Phonon-Dressed Third-Harmonic Generation in Diamond

We demonstrate a strong, elastic, frequency-upshifting optical polarizability that accompanies sum-frequency driving of Raman phonons and two-photon absorption, extending the duality of structural and optical property modification recently observed in light-driven phononics. The effect, phonon-dressed third-harmonic generation (THG), is a degenerate low-frequency analog of coherent anti-Stokes Raman scattering. In diamond, it has a cubic electric susceptibility at least 58 times larger than purely electronic THG, with polarization and frequency dependence reflecting strong coupling to Raman phonon degrees of freedom.