Fluctuating nanomechanical system in a high finesse optical microcavity

The idea of extending cavity quantum electrodynamics experiments to sub-wavelength sized nanomechanical systems has been recently proposed in the context of optical cavity cooling and optomechanics of deformable cavities. Here we present an experiment involving a single nanorod consisting of about 10$^{9}$ atoms precisely positioned into the confined mode of a miniature high finesse Fabry-Pérot microcavity. We show that the optical transmission of the cavity is affected not only by the static position of the nanorod but also by its vibrational fluctuation. The Brownian motion of the nanorod is resolved with a displacement sensitivity of 200 fm/√Hz at room temperature. Besides a broad range of sensing applications, cavity-induced manipulation of optomechanical nanosystems and back-action is anticipated.