  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.
