Electromagnetically induced absorption in a four-state system

Summary form only given. It is well-known that nonlinear interference effects in the resonant atom-light interaction can lead to interesting and important phenomena, such as electromagnetically induced transparency (EIT) of atomic medium, coherent population trapping, lasing without inversion, and others. Common to all these phenomena is the appearance of light-induced coherence between atomic levels, which are not coupled by dipole transitions. Recently Akulshin and co-workers (1998-9) have observed subnatural-width resonances in the absorption on the D/sub 2/ line of rubidium vapor under excitation by two copropagating optical waves with variable frequency offset. It is remarkable that, apart from expected EIT-resonances with negative sign, authors detected positive resonances termed as electromagnetically induced absorption (EIA). More recently, similar effects in a (3+5)-state system have been described as a result of constructive interference of the dipole excitation channels. We propose a simple theoretical model for EIA - a four-state N-atom. Remember that three-state /spl Lambda/- and V-systems give the EIT-resonances only.