  The creation and manipulation of coherence continues to capture the attention
of scientists and engineers. The optical laser is a canonical example of a
system that, in principle, exhibits complete coherence. Recent research has
focused on the creation of coherent, laser-like states in other physical
systems. The phonon laser is one example where it is possible to amplify
self-sustained mechanical oscillations. A single mode phonon laser in a
levitated optical tweezer has been demonstrated through appropriate balance of
active feedback gain and damping. In this work, coherent control of the
dynamics of an optical tweezer phonon laser is used to share coherence between
its different modes of oscillation, creating a multimode phonon laser. The
coupling of the modes is achieved by periodically rotating the asymmetric
optical potential in the transverse focal plane of the trapping beam via trap
laser polarization rotation. The presented theory and experiment demonstrate
that coherence can be transferred across different modes of an optical tweezer
phonon laser, and are a step toward using these systems for precision
measurement and quantum information processing.
