  In this work, we study the dispersive coupling between optical
quasi-bound-states in the continuum at telecom wavelengths and GHz-mechanical
modes in high-index wavelength-sized disks. We show that such cavities can
display values of the optomechanical coupling rate on par with optomechanical
crystal cavities ($g_{0}/2\pi\backsimeq$ 800 kHz). Interestingly,
optomechanical coupling of optical resonances with mechanical modes at
frequencies well above 10 GHz seems attainable. We also show that mechanical
leakage in the substrate can be extremely reduced by placing the disk over a
thin silica pedestal. Our results suggest a new route for ultra compact
optomechanical cavities which can potentially be arranged in massive arrays
forming optomechanical metasurfaces.
