Nonlinear Absorption-Gain Response and Population Dynamics in a Laser-Driven Four-Level Dense Atomic System

We theoretically investigate the response of nonlinear absorptionand population dynamics in optically dense media of four-level atoms driven by a single-mode probe laser, via taking the density-dependent near dipole-dipole (NDD) interactions intoconsideration. The influence of the NDD effects on the absorption of the probe field and population dynamics is predicted via numerical calculations. It is shown that the NDD effects canreduce gradually to transient absorption with the increase of thestrengths of the NDD interactions, and transient amplification canbe achieved. In the steady-state limit, the probe field exhibitstransparency for strong NDD interactions. Alternatively, thepopulation entirely remains at the ground state due to the NDD effects.