  The next generation of gravitational wave (gw) detectors is expected to fully
enter into the quantum regime of force and displacement detection. With this
aim, it is important to scale up the experiments on opto-mechanical effects
from the microscopic regime to large mass systems and test the schemes that
should be applied to reach the quantum regime of detection. In this work we
present the experimental characterization of a prototype of massive gw
detector, composed of two oscillators with a mass of the order of the kg, whose
distance is read by a high finesse optical cavity. The mechanical response
function is measured by exciting the oscillators though modulated radiation
pressure. We demonstrate two effects crucial for the next generation of
massive, cryogenic gw detectors (DUAL detectors): a) the reduction of the
contribution of 'local' susceptibility thanks to an average over a large
interrogation area. Such effect is measured on the photo-thermal response
thanks to the first implementation of a folded-Fabry-Perot cavity; b) the
'back-action reduction' due to negative interference between acoustic modes.
Moreover, we obtain the active cooling of an oscillation mode through radiation
pressure, on the described mechanical device which is several orders of
magnitude heavier than previously demonstrated radiation-pressure cooled
systems.
