  To date, two types of coupling between electromagnetic radiation and a
crystal lattice have been identified experimentally. One is direct, for
infrared (IR)-active vibrations that carry an electric dipole. The second is
indirect, it occurs through intermediate excitation of the electronic system
via electron-phonon coupling, as in stimulated Raman scattering. Nearly 40
years ago, proposals were made of a third path, referred to as ionic Raman
scattering (IRS). It was posited that excitation of an IR-active phonon could
serve as the intermediate state for a Raman scattering process relying on
lattice anharmonicity as opposed to electron phonon interaction. In this paper,
we report an experimental demonstration of ionic Raman scattering and show that
this mechanism is relevant to optical control in solids. The key insight is
that a rectified phonon field can exert a directional force onto the crystal,
inducing an abrupt displacement of the atoms from the equilibrium positions
that could not be achieved through excitation of an IR-active vibration alone,
for which the force is oscillatory. IRS opens up a new direction for the
coherent control of solids in their electronic ground state, different from
approaches that rely on electronic excitations.
