Sum-frequency ionic Raman scattering

Coherently excited phonons are a unique tool to control the properties of materials in their electronic ground state on the picosecond timescale. Here, the authors complete the map of four excitation mechanisms for Raman-active phonons, which are either directly mediated by photons or work with the help of nonlinearly coupled phonons. Each process can be achieved by using a sum- or difference-frequency mixing of the scattering particles. The general model here captures all four mechanisms, which is demonstrated for different types of material using density functional theory. This study may serve as a guide for the selective excitation of phonons and Raman-active modes in general.