The combination of large per-photon optical force and small motional mass attainable in nanocavity optomechanical systems results in strong dynamical back-action between mechanical motion and the cavity light field. In this work we study the optical control of mechanical motion within two different nanocavity structures, a zipper nanobeam photonic crystal cavity and a double-microdisk whispering-gallery resonator. The strong optical gradient force within these cavities is shown to introduce signifcant optical rigidity into the structure, with the dressed mechanical states renormalized into optically-bright and optically-dark modes of motion. With the addition of internal mechanical coupling between mechanical modes, a form of optically-controlled mechanical transparency is demonstrated in analogy to electromagnetically induced transparency of three-level atomic media. Based upon these measurements, a proposal for coherently transferring RF/microwave signals between the optical field and a long-lived dark mechanical state is described.
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