  We present a direct comparison between resonant terahertz (THz) and
nonresonant impulsive stimulated Raman scattering (ISRS) excitation of
phonon-polaritons in ferroelectric lithium niobate. THz excitation offers
advantages of selectively driving only the forward propagating phonon-polariton
mode to exceedingly high amplitudes, without complications due to nonlinear
processes at the high 800 nm pump fluences used in Raman excitation. At
peak-to-peak THz electric field strengths exceeding 1 MV/cm, the ferroelectric
lattice is driven into the anharmonic regime, allowing experimental
determination of the shape of the potential energy surface.
