Electromagnetically induced absorption in a four-state system

A simple theoretical model describing the positive sign of subnatural-width absorption resonances in the recent experiment of Akulshin and co-workers [Phys. Rev. A 57, 2996 (1998)] is proposed. An analytical expression for the linear response to the weak probe field is found in the low-saturation limit with respect to the control field. It is shown that the positive sign of subnatural resonance is caused by the spontaneous transfer of the light-induced coherence from the excited level to the ground one.