Ultrafast Control of Material Optical Properties via the Infrared Resonant Raman Effect

The Raman effect -- inelastic scattering of light by lattice vibrations (phonons) -- is one of the workhorses in optical physics and a ubiquitous tool for characterization of crystalline materials. In the typical experimental frequency range, the Raman effect is dominated by changes to electronic dipoles by Raman-active phonons, but when light is tuned into the mid- and far-infrared, the Raman effect is rich with potential physical pathways due to the presence of additional material dipoles through the lattice. Here we derive symmetry relations and complete expressions for the optical susceptibility including all electronic and lattice mediated pathways for the Raman effect using a perturbative approach. We show that in insulating materials, when light is tuned to resonantly excite infrared active phonons, the Raman effect may be dominated by direct changes to the lattice polarizability induced by Raman-active phonons, a mechanism distinct from the recently investigated ionic Raman scattering. Using first-principles techniques we show that, in an archetypal insulating perovskite SrTiO$_3$, this infrared-resonant Raman effect can induce optical symmetry breaking and giant shifts to the refractive index which are tailored by the incident light polarization and infrared active phonon excited. Additionally, the nonlinear polarization pathway responsible for the infrared-resonant Raman effect contributes to the quasistatic control of crystalline structure that has been the focus of recent nonlinear phononics studies.