The ladder three-level system including ground state, excited state and high excited state is investigated by using the density matrix equations. The coherence effect of ladder three-level system is analyzed, and absorption and dispersion curves of weak probe laser are obtained. The effects of interaction between high excited atoms and external electric field on the coherence effect are also investigated. The result shows that the interaction between high excited atoms or the external field can make the absorption curve and the dispersion curve shifted.

The highly excited atoms have a strong Van der Waals interaction as compared with the ground-state atoms, which can block the excitation of neighboring atoms and forming the blockade effect. In this work, the interaction between highly excited atoms is calculated using the perturbation theory, the interaction of nS and nD pair states and the dispersion coefficients are obtained. This shows that the interactions of the SS for both Rb and Cs atoms are repulsive, whereas the interaction of DD states is repulsive for Cs and attractive for Rb.