Nonlinear phononics: A new ultrafast route to lattice control

TL;DR

This paper experimentally demonstrates ionic Raman scattering, revealing a new ultrafast method to control lattice structures in solids via rectified phonon fields, distinct from electronic excitation-based techniques.

Contribution

It provides the first experimental evidence of ionic Raman scattering as a mechanism for ultrafast lattice control, expanding the understanding of phonon-mediated optical control.

Findings

Ionic Raman scattering can induce atomic displacements beyond IR-active vibrations.

Rectified phonon fields exert directional forces on the crystal lattice.

This mechanism enables ultrafast control of lattice structures without electronic excitation.

Abstract

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.