The role of atomic cooperation in producing optical bistability in a system consisting of resonant atoms in a low-Qcavity driven on resonance is investigated. It is shown that while completely correlated atomic cooperation precludes steady-state bistability, the assumption that each atom behaves independently and is affected only by the average behavior of the others leads to a bistable steady state. Certain physical and mathematical aspects of the two kinds of cooperation are discussed, and the relationship of the results to the problem of optical bistability without a resonator is indicated.
The optical bistability of a four-level system with triplet-state absorption typical of organic media is studied. We derive the state equation of optical bistability with triplet-state absorption. Different bistabilities with resonant absorption or dispersion of the triplet state are numerically discussed. The effects of the cooperativity parameter and the atomic- and cavity-detuning parameters on optical bistable behavior are analyzed. It is found that the hysteresis loops decrease with an increase of both the cooperativity parameter and the atomic-detuning parameter of the triplet state and that under some conditions reverse saturable-absorptive media can display bistability.