  Integrated optomechanical cavities stand as a promising means to interface
mechanical motion and guided optical modes. State-of-the-art demonstrations
rely on optical and mechanical modes tightly confined of in micron-scale areas
to achieve strong optomechanical coupling. However, the need for tight
optomechanical confinement and the general use of suspended devices hinders
interaction with external devices, limiting the potential for the
implementation of complex circuits. Here, we propose and demonstrate a new
approach for optomechanical cavities coupling free-propagating surface acoustic
waves (SAWs) and guided optical modes. The cavity is formed by a periodic array
of silicon nanopillars with subwavelength separation, implemented in
silicon-on-insulator substrate. Optical pumping yields a strong radiation
pressure that drives the harmonic vibration of the pillars, periodically
deforming the silica under-cladding and exciting the SAW. The propagation of
the SAW deforms the cavity period, modulating the resonance wavelength to close
the optomechanical coupling loop. Based on this concept, we experimentally
demonstrate a phonon laser at room temperature and ambient conditions with
optical pump power as low as 1 mW. We also show the possibility to cascade this
process, achieving a frequency comb generation with more than 30 harmonic
lines. These results open a new path to achieve strong bidirectional coupling
between integrated waveguides and SAW, with a great potential for a wide range
of applications in quantum and classical domains.
