the infrared-resonant Raman effect*

The Raman effect - the inelastic scattering of light by lattice vibrations - provides an important tool for conversion of light from one color to another in optical physics, and is ubiquitous in materials characterization because of its fundamental connection to crystal symmetry. The Raman effect is dominated by changes to the electronic susceptibility in the UV and visible frequency ranges. However, in the mid- and far-IR, nonlinear contributions to the lattice polarization provide additional Raman pathways, which have been little explored.

Using a combination of theory and first-principles calculations, we derive and evaluate symmetry relations and complete expressions for the Raman effect in insulating crystalline materials including all electronic and lattice-mediated pathways. We show that when infrared-active phonons are resonantly excited, the Raman effect may be dominated by direct changes to the lattice polarizability induced by Raman phonons. Applying this approach to the archetypal perovskite SrTiO₃, we show that this infrared-resonant Raman effect can induce optical symmetry breaking and giant refractive index shifts that are tailored by the incident light polarization and which infrared active phonons are excited. (arXiv:2011.02010)