Abstract
The Raman effect -- inelastic scattering of light by lattice vibrations (phonons) -- produces an optical response closely tied to a material's crystal structure. Here we show that resonant optical excitation of IR and Raman phonons gives rise to a Raman scattering effect that can induce giant shifts to the refractive index and induce new optical constants that are forbidden in the equilibrium crystal structure. We complete the description of light-matter interactions mediated by coupled IR and Raman phonons in crystalline insulators -- currently the focus of numerous experiments aiming to dynamically control material properties -- by including a forgotten pathway through the nonlinear lattice polarizability. Our work expands the toolset for control and development of new optical technologies by revealing that the absorption of light within the terahertz gap can enable control of optical properties of materials over a broad frequency range.
Highlights
The Raman effect arises from the inelastic scattering of light by phonons in crystals or vibrations in molecules
We show that resonant optical excitation of IR phonons strongly contributes to the optical polarizability via a Raman-scattering mechanism mediated by the displacements of IR phonons and that such excitations can be exploited to significantly modify a material’s optical properties, including inducing new optical constants that are forbidden in the equilibrium crystal structure
We develop a theory of infrared-resonant Raman scattering (IRRS) as follows: (1) We define a potential for describing a centrosymmetric crystalline lattice driven to large phonon mode displacements by IR light including the lowest-order nonlinear term in the lattice polarizability, which gives rise to IRRS. (2) We summarize the dependence of the linear electric susceptibility tensor on Raman phonon mode displacements and related symmetry considerations
Summary
The Raman effect arises from the inelastic scattering of light by phonons in crystals or vibrations in molecules. We show that resonant optical excitation of IR phonons strongly contributes to the optical polarizability via a Raman-scattering mechanism mediated by the displacements of IR phonons and that such excitations can be exploited to significantly modify a material’s optical properties, including inducing new optical constants that are forbidden in the equilibrium crystal structure. (3) We employ a perturbation method approach to derive a third-order polarization and a Raman-scattering susceptibility that capture the IRRS effect These quantities may be used to explore the intensity-dependent modification of the linear susceptibility tensor for a second laser as a result of resonant or near-resonant IR pumping by a first laser. These quantities may be used to explore the intensity-dependent modification of the linear susceptibility tensor for a second laser as a result of resonant or near-resonant IR pumping by a first laser. (4) We employ first-principles computational techniques for the perovskite SrTiO3 to investigate the physical mechanism of IRRS and opportunities for ultrafast material optical property control
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