  We present here an optomechanical system fabricated with novel stress
management techniques that allow us to suspend an ultrathin defect-free silicon
photonic-crystal membrane above a Silicon-on-Insulator (SOI) substrate with a
gap that is tunable to below 200 nm. Our devices are able to generate strong
attractive and repulsive optical forces over a large surface area with simple
in- and outcoupling and feature the strongest repulsive optomechanical coupling
in any geometry to date (gOM/2{\pi} ~ -65 GHz/nm). The interplay between the
optomechanical and photo-thermal-mechanical dynamics is explored, and the
latter is used to achieve cooling and amplification of the mechanical mode,
demonstrating that our platform is well-suited for applications in low-power
mass, force, and refractive index sensing as well as and optomechanical
accelerometry.
