Optically Defined Phononic Crystal Defect

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 band gap, 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. This fast, reversible, all-optical control represents a qualitatively new way to engineer mechanical defects, advantageously with in situ reconfigurability.