Nonreciprocal coupling modulated difference-sideband generation in a double-cavity optomechanical system.

We theoretically study the difference-sideband generation in a double-cavity optomechanical system with nonreciprocal coupling. Beyond the conventional linearized description of optomechanical interactions, we derive analytical expressions for the efficiency of difference-sideband generation by using a perturbation method. Here we investigate bistable behaviors of the system and show the difference-sideband generation modulated by the nonreciprocal coupling strength between the two cavities. We find that the nonreciprocal coupling strength can not only affect the bistability of the system but also lead to different efficiencies of difference-sideband generation at low power. To achieve high efficiency of difference-sideband generation, we give the optimal matching conditions under different parameter mechanisms. Especially as the power increases, we find new matching conditions with remarkable difference-sideband generation emerging, which is attribute to the strong coherence between the cavity field and the mechanical oscillator. Furthermore, a feasible scheme to obtain difference-sideband generation by employing multiple adjustable variables is proposed. Our results may find applications in nonreciprocal optical frequency combs and communications, and provide a potential method for precision measurements and on-chip manipulation of light transmission.