  In stimulated Raman scattering, two incident optical waves induce a force
oscillating at the difference of the two light frequencies. This process has
enabled important applications such as the excitation and coherent control of
phonons and magnons by femtosecond laser pulses. Here, we experimentally and
theoretically demonstrate the so far neglected up-conversion counterpart of
this process: THz sum-frequency excitation of a Raman-active phonon mode, which
is tantamount to two-photon absorption by an optical transition between two
adjacent vibrational levels. Coherent control of an optical lattice vibration
of diamond is achieved by an intense terahertz pulse whose spectrum is centered
at half the phonon frequency of 40 THz. Remarkably, the carrier-envelope phase
of the driving pulse is directly imprinted on the lattice vibration. New
prospects in infrared spectroscopy, light storage schemes and lattice
trajectory control in the electronic ground state emerge.
