Tailoring the Dynamics of a Nanomechanical resonator with (Anti)-squashed light

We have designed and implemented a phase–sensitive closed–loop control scheme to engineer the fluctuations of the pump field which drives an optomechanical system. The feedback loop acts on the driving field and it can be engineered to modify the effect of radiation pressure on the mechanical resonator. We first show that, operating in the counter–intuitive “anti–squashing” regime of positive feedback and increased field fluctuations, sideband cooling of a nanomechanical membrane within an optical cavity is improved by 7.5 dB with respect to the case without feedback. Close to the quantum regime of reduced thermal noise, such scheme would allow going well below the quantum backaction cooling limit, even better than what can be achieved by injecting squeezed light. A further unexpected application of anti-squashed light is that it enables a weakly coupled optomechanical system to display normal-mode splitting, which is a typical signature of strongly-coupled system. In fact in this case the resonator interacts with an effective very narrow cavity mode modified by feedback.