  We propose and evaluate a new type of optical force microscope based on a
standing wave optical trap. Our microscope, calibrated in-situ and operating in
a dynamic mode, is able to trap, without heating, a single metallic
nanoparticle of 150 nm that acts as a highly sensitive probe for external
radiation pressure. An Allan deviation-based stability analysis of the setup
yields an optimal 0.1 Hz measurement bandwidth over which the microscope is
thermally limited. Over this bandwidth, and with a genuine sine-wave external
drive, we demonstrate an optical force resolution down to 3 fN in water at room
temperature with a dynamical range for force detection that covers almost 2
orders of magnitude. This resolution is reached both in the confined and freely
diffusing regimes of the optical trap. In the latter, we measure $10^{-11}$ m
induced displacements on the trapped nanoparticle, spatially confined within
less than 25 nm along the optical axis.
