Using a transition as an analytically tractable model, we discuss in detail magneto-optical resonances of both electromagnetically induced absorption (EIA) and electromagnetically induced transparency (EIT) types in the Hanle configuration. The analysis is made for an arbitrary rate of depolarizing collisions in the excited state and an arbitrary elliptical field polarization. The obtained results clearly show that the main reason for the EIA subnatural resonance is the spontaneous transfer of anisotropy from the excited level to the ground level. In the EIA case we predict the negative structures in the absorption resonance at large field detuning. The role of the finite atom-light interaction time is briefly discussed. In addition we study nontrivial peculiarities of the resonance line shape related to the velocity spread in a gas.
© 2005 Optical Society of America