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A novel design for an optomechanical cavity consisting of a series of rectangular silicon nanobricks, with each brick acting as an independent mechanical resonator but all coupled to a same optical field, is proposed, and numerically demonstrated. Each brick is placed on top of a thin silica pillar that provides mechanical support whilst isolates the individual mechanical resonances. The mass sensing capabilities of this cavity are studied through numerical simulations, proving that a point mass approximation can be used for silica nanoparticles with radius smaller than 100 nm, and that different nanoparticles can be measured independently but simultaneously.

Cavity optomechanics has recently emerged as a new paradigm enabling the manipulation of mechanical motion via optical fields tightly confined in deformable cavities. When driving an optomechanical (OM) crystal cavity with a laser blue-detuned with respect to the optical resonance, the mechanical motion is amplified, ultimately resulting in phonon lasing at MHz and even GHz frequencies. In this work, we show that a silicon OM crystal cavity performs as an OM microwave oscillator when pumped above the threshold for self-sustained OM oscillations. To this end, we use an OM cavity designed to have a breathing-like mechanical mode at 3.897 GHz in a full phononic bandgap. Our measurements show that the first harmonic of the detected signal displays a phase noise of ≈−100 dBc/Hz at 100 kHz. Stronger blue-detuned driving leads eventually to the formation of an OM frequency comb, whose lines are spaced by the mechanical frequency. We also measure the phase noise for higher-order harmonics and show that, unlike in Brillouin oscillators, the noise is increased as corresponding to classical harmonic mixing. Finally, we present real-time measurements of the comb waveform and show that it can be fitted to a theoretical model recently presented. Our results suggest that silicon OM cavities could be relevant processing elements in microwave photonics and optical RF processing, in particular in disciplines requiring low weight, compactness and fiber interconnection.

In this work we show that a silicon optomechanical crystal cavity can be used as an optomechanical oscillator when driven to the phonon lasing condition with a blue-detuned laser. The optomechanical cavity is designed to have a breathing like mode vibrating at Ωm/2π =3.897 GHz in a full phononic bandgap. Our measurements show that the first harmonic displays a phase noise of -100 dBc/Hz at 100 kHz. Stronger bluedetuned driving leads eventually to the formation of an optomechanical frequency comb, with lines spaced by the mechanical frequency. The measured phase noise grows up with the harmonic number, as in classical harmonic mixing. We present real-time measurements of the comb waveform and show that it can be adjusted to a theoretical model recently presented. Our results suggest that silicon optomechanical cavities can play a role in integrated microwave photonics.

Optomechanical cavities can be used as highly sensitive mass sensors actuated by an optical field. In this work, we introduce and numerically demonstrate a new design for an optomechanical cavity consisting of a series of asymmetrically distributed rectangular silicon nanobricks, with each brick acting as an independent mechanical resonator but all coupled to the same optical field. Each silicon brick is placed on top of a thin silica pillar that ensures mechanical support whilst providing enough acoustic isolation between the individual mechanical resonances - at GHz frequencies - of each brick. The mass sensing capabilities of this cavity are studied through numerical simulations, proving that a point mass approximation can be used for silica nanoparticles with a radius smaller than 100 nm and that different nanoparticles can be measured independently but simultaneously and in real-time.