  We demonstrate a mechanical crystal with an optically programmable defect
mode. By applying an optical spring to a single unit cell of a phononic crystal
membrane, we smoothly transfer a single mechanical mode into the bandgap,
thereby localizing its spatial profile from one spanning the entire crystal to
one confined within a few unit cells. This localization is evidenced by an
enhanced mechanical frequency shift commensurate with a 37-fold reduction in
the mode's participating mass. Our results lay groundwork for a new class of
optomechanical systems that control mechanical mode profile and participating
mass.
