We present an experiment where the motion of a silicon micro-mechanical resonator is optically monitored with
a very high-finesse optical cavity, down to a quantum-limited sensitivity at the 10-19m/√Hz level. We have
observed the thermal noise of the resonator at room temperature over a wide frequency range, and fully characterized
its optomechanical behaviour, in good agreement with theoretical models. We have also demonstrated a
direct effect of intracavity radiation pressure upon the dynamics of the micro-resonator in a detuned high-finesse
optical cavity: depending on the sign of the detuning, we have obtained both cooling and heating, with an
effective temperature ranging from 10 to 2000 K. We have also observed a related radiation-pressure induced
instability of the resonator. This experiment opens the way to radiation pressure-driven quantum optics effects,
with silicon resonators offering high resonance frequencies, low effective masses, and a high displacement sensitivity.
Possible experiments include QND measurement of light intensity or optomechanical squeezing of the
optical field, as well as the optical observation of the quantum ground state of a macroscopic oscillator.