  We experimentally demonstrate an optomechanical cavity based on an air-slot
photonic crystal cavity with optical quality factor Qo=4.2\times10^4 and a
small modal volume of 0.05 cubic wavelengths. The optical mode is coupled with
the in-plane mechanical modes with frequencies up to hundreds of MHz. The
fundamental mechanical mode shows a frequency of 65 MHz and a mechanical
quality factor of 376. The optical spring effect, optical damping and
amplification are observed with a large experimental optomechanical coupling
rate gom/2pi of 154 GHz/nm, corresponding to a vacuum optomechanical coupling
rate g*/2pi of 707 kHz. With sub-mW or less input power levels, the cavity
exhibits strong parametric oscillations. The phase noise of the photonic
crystal optomechanical oscillator is also measured.
