Effect of spontaneously generated coherence (SGC) on the line shapes of absorption, transparency, dispersion and group index of a four-level inverted Y-type atom–lasers coupling system

Abstract

In this article, we have studied the effects of a coherent phenomenon called spontaneously generated coherence (SGC) in a four-level inverted Y-type atomic system formed by a weak probe laser field, two strong coupling (pump and control) laser fields and an incoherent pumping field. The underlying foundation of our studies on the nonlinear optical responses of the atomic medium is based on the derivation of the density matrix equations from the well-known Liouville’s equation and subsequent numerical solution of them. We have first presented the EIT and related dispersion, ‘with’ and ‘without’ the SGC effect. We have observed a feature of SGC-induced small sharp peak at the line centre accompanied by probe amplification. Besides, the dispersive curve displays a small positive slope at the line centre along with positive and negative slopes away from the line centre. We have then explored the dependence of the control field strengths, relative phase angles and the angle between the non-orthogonal dipole matrix elements on the absorption, dispersion and group index of the medium under the SGC effect. Some contour plots have been illustrated for getting a visual insight into the numerically obtained spectra. Further, the nature of the group index behaviour has been investigated under the above-mentioned angular dependences to justify the manipulation of group velocities of light in the SGC-induced EIT medium. Finally, we have shown the possible utilisation of the SGC effect and the relative phase angle in the EIT medium for the proposed scheme to explain the switching action between subluminal and superluminal modes of light propagation.