  A stable optical resonator has been built using a 30 micrometer-wide,
metal-coated microcantilever as one mirror. The second mirror was a 12.7
mm-diameter concave dielectric mirror. By positioning the two mirrors 75 mm
apart in a near-hemispherical configuration, a Fabry-Perot cavity with a
finesse equal to 55 was achieved. The finesse was limited by the optical loss
in the cantilever's metal coating; diffraction losses from the small mirror
were negligible. The cavity achieved passive laser cooling of the cantilever's
Brownian motion.
