Enhancement of electromagnetically induced absorption and four-wave mixing in a double two-level system

We present a theoretical investigation of the electromagnetically induced absorption (EIA) due to transfer of population (TOP) in the double two-level system (TLS). It shows that one TLS is responsible for the sub-natural absorption part of EIA, and the other TLS is responsible for the natural absorption part of EIA. We propose a scheme in which the sub-natural absorption part of EIA is governed by the effect of coherent hole burning (CHB) and achieves an enhancement of at least two orders of magnitude with the detuned coupling field, while the natural absorption part is dominated by the effect of Mollow absorption (MA) and does not change with the detuned coupling field. Due to the effects of CHB and MA, the magnitude of four-wave mixing (FWM) achieves a significant increase for double TLS. We show in detail the evolution of the magnitude of the FWM signal with coupling detuning and Rabi frequency. It is demonstrated that strong resonances occur in the FWM profile at frequencies symmetrically displaced from the frequency of the coupling field by coupling detuning.