  We describe a method for feedback-regulation of a microcantilever's response
using optical radiation pressure. One laser measures the position of the
cantilever and another laser applies a force that is a phase-shifted function
of that position. The force is due solely to the momentum of the photons in the
laser. The feedback changes the microcantilever's effective quality factor Qeff
and effective temperature Teff. Reduction of both Qeff and Teff by more than a
factor of 15 is demonstrated. Additionally, we suggest a method for
determination of a microcantilever's spring constant using the known force
exerted on it by radiation pressure.
