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 ...

Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy. However, because many important excitations of solids are found at low energies, much can be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies. For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions. So far it has only been possible to exploit second-order phonon nonlinearities at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A (transverse optical) phonon mode in the ferroelectric material lithium niobate. By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material. Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.

The chemical bond is one of the most powerful, yet much debated concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding was identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking, and sharing of about one electron between adjacent atoms. This metavalent bond is a fundamental type of bonding in solids, besides covalent, ionic, and metallic bonding, raising the pertinent question as to whether there is a well‐defined transition between metavalent and covalent bonds. Here, three different pseudo‐binary lines, namely, GeTe1−xSex, Sb2Te3(1−x)Se3x, and Bi2−2xSb2xSe3, are studied, and a sudden change in several properties, including optical absorption ε2(ω), optical dielectric constant ε∞, Born effective charge Z*, electrical conductivity, as well as bond breaking behavior for a critical Se or Sb concentration, is evidenced. These findings provide a blueprint to experimentally explore the influence of metavalent bonding on attractive properties of phase‐change materials and thermoelectrics. Particularly important is its impact on optical properties, which can be tailored by the amount of electrons shared between adjacent atoms. This correlation can be used to design optoelectronic materials and to explore systematic changes in chemical bonding with stoichiometry and atomic arrangement.

Glass forming materials are employed in information storage technologies making use of the transition between a disordered (amorphous) and an ordered (crystalline) state. With increasing temperature the crystal growth velocity of these phase-change materials becomes so fast that prior studies have not been able to resolve these crystallization dynamics. However, crystallization is the time limiting factor in the write speed of phase-change memory devices. Here, for the first time, we quantify crystal growth velocities up to the melting point by using the relaxation of photo-excited carriers as an ultrafast heating mechanism.During repetitive femtosecond optical excitation, each pulse enables dynamical evolution for tens of picoseconds before the intermediate atomic structure is frozen-in as the sample rapidly cools. We apply this technique to Ag 4 In 3 Sb 67 Te 26 (AIST) and compare the dynamics of as-deposited and application-relevant melt-quenched glass. Both glasses retain their different kinetics even in the supercooled liquid state, thereby revealing differences in their kinetic fragilities. This approach enables the characterization of application-relevant properties of phase-change materials up to the melting temperature, which has not been possible before.Mankind has utilized glasses during the last five thousand years. They can be prepared by cooling a liquid fast and far enough below the glass transition temperature -to a temperature where its viscosity is sufficiently high that the atomic arrangement is kinetically frozen-in 1 . Until recently, research and technology have focused on good glass formers, i.e. materials which can be vitrified by cooling their liquid state at moderate rates. But in recent decades poor glass formers such as metallic glasses and certain chalcogenide glasses have gained interest due to their remarkable property portfolio 2,3 . These materials need to be cooled at rates in excess of around 3*10 9 K/s to bypass crystallization and to quench the atoms in an amorphous arrangement 4 . This so-called glass transition at temperature is commonly observed at a