Over the past 50 years, nonlinear photonics has revolutionized the generation and manipulation of light; second-harmonic generation, first observed by Franken and co-workers in the Randall Laboratory at the University of Michigan, is just one of many prominent examples. Now a team of researchers that includes Michael Först and Andrea Cavalleri (Max Planck Research Department for Structural Dynamics, Hamburg, Germany), Yoshinori Tokura (AIST, Tsukuba, Japan) and Roberto Merlin (University of Michigan) 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. The study is reported online this week in Nature Physics.

The 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 color”.

Nonlinear phononics, as this work demonstrates for the first time, 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.

Nonlinear phononics as a new ultrafast route to lattice control 
M. Först, C. Manzoni, S. Kaiser, Y. Tomioka, Y. Tokura, R. Merlin and A. Cavalleri 
Nature Physics, online 07. August (2011)