We report on the detection of free nanoparticles in a micromachined, open-access Fabry-Perot microcavity. With a mirror separation of $130,mu$m, a radius of curvature of $1.3,$mm, and a beam waist of $12,mu$m, the mode volume of our symmetric infrared cavity is smaller than $15,$pL. The small beam waist, together with a finesse exceeding 34,000, enables the detection of nano-scale dielectric particles in high vacuum. This device allows monitoring of the motion of individual $150,$nm radius silica nanospheres in real time. We observe strong coupling between the particles and the cavity field, a precondition for optomechanical control. We discuss the prospects for optical cooling and detection of dielectric particles smaller than $10,$nm in radius and $1times10^7,$amu in mass.
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