Over the past decades nonlinear photonics have revolutionized the generation and manipulation of light; changing its color at high intensities is just one of many prominent examples. In this study we have demonstrated an analogous effect for vibrational modes in crystal lattices. High-amplitude coherent excitation of a distinct phonon mode led to the generation of a lower-frequency coherent phonon mode driven through nonlinearities of the crystal lattice.

Our observations are explained by the Ionic Raman Scattering effect – predicted 40 years ago, but hitherto never observed. Comparable to optical rectification in nonlinear photonics, this mechanism rectifies the high-amplitude phonon field to exert a directional force onto the crystal. This force induces an abrupt displacement of the atoms from their equilibrium positions to manipulate the lattice in an unprecedented fashion in order to generate the “new colour”.

Nonlinear phononics, as demonstrated here, can be used to control crystal structures in a new way, opening the path to selective lattice modifications impossible with electronic excitations. It leads to new avenues for the control of condensed matter with light, as it can explain recent breakthroughs in vibrationally induced superconductivity, insulator-metal transitions, and magnetic switching.