Coherent control of optical bistability and multistability via double dark resonances (DDRs)

We analyze theoretically optical bistability (OB) and optical multistability (OM) in a medium consisting of four-level cascade-type cold atoms by means of a unidirectional ring cavity. Due to existence of a radio-frequency (RF) field, upper two-folded levels are coupled and double dark resonances (DDRs) can arise. We show that by proper tuning of the RF field the threshold and the hysteresis cycle shape of OB and OM can be engineered. Also, the effect of intensity and frequency detuning of continuous-wave (cw) control laser field on bistable behavior of the medium is discussed. In addition, we explore the influence of different parameters on switching between OB and OM in this medium, which is applicable in all optical switching.