  The implementation of optomechanical devices in silicon-on-insulator (SOI),
the canonical silicon photonics technology is seriously hampered by the strong
phonon leakage into the silica under-cladding. This limitation has been
partially circumvented by total or partial removal of the silica under-cladding
to form Si membranes or pedestal waveguides. However, this approach complicates
integration with standard silicon optoelectronics circuitry, limiting the
versatility and application of the strategy. Here, we propose and demonstrate a
new strategy to confine photons and phonons in SOI without removing the silica
under-cladding. Inspired by end-fire antenna arrays, we implement a periodic
nanostructuration of silicon that simultaneously enables cancelling phonon
leakage by destructive interference and guiding of photons by metamaterial
index confinement. Based on this concept, we implement SOI optomechanical
micro-resonators yielding remarkable optomechanical coupling (go=49 kHz)
between 0.66 GHz mechanical modes and near-infrared optical modes. The
mechanical mode exhibits a measured quality factor of Qm = 730, the largest
reported for SOI optomechanical resonators, without silica removal. This value
compares favorably with state-of-the-art Si membrane waveguides recently used
to demonstrate remarkable Brillouin interactions in silicon (Qm ~ 700). These
results open a new path for developing optomechanics in SOI without the need
for silica removal, allowing seamless co-integration with current Si
optoelectronics circuits, with a great potential for applications in
communications, sensing, metrology, and quantum technologies.
