Mechanical oscillation and cooling actuated by the optical gradient force.

In this work, we combine the large per-photon optical gradient force with the sensitive feedback of a high quality factor whispering-gallery microcavity. The cavity geometry, consisting of a pair of silica disks separated by a nanoscale gap, shows extremely strong dynamical backaction, powerful enough to excite coherent oscillations even under heavily damped conditions (mechanical Q approximately 4). In vacuum, the threshold for regenerative mechanical oscillation is lowered to an optical input power of only 270 nW, or roughly 1000 stored cavity photons, and efficient cooling of the mechanical motion is obtained with a temperature compression factor of nearly 14 dB with an input optical power of only 11 microW.