Optomechanical systems hold great promise for applications in quantum information transduction and routing. To get the system in the required mechanical ground state using optomechanical cooling, the optical linewidth needs to be smaller than the mechanical frequency (the so-called resolved sideband condition), which avoids the detrimental heating by quantum backaction that gives rise to the Doppler limit in sideband-unresolved cavities.

However, by parametrically coupling a low frequency mechanical mode to an auxiliary high frequency mechanical mode, the low frequency mode can be efficiently cooled, even when the optical linewidth is larger than its frequency [1].

We realize an optomechanical device possessing mechanical modes in both the MHz and GHz range, coupled to the same NIR optical mode. We theoretically and experimentally demonstrate linewidth broadening due to the induced interaction, indicating potential for cooling beyond the Doppler limit. Moreover, we show that this interaction can be used to reverse the effect of dynamical backaction, leading to amplification instead of cooling. We find a sharp transition between both regimes indicating great responsivity to the control tone.

This shows that parametric control in multimode optomechanical systems can dramatically alter the dynamics, suggesting potential for improved control of resonators and their couplings.

[1] T. Ojanen and K. Børkje. Ground-state cooling of mechanical motion in the unresolved sideband regime by use of optomechanically induced transparency. Physical Review A, 90(1):013824, July 2014.