Effect of spontaneously generated coherence on the dynamics of a four level N-system

Effect of spontaneously generated coherence (SGC) on the optical response of a medium of four-level atoms in N-configuration with near degenerate ground or excited levels is investigated. Analytical expressions valid for arbitrary interaction parameters are obtained for the first and third order susceptibilities, and are used to discuss linear and nonlinear absorption and Kerr nonlinearity. SGC is observed to enhance the Kerr nonlinearity while decreasing or increasing the linear/nonlinear absorption depending on the Rabi frequencies and detuning. Model system with closely spaced excited levels is found to exhibit gigantic Kerr nonlinearity accompanied by amplification in the linear and non-linear regimes. Role of SGC on the Doppler averaged linear/nonlinear absorption spectra is studied to reveal narrowing of electromagnetic induced transparency/absorption (EIT/EIA), and transformation of EIT into EIA. Results are discussed in terms of the buildup of extra coherences due to the presence of two SGC channels.