  Nanophotonic devices take advantage of geometry-dependent optical properties
to confine and enhance the interaction of light with matter on small scales. By
carefully patterning nanoscale geometries, coupling of responses across
distinct physical domains can be achieved, such as photon and phonon modes in
optomechanical crystals. Extreme optical confinement of plasmonics can localize
strong cross-domain interactions into nanometer-scale volumes, smaller than
many nanosystem's thermal and mechanical eigenmodes, opening new regimes for
engineering reconfigurable, dynamic nanosystems. Here, we present a nanosystem
based on an electrically actuated, mechanically coupled gap-plasmon
metamolecule resonator. Integrated electrical actuation reveals distinct
dispersive and reactive optomechanical coupling regimes of the metamolecules,
and enables both broad spectral tuning of individual plasmonic resonances and
selective optical transduction of nanomechanical modes. Strong thermomechanical
backaction from a single point-like metamolecule yields optically driven
self-oscillation, and we show that this metamolecule phonon laser can be
injection-locked to enhance weak mechanical stimulii. This multi-domain
nanosystem advances nanomechanical sensing and optical modulation, opens new
avenues for studying nanoscale nonlinear optomechanics, and may serve as a
building block for future reconfigurable "smart" metamaterials.
