Phase control of optical bistability and multistability in a tripod four-level atomic medium

Optical bistability and multistability behaviors are investigated in a tripod four-level atomic configuration, including a unidirectional ring cavity. We find that optical bistability and multistability can be obtained by changing the relative phase of the driven fields and the intensity of the microwave field. In addition, we can switch optical bistability to optical multistability by adjusting the relative phase of the driven fields or vice versa, and the threshold can be controlled via changing the parameters of the scheme.