Population distribution in three-level electromagnetically induced transparent system

Abstract

The interaction between light and matter can be described by the theory of density matrix equations. Based on the theory, the population distribution in a ladder type three-level system under the driven of two laser fields is simulated analytically. It is found that as tuning the frequency of the probe field, the variation of population in the upper level (labeled 2) of probe absorption transition is similar to the one of probe absorption. When the phenomenon of electromagnetically induced transparency (EIT) occurs, there is no population in level 2 at the resonant frequency. This is attributed to the fact that the population in the upper level of absorption transition is related to the absorption cross section closely. The magnitude of the peak of both population and probe absorption are independent of the intensity of the coupling field. Although there is no population in level 2 as full EIT occur, but atoms distributed in the ground level (labeled 1) can be excited to the highest one (labeled 3) in the manner of two-photon resonant absorption by absorbing the probe and the coupling photons. Thus there is obvious population in level 3, and population inversion is formed between level 3 and level 2. This can be used to realize light amplification. Increasing the intensity of the probe field, the population of level 2 grows gradually until it is saturated, while the magnitude of probe absorption will undergoes a process of increase first and then decrease.