Nanoparticle detection in an open-access silicon microcavity

We report on the detection of free nanoparticles in a micromachined, open-access Fabry-Perot microcavity. With a mirror separation of 130 μm, a radius of curvature of 1.3 mm, and a beam waist of 12 μm, 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 nanoscale 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 1 × 107 amu in mass.