Integrated optomechanical cavities allow precise control of optical and mechanical modes and enable strong photonphonon interactions in micron-scale volume, key for the implementation of microwave-photonic oscillators and quantum transducers. Silicon photonics provides low production cost and compatibility with the state-of-art optoelectronic circuitry. Thus, it is particularly interesting for the implementation of on-chip optomechanics. However, silicon has higher stiffness and acoustic velocity than the silica cladding, hampering phonon confinement in silicon-on-insulator (SOI) waveguides. Here, we present our most recent results on SOI optomechanical systems coupling mechanical and guided optical modes. The cavities use silicon pillars with subwavelength period. Strong radiation pressure is exploited to drive the optomechanical coupling. Based on this concept, we experimentally demonstrate the optomechanical coupling between photons and high-quality factor phonons in non-suspended cavities, with a great potential for applications in quantum and classical photonics.

Flexible control of photons and phonons in silicon nanophotonic waveguides is a key feature for emerging applications in communications, sensing and quantum technologies. Strong phonon leakage towards the silica under-cladding hampers optomechanical interactions in silicon-on-insulator. This limitation has been circumvented by totally or partially removing the silica under-cladding to form pedestal or silicon membrane waveguides. Remarkable optomechanical interactions have been demonstrated in silicon using pedestal strips, membrane ribs, and photonic/phononic crystal membrane waveguides. Still, the mechanical frequencies are limited to the 1-5 GHz range. Here, we exploit the periodic nanostructuration in Si membrane gratings to shape GHz phononic modes and nearinfrared photonic modes, achieving ultrahigh mechanical frequency (10 GHz) and strong photon-phonon overlap (61.5%) simultaneously. Based on this concept, we experimentally demonstrate a one-dimension optomechanical micro-resonator with a high mechanical frequency of 10 GHz and a quality factor of 1000. These results were obtained at room temperature and ambient conditions with an intracavity optical power below 1 mW, illustrating the efficient optical driving of the mechanical mode enabled by the proposed approach.