It is well known that the ground-state cooling of a single mechanical oscillator is limited by some strict conditions in the conventional sideband cooling method, such as small cavity decay, weak effective coupling, and minimum cooling phonon occupancy. Here we propose an effective scheme to greatly improve the ground-state cooling of two mechanical oscillators in a compound three-mode optomechanical system. By introducing the frequency modulation and the bias gate voltage modulation switch into the system, it is demonstrated that the scheme is well suitable for simultaneously cooling the two mechanical oscillators whether their frequencies are identical or largely different. Compared with previous schemes without modulation, our scheme can be achievable in a much wider parameter zone but shows a more ideal cooling effect, i.e., ranging from weak coupling to ultrastrong coupling and from the resolved-sideband regime to the unresolved-sideband regime. Moreover, the minimum cooling phonon occupancies obtained in our scheme for identical mechanical frequencies successfully break the quantum backaction limit defined in the absence of modulation. More importantly, even in the conventional unstable zone, our scheme is still workable and the steady ground-state cooling can be achieved perfectly. Our scheme opens up an alternative method for simultaneously cooling multiple mechanical oscillators with lower cooling phonon occupancy even breaking the quantum backaction limit but with fewer parametric limitations.

• Received 30 June 2020
• Revised 14 February 2021
• Accepted 22 February 2021

DOI:https://doi.org/10.1103/PhysRevA.103.033508