ABSTRACT
Effective coupling of a microscopic transducer to an external, bulk optical cavity is technically challenging due to alignment, drift and the need for high reflectivity coating on the transducer.14 Because of the diffraction limit, in such a system the light cannot be focused to a spot much smaller than the optical wavelength. When the transducer (cantilever, nanobeam, etc.) size is reduced below the wavelength of light, the coupling and the sensitivity deteriorate dramatically. Here, these challenges are overcome by integrating the nanomechanical transducer in the near field of a nanophotonic cavity microfabricated together on the same chip.17-19 The cavity can be self-aligned to the nanobeam with the accuracy of a few nanometers, and the mechanical beam can interact strongly with the optical field in the cavity without mechanical contact. The optical quality of the cavity can be maintained as well. This fully integrated, stable, and practical optomechanical device can be fiber connectorized and implement the readout of the mechanical motion of the nanoscale transducer with GHz bandwidth and the imprecision near the standard quantum limit with only microwatts of optical power.The combined sensor can achieve the fundamental thermomechanical noise floor not only near the mechanical resonance frequency, but over a broad frequency range from near DC to several times the mechanical resonance.19 Furthermore, the dynamics of the mechanical transducer can be modified and tailored to specific needs by either or both using optical forces or a suitable other actuation mechanism, such as an integrated electrostatic actuator. Both a classical feedback control scheme with virtually no excess noise injected into the transducer, and a quantum control scheme through optical forces, whereby the transducer acts directly on the cavity optical field, can be implemented. Such schemes can be used to widely tune the mechanical frequency, implement a
regenerative oscillator or cold-dampen a high mechanical Qm, low noise transducer and produce a flat transfer function for dynamic operation over a broad frequency range.
