New pathways for ultrafast structural control via nonlinear phononics

The coherent excitation of infrared active phonons with modern terahertz sources has enabled the study of energy transfer through the crystal lattice on ultrashort timescales. Along with the dynamical structural changes induced, dramatic changes to functional properties like ferroelectricity, magnetism, and superconductivity have been observed. At the heart of theoretical descriptions of these studies are special symmetry-constrained anharmonic lattice potential energy terms, because they can describe the quasistatic and unidirectional displacement of Raman phonons seen experimentally. Recent theoretical work (Phys. Rev. X 11, 021067 (2021)) has emphasized an additional pathway, the nonlinear lattice polarizability, because of its large effect on optical properties when infrared active phonons are excited.

In this theoretical work, we show that the nonlinear lattice polarizability gives additional strategies for selective structural control through tuned polarization, frequency, and duration/shaping of the THz pulse. This approach expands the toolbox for ultrafast structural control of crystalline materials in the mid- and far-infrared, enabling a new direction-specific handle on Raman-active phonons and silent phonons normally inaccessible to the conventional nonlinear phononics pathway.