Effect of Spontaneously Generated Coherence and Dynamically Induced Coherence on the Susceptibility and Group Index of a Three Level Closed L System
The susceptibility and the group index in a three-level system with incoherent pumping has been studied when both the dynamically induced coherence (DIC) and spontaneously generated coherence (SGC) play a significant role. I have shown that by varying the probe field Rabi frequency one can control the interference between these two coherences which leads to different non-linear response (amplification without inversion, electromagnetically induced transparency and electromagnetically induced absorption) leading to different (positive and negative) dispersion. Hence control over switching of group velocity from subluminal to superluminal and vice versa can be achieved. By changing the relative phase of two fields the group velocity switches over from superluminal velocity to subluminal velocity and vice-versa which can be used as optical switch. Choosing appropriate value of the relative phase and the probe field Rabi frequency, the dispersion and hence the propagation of the probe pulse can be controlled efficiently. Therefore these parameters can be used as a knob to manipulate light propagation from subluminal to superluminal. Going beyond the weak probe field approximation, I have derived analytical expressions for group velocity and hence the group index in the steady state limit (keeping all orders of system parameters) to generalize the analysis, and these expressions can be used for any set of system parameters without any restriction. Numerical values obtained by solving the density matrix equations agree well with these exact analytical values at large time limit.
Keywords: Spontaneously generated coherence, group velocity, subluminal, superluminal.
PACS numbers: 42.50.Gy, 42.50.Hz