  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.
