Optomechanical feedback cooling of a 5 mm long torsional mode

We report three orders of magnitude optical cooling of the fundamental torsional mode of a 5 mm long,550 nm diameter optical nanofiber.The rotation of the nanofiber couples to the polarization of guided laser fields.We use a weak laser probe to monitor the rotation and use feedback to modulate the polarization of an auxiliary drive laser providing torque.Our results present a tool for the optomechanical control of large-scale torsional resonators,with metrological applications and potential implications for studying macroscopic objects in quantum states.