  In the field of optomechanics, radiation forces have provided a particularly
high level of control over the frequency and dissipation of mechanical
elements. Here we propose a class of optomechanical systems in which light
exerts a similarly profound influence over two other fundamental parameters:
geometry and mass. By applying an optical trap to one lattice site of an
extended phononic crystal, we show it is possible to create a tunable,
localized mechanical mode. Owing to light's simultaneous and constructive
coupling with the structure's continuum of modes, we estimate that a trap power
at the level of a single intracavity photon should be capable of producing a
significant effect within a realistic, chip-scale device.
