CAVITY OPTOMECHANICAL SENSORS
Houxun Miao, Kartik Srinivasan, Matthew T. Rakher, Marcelo Davanco and Vladimir Aksyuk
The performance of many micromechanical sensors and the ability to scale them down to nanoscale dimensions are often limited by the precision of the available techniques for mechanical motion measurement. For example, for high speed Atomic Force Microscopy (AFM), a small cantilever is desired to obtain a high mechanical frequency and a small spring constant simultaneously . Commonly used integrated electrostatic readout is limited by the available sensor capacitance, often dictated by footprint and mechanical speed requirements. For ultrahigh sensitivity, MEMS have been coupled to external optics to form high quality factor (Q) optical resonators demonstrating near quantum-limited displacement sensitivity . In addition to large size and cost, these systems do not scale well to NEMS devices due to the diffraction limit. Approaches involving movable periodic nanostructures and photonic crystals  are very promising, however they require external optical excitation and readout, so far have not achieved very high optical Q and it is not clear how to scale them to measure individual NEMS.
By microfabricating on a single chip a high Q (106) photonic resonator near-field coupled to a mechanically separate movable structure, we achieve a novel class of optomechanical transducers that combine high precision optical interferometry with compactness, stability and robustness of integrated photonics. The approach allows for separate engineering of the mechanical part of the transducer, making it widely applicable for a variety of MEMS and NEMS sensing situations requiring high precision, high bandwidth and small footprint.
We have demonstrated the principle using two examples. Both use the same 10 um diameter Si optical microdisk resonator and have the total sensor footprint below 15 um x 15 um. The first is an MEMS transducer integrating the optomechanical sensor, an electrostatic actuator and an optical waveguide for convenient fiber connectorization. The second is a simpler device to demonstrate the scaling of the technique. It achieves precision optical readout (4.4x10-16±0.3x10-16 m/√Hz) of an integrated NEMS cantilever probe ( 65 nm x 260 nm x 20 um) designed for application in high speed AFM.