Journal Article

Probing the interatomic potential of solids with strong-field nonlinear phononics

von Hoegen, A., Mankowsky, R., Fechner, M., Först, M., Cavalleri, A.

2018

Overview

Ultrashort mid-infrared laser pulses can drive atoms far from their equilibrium positions in LiNbO 3 , exciting high phonon harmonics and providing a way to map the interatomic potential. Solid display of phonon harmonics High-harmonic generation of electromagnetic radiation is a well-known example of a nonlinear process. It occurs when photons—usually from a strong laser pulse—interact with nonlinear systems, such as a gas, plasma or solid, in a way that generates new photons with energies that are multiples of the original. This idea can also be applied to phonons—the quasiparticles associated with lattice vibrations—but high-order phonon modes are much harder to generate. Andrea Cavalleri and colleagues now show that ultrashort mid-infrared laser pulses can induce field strengths in lithium niobate (LiNbO 3 ) that are large enough to drive atoms far away from their equilibrium positions. Such strong fields can excite up to five phonon harmonics and provide a way to map the interatomic potential, which can be used to benchmark ab initio calculations. Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy 1 . 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 2 , 3 . For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions 4 , 5 , 6 , 7 , 8 . So far it has only been possible to exploit second-order phonon nonlinearities 9 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 1 (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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/301/119 / 639/766/400/3923 / 639/766/400/584 / Dynamic control / Ferroelectric materials / Ferroelectricity / Field strength / Humanities and Social Sciences / Infrared lasers / Lattice vibration / letter / Light / Lithium / Lithium niobate / Lithium niobates / Methods / multidisciplinary / Nonlinear optics / Observations / Optical techniques / Optical tomography / Optics / Phase transitions / Phonons / Propagation / Science / Vibrations