Directional optical switch using interacting dark states

Restricted Access.We experimentally observe a non-linear absorption resonance in one of the two probe beams of a double Λ system which is interacting with three optical beams. This probe beam counterpropagates to the other two beams, both of which are co-propagating. The absorption resonance in this probe beam arises due to interaction between the dark states of the double Λ created by the co-propagating beams. The double Λ configuration of levels is in the D2 hyperfine manifold of 87Rb atoms, and the interaction is studied at room temperature. The non-linear absorption resonance is shown to be a three-photon process arising whenever the two co-propagating beams satisfy the two-photon Raman condition, necessary for electromagnetically induced transparency. Through a dressed state analysis, we show that the absorption feature is a result of interference between the absorption pathways to the excited states, from the ground states of the double Λ. The non-linear nature of the probe beam absorption enables it to perform like a directional optical switch. This absorption switch operates conditioned on non-absorption of a single photon of the same frequency due to a coherent process, in a beam traveling in the opposite direction