Phonon-Mediated Third-Harmonic Generation in Diamond

We observe strongly anisotropic third-harmonic generation mediated by resonant sum-frequency driving of Raman phonons with THz light, extending light-induced dual control of structural and optical properties in solids. Either strong enhancement or strong suppression of the third harmonic covering six orders of magnitude can be achieved, a result of interference between purely electronic and phonon-mediated contributions to the polarization field. These findings enrich capabilities for tailoring nonlinear optics via phononics and for the spectroscopy of crystalline structural dynamics.