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Dark mode in metamaterials has become a vital component in determining the merit of the Fano type of interference in the system. Its strength dictates the enhancement and suppression in the amplitude and Q-factors of resulting resonance features. In this work, we experimentally probe the effect of strong near-field coupling on the strength of the dark mode in a concentrically aligned bright resonator and a dark split ring resonator (SRR) system exhibiting the classical analog of the electromagnetically induced transparency effect. An enhanced strong magnetic field between the bright-dark resonators destructively interferes with the inherent magnetic field of the dark mode to completely annihilate its effect in the coupled system. Moreover, the observed annihilation effect in the dark mode has a direct consequence on the disappearance of the SRR effect in the proposed system, wherein under the strong magnetic interactions, the LC resonance feature of the split ring resonator becomes invisible to the incident terahertz wave. MOE (Min. of Education, S’pore) Accepted version

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.