Coherent perfect absorption accompanying subnatural narrowing by vacuum-induced coupling

We present an alternative route to realize coherent perfect absorption (CPA) and subnatural narrowing by vacuum-induced coupling (VIC) between excited levels of an atomic ensemble in an optical nanofiber (ONF) cavity quantum electrodynamics (CQED) system. By employing the VIC arising from quantum interference between the spontaneous emission pathways from the upper excited doublet to one common ground state in the three-level $V$-type atoms, a very narrow absorption resonance can be observed in the generated dark-state spectral window owing to quantum destructive interference between the spontaneous emission pathways. The spectroscopic output feature exhibits a reduced linewidth, which is much narrower than both the empty-cavity linewidth and the atomic transition linewidth. As a check, the approximate analytical results for the ONF cavity-output power and linewidth of interest agree well with the numerical simulations. Furthermore, we find that the CPA can be used to probe the existence of the VIC effect. By tuning the relative phase between the two input light fields appropriately, the conversion between the CPA and coherent perfect transmission (CPT) can be realized in both the linear and nonlinear excitation regimes, i.e., the relative phase can switch the ONF CQED system from a perfect photon absorber to a nearly perfect photon transmitter. Also, we analyze the optical bistability of the ONF CQED system and its phase dependence in the nonlinear excitation regime. Such interference control of the CPA-to-CPT (or vice versa) scheme proposed here may have potential applications in optical logical devices for optical computing and quantum information processing.