  We report the development of an ultrasensitive optomechanical sensor designed
to improve the accuracy and precision of force measurements with atomic force
microscopy. The sensors reach quality factors of 4.3x10^6 and force resolution
on the femtonewton scale at room temperature. Self-calibration of the sensor is
accomplished using radiation pressure to create a reference force.
Self-calibration enables in situ calibration of the sensor in extreme
environments, such as cryogenic ultra-high vacuum. The senor technology
presents a viable route to force measurements at the atomic scale with
uncertainties below the percent level.
