kg-mass prototype demonstrator for DUAL gravitational wave detector: Optomechanical excitation and cooling

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 through 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 ’backaction 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.