Tunable slow and fast light in an atom-assisted optomechanical system

We theoretically investigate the slow and fast light effects of a probe laser in an optomechanical double-ended cavity filled with a bunch of identical two-level cold 87Rb atoms. It is shown that the presence of atoms can transfer the optomechanically induced transparency (OMIT) window into the steeper Fano shapes and the broader normal-mode splitting shapes. We find that the transmission group delay related to the Fano resonance is much greater than that of the OMIT and normal-mode splitting at the resonances, meanwhile, with the growing of the atomic number, the slow light becomes increasingly significant. The value of reflection group delay related to the OMIT and normal-mode splitting is positive, while the value related to the Fano resonance is significantly negative. This indicates that the reflection parts of output field contain both slow light and fast light. Therefore, via changing the atomic number and detuning, we can realize not only the manipulation of the magnitudes of the group delay and advance, but also a tunable switch from slow light to fast light.