We experimentally identify a strong, elastic, frequency-upshifting optical effect in diamond, phonon-dressed third-harmonic generation (PD-THG). Unlike traditional electronic THG, PD-THG shows strong sensitivity to pump polarization and frequency, emphasizing its deep connection to Raman phonon degrees of freedom. We demonstrate that its cubic electric susceptibility is at least 58 times larger than the regular electronic route, boosting THG efficiency by up to three orders of magnitude. The effect is a degenerate sum-frequency analog of coherent anti-Stokes Raman scattering that works in the mid-IR and THz range, and has implications for infrared nonlinear optics and the burgeoning realm of light-driven structural control. In addition to its relevance to diamond photonics, we anticipate applications of PD-THG including THz spectroscopy, infrared-controlled nonlinear optical switching, and orientation diagnostics, as well as a temporal diagnostic for coherent structural excitation.
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