Towards ultrafast control of dielectric response through nonlinear lattice excitation

The development of intense laser sources in the infrared has created an opportunity for unprecedented ultrafast selective control of crystal structure. Striking changes in material properties have been achieved by the optical nonlinear phononics effect - where large amplitude infrared (IR) phonons drive Raman active phonons through nonlinear lattice coupling. [Rep. Prog. Phys. 79, 064503 (2016)] Excitation of IR phonons with light is conventionally described by coupling of the infrared electric field to a linear dipole induced by the IR phonon. When IR phonons are excited to large amplitude, when multiple optical phonons are displaced simultaneously, or when multiple laser sources are used concurrently, the conventional linear description of the crystal’s dipole may be insufficient.

Using theory and first-principles calculations, we explore the implications of the nonlinear dipole moment to optical experiments. When considered in concert with nonlinear lattice coupling, we find sizable polarization dependent dielectric changes can span a broad frequency range, including frequencies far above the IR resonance. In addition to changes in the optical response, the nonlinear dipole moment provides an additional strategy for ultrafast structural control.