Manipulating multi-frequency light in a five-level cascade EIT medium under Doppler broadening

An electromagnetically induced transparency (EIT) medium consisting of five-level-cascade atoms is proposed to control the group velocity of multi-frequency light under Doppler broadening. An analytic expression for the group index of probe light is derived as a function of the parameters of coupling light and temperature of the medium. It is shown that by adjusting intensity and/or frequency of the solely coupling light one may manipulate simultaneously group velocity of the probe light in three separated frequency regions, each of which enables to switch between the subluminal and superluminal modes. On the other hand, the effect of Doppler broadening increases the group velocity and enlarges the tuning range of the coupling intensity to switch between the superluminal and subluminal modes. The model is helpful to find applications related to the group velocity manipulation of multi-frequency light, and to serve future experimental studies at various temperature conditions.