  Direct manipulation of the atomic lattice using intense long-wavelength laser
pulses has become a viable approach to create new states of matter in complex
materials. Conventionally, a high frequency vibrational mode is driven
resonantly by a mid-infrared laser pulse and the lattice structure is modified
through indirect coupling of this infrared-active phonon to other, lower
frequency lattice modulations. Here, we drive the lowest frequency optical
phonon in the prototypical transition metal oxide SrTiO3 well into the
anharmonic regime with an intense terahertz field. We show that it is possible
to transfer energy to higher frequency phonon modes through nonlinear coupling.
Our observations are carried out by directly mapping the lattice response to
the coherent drive field with femtosecond x-ray pulses, enabling direct
visualization of the atomic displacements.
