  In cavity optomechanics, radiation pressure and photothermal forces are
widely utilized to cool and control micromechanical motion, with applications
ranging from precision sensing and quantum information to fundamental science.
Here, we realize an alternative approach to optical forcing based on superfluid
flow and evaporation in response to optical heating. We demonstrate optical
forcing of the motion of a cryogenic microtoroidal resonator at a level of 1.46
nN, roughly one order of magnitude larger than the radiation pressure force. We
use this force to feedback cool the motion of a microtoroid mechanical mode to
137 mK. The photoconvective forces demonstrated here provide a new tool for
high bandwidth control of mechanical motion in cryogenic conditions, and have
the potential to allow efficient transfer of electromagnetic energy to motional
kinetic energy.
