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.

在本文中，我们研究了一种称为自发相干 (SGC) 的相干现象在由弱探测激光场、两个强耦合（泵浦和控制）激光场和一个不连贯的泵场。我们研究原子介质非线性光学响应的​​基础是基于密度矩阵方程的推导，从著名的刘维尔方程和随后的数值解。我们首先介绍了 EIT 和相关色散，“有”和“没有”SGC 效应。我们已经观察到 SGC 诱导的线中心小尖峰的特征，伴随着探针扩增。除了，色散曲线在线中心处显示一个小的正斜率以及远离线中心的正负斜率。然后，我们探索了控制场强、相对相位角和非正交偶极矩阵元素之间的角度对 SGC 效应下介质的吸收、色散和群指数的依赖性。已经说明了一些等高线图，以便直观地了解数字获得的光谱。此外，已经在上述角度依赖性下研究了群指数行为的性质，以证明在 SGC 诱导的 EIT 介质中操纵光的群速度是合理的。最后，