump = 7 THz two lines correspond to opposite signs of the pump pulse electric field. Experimental data are from [11] ences above a threshold value of 60 mJ/cm 2 the secondharmonic intensity (and thus the ferroelectric mode ) was observed to vanish completely. We see, however, only a reduction of it at this fluence.…”

Perspective: THz-driven nuclear dynamics from solids to molecules

“…Nonlinear phononic effects can also create transient magnetic fields, thereby exciting magnetic excitations 131 . In the context of control over ferroelectrics, there is even a recent experiment that sees some indications of possible nonlinear-phononics-induced transient reversal 132 . For interested readers, who would like to know more on this subject, we refer to some recently published review articles in Refs.…”

Ultrafast polarization switching in ferroelectrics

“…However the present sources do not provide the strength of a pulse sufficient for completely switching the polarization. Experiments [36] have shown that the reversal can be only 40% of its equilibrium value. Although the reversal is quite fast, occurring in about 10 −13 s, but the reversed polarization rapidly, during the same 10 −13 s, returns to the initial state, similarly to the dynamics induced by terahertz pulses, when the reversal happens over a picosecond time scale, followed by its fast complete retrieval [37].…”