Phase-enhanced directional asymmetry in photon-phonon conversion via coupled optomechanical cavities.

Nonreciprocity, characterized by direction-dependent signal propagation, is fundamental to technologies such as isolators, signal routing, and precision sensing. In this Letter, we theoretically investigated the phonon transport and the conversion between photonic and acoustic phonon signals within coupled optomechanical cavities that are driven by phase-controlled optical fields. We demonstrate that, unlike phonon transport-which necessitates both dissipation and phase-induced breaking of time-reversal symmetry (TRS) to exhibit nonreciprocity-photon-phonon conversion shows a significant direction-dependent asymmetry, even without TRS breaking. This phenomenon arises from the interference between nonidentical forward and backward conversion pathways associated with different input-output ports. We clarify that this effect represents asymmetric mode conversion rather than strict nonreciprocity. Additionally, the asymmetry in conversion can be notably enhanced, resulting in isolation levels of up to 40 dB, by adjusting the phase difference of the driving lasers.