  We realise a phase-sensitive closed-loop control scheme to engineer the
fluctuations of the pump field which drives an optomechanical system, and show
that the corresponding cooling dynamics can be significantly improved. In
particular, 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 can be improved by 7.5~dB with
respect to the case without feedback. Close to the quantum regime of reduced
thermal noise, such feedback-controlled light would allow going well below the
quantum backaction cooling limit.
