- Integrated Silicon Optomechanical Transducers and Their Application in Atomic Force Microscopy

Abstract

Physics experiments measuring unknown quantities by transducing them to mechanical motion have a long and distinguished history 1 .The advent of micro-and now nano-fabricated mechanical transducers has continued this trend over the last two decades, where miniaturization enabled better coupling and measurement of microscopic physical phenomena, from electron and nuclear spins 2,3 to individual vortices in superconductors 4 , from quantum vacuum fluctuations in nanoscale optical cavities 5,6 to shapes and masses of individual molecules 7 .While micromechanical rotation and acceleration sensors are now ubiquitous in both cars and cell phones, in the physics laboratory nanoscale mechanical devices have now been cooled to their quantum mechanical ground state 8,9 and continue to enable measurements with an unprecedented degree of precision and control.Far from being confined to a narrow set of unique applications, micromechanical measurement is at the center of the Atomic Force Microscope (AFM) 10,11 , with its wide variety of operation modes and applications in physics, biology, and industrial inspection 12 .Fabrication techniques for nanobeams and more complicated mechanical transducers with one or more critical dimensions below 100 nm are now well established.Due to their small size and low mass, such devices can achieve a unique combination of high speed, high sensitivity, and local coupling to nanoscale systems and phenomena.Mechanical resonance frequencies above 100 MHz can be achieved without sacrificing mechanical compliance and force sensitivity 2-7 .The ultimate limit on the performance of such transducers is imposed by the fundamental thermodynamic mechanical force noise in accordance with the fluctuation dissipation theorem.It is therefore critical to minimize mechanical dissipation in the transducer.With proper materials and fabrication techniques, a very high mechanical quality factor (Q m ) in the range of 10 4 to 10 6 can be achieved in vacuum environment.In combination with the very low mass and stiffness, phenomenally low intrinsic noise can be achieved, even at room temperature.Although when operated in ambient, the air damping significantly reduces the Q m , the absolute value of the damping coefficient and the corresponding force sensitivity can scale favorably compared to larger mechanical transducers.