Title: Probing the Interatomic Potential of Solids by Strong-Field Nonlinear Phononics,
Abstract: Femtosecond optical pulses at mid-infrared frequencies have opened up the
nonlinear control of lattice vibrations in solids. So far, all applications
have relied on second order phonon nonlinearities, which are dominant at field
strengths near 1 MVcm-1. In this regime, nonlinear phononics can transiently
change the average lattice structure, and with it the functionality of a
material. Here, we achieve an order-of-magnitude increase in field strength,
and explore higher-order lattice nonlinearities. We drive up to five phonon
harmonics of the A1 mode in LiNbO3. Phase-sensitive measurements of atomic
trajectories in this regime are used to experimentally reconstruct the
interatomic potential and to benchmark ab-initio calculations for this
material. Tomography of the Free Energy surface by high-order nonlinear
phononics will impact many aspects of materials research, including the study
of classical and quantum phase transitions.