A hybrid on-chip opto-nanomechanical transducer for ultra-sensitive   force measurements
  Nanomechanical oscillators have been employed as transducers to measure force, mass and charge with high sensitivity. They are also used in opto- or electromechanical experiments with the goal of quantum control and phenomena of mechanical systems. Here, we report the realization and operation of a hybrid monolithically integrated transducer system consisting of a high-$Q$ nanomechanical oscillator with modes in the MHz regime coupled to the near-field of a high-$Q$ optical whispering-gallery-mode microresonator. The transducer system enables a sensitive resolution of the nanomechanical beam's thermal motion with a signal-to-noise of five orders of magnitude and has a force sensitivity of $74\,\rm{aN}\,\rm{Hz}^{-1/2}$ at room temperature. We show, both theoretically and experimentally, that the sensitivity of continuous incoherent force detection improves only with the fourth root of the averaging time. Using dissipative feedback based on radiation pressure enabled control, we explicitly demonstrate by detecting a weak incoherent force that this constraint can be significantly relaxed. We achieve a more than 30-fold reduction in averaging time with our hybrid transducer and are able to detect an incoherent force having a force spectral density as small as $15\,\rm{aN}\,\rm{Hz}^{-1/2}$ within $35\,\rm{s}$ of averaging. This corresponds to a signal which is 25 times smaller than the thermal noise and would otherwise remain out of reach. The reported monolithic platform is an enabling step towards hybrid nanomechanical transducers relying on the light-mechanics interface. 
Emanuel Gavartin, Pierre Verlot, Tobias J. Kippenberg
physics.optics