  We introduce a novel ultra-compact nanocapacitor of coherent phonons formed
by high-finesse interference mirrors based on atomic-scale semiconductor
metamaterials. Our molecular dynamics simulations show that the nanocapacitor
stores THz monochromatic lattice waves, which can be used for phonon lasing -
the emission of coherent phonons. Either one- or two-color phonon lasing can be
realized depending on the geometry of the nanodevice. The two color regimes of
the capacitor originates from the distinct transmittance dependance on the
phonon wave packet incident angle for the two phonon polarizations at their
respective resonances. Phonon nanocapacitor can be charged by cooling the
sample equilibrated at room temperature or by the pump-probe technique. The
nanocapacitor can be discharged by applying tunable reversible strain,
resulting in the emission of coherent THz acoustic beams.
