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

Abstract

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.

Laser‐Driven Strong‐Field Terahertz Sources

“…For instance, excitation of infrared-optical phonons with intense THz pulses and tracing the anharmonic response [161] allowed one to reconstruct the atomic-scale potential the ions are moving in. [162,163] Even if the phonon of interest is infrared-inactive yet Ramanactive does THz radiation provide new means of phonon excitation by a Raman-type sum-frequency process. [164] Here, two frequencies of the incident THz pulse induce a force on the lattice oscillating at the sum frequency, rather than the difference frequency that occurs in standard Raman-type excitations.…”

Ultrafast polarization switching in ferroelectrics

“…(27), and the existence of the small parameters described in Eqs. (26), (28), and (29), we notice that the variables u and h can be classified as fast, while the variables w and s, as slow. In that case, for solving the given system of equations, it is admissible to resort to the averaging techniques [54,55].…”

Experimental station Bernina at SwissFEL: condensed matter physics on femtosecond time scales investigated by X-ray diffraction and spectroscopic methods

“…By exploiting selective excitation and control of higherenergy modes in the frequency range 15-20 THz (20-15 mm), recent experiments have demonstrated efficient non-linear coupling from the resonantly excited IR-active mode to Raman modes along the symmetry breaking coordinate (Fö rst et al, 2011a). The suppression of magnetic and orbital order (Fö rst et al, 2011b(Fö rst et al, , 2015, enhanced superconductivity in high-T c superconductors (Mankowsky et al, 2014;Mitrano et al, 2016;Fausti et al, 2011) or ultrafast switching of the ferroelectric polarization von Hoegen et al, 2018) has been demonstrated. In one case non-linear electron-phonon coupling was found to dominate directly the charge order melting in a doped manganite (Esposito et al, 2017).…”