Investigation of electromagnetically induced transparency in the strong probe regime

We study the response of an inhomogeneously broadened atomic three-level ladder system to a strong probe beam under conditions in which electromagnetically induced transparency would be induced on a weak probe beam. Theoretical analysis and experimental data show that saturation effects associated with the strong probe can substantially alter the behavior of the system and can result in enhanced probe absorption under exactly the same circumstances in which a weak probe would experience enhanced transparency. Strong probe effects are also shown to lead to a breakdown of the correspondence between population inversion and gain. A power-series solution of the density-matrix equations describing the ladder system provides a physical picture for strong probe effects in terms of interfering multiphoton transition pathways.