Nonlinear phononics: origin of lattice anharmonicity in perovskites and strategies for control via pressure and strain

Nonlinear phononics provides an avenue for ultrafast control of crystal structure and properties. Anharmonic coupling between IR and Raman phonons in the lattice potential opens pathways for light to induce sizable unidirectional excitations of Raman phonons, altering the crystal structure and therefore its properties. The magnitude of the change in structure and properties is controlled by a combination of key intrinsic microscopic quantities. Strategies for tuning these microscopic quantities, and for engineering an enhanced response, are currently unknown.

Using theory and first-principles calculations, we investigate how the nonlinear phononics response evolves with pressure and strain in SrTiO₃ and LaAlO₃. Specifically, we track how the strength of the anharmonic coupling between, and force constants of, the IR and Raman modes, and the strength of the coupling between the light pulse and the excited IR mode evolve with pressure and epitaxial strain. Our results suggest that the amplitude of key structural distortions at equilibrium determines the strength of dynamical anharmonicity in the optically excited material, and more generally, that the nonlinear phononics response can be tailored via pressure and epitaxial strain.