We report on the detection of free nanoparticles in a micromachined,
open-access Fabry-P\'erot microcavity. With a mirror separation of 130ÎĽm,
a radius of curvature of 1.3mm, and a beam waist of 12ÎĽm, the mode
volume of our symmetric infrared cavity is smaller than 15pL. 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 150nm 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 10nm in radius
and 1Ă—107amu in mass.Comment: 4 pages, 3 figure