Enhanced self-Kerr nonlinearity in spontaneous emission

We propose a new scheme for achieving giant enhancement of the self-Kerr nonlinearity in a four-level Y-type atom with two highest nearly degenerate lying levels. We have found that, owing to the quantum interference effect that originates from spontaneous emission from two closely lying levels, large Kerr nonlinearity can be achieved under appropriate conditions. In particular, in addition to the amplitudes, the phases of the applied fields also affect the Kerr nonlinearity. We attribute the enhancement of Kerr nonlinearity mainly to the presence of some extra atomic coherence terms induced by the spontaneously generated coherence. We present a physical understanding of our numerical results using analytical explanation. (C) 2009 Optical Society of America