  We propose a novel type of optomechanical coupling which enables a tripartite
interaction between a quantum emitter, an optical mode and a macroscopic
mechanical oscillator. The interaction uses a mechanism we term mode field
coupling: mechanical displacement modifies the spatial distribution of the
optical mode field, which in turn modulates the atom-photon coupling rate. In
properly designed multimode optomechanical systems, we can achieve situations
in which mode field coupling is the only possible interaction pathway for the
system. This enables, for example, swapping of a single excitation between
emitter and phonon, creation of nonclassical states of motion and mechanical
ground-state cooling in the bad-cavity regime. Importantly, the emitter-phonon
coupling rate can be enhanced through an optical drive field, allowing active
control of strong atom-phonon coupling for realistic experimental parameters.
