  We investigate the self-Kerr nonlinearity of a four-level N-type atomic
system in 87Rb and observe its reversible property with the unidirectional
increase of the switching field. For the laser arrangement that the probe field
interacts with the middle two states, the slope and the sign of the self-Kerr
nonlinearity around the atomic resonance can not only be changed from negative
to positive, but also can be changed to negative again with the unidirectional
increasing of the switching field. Numerical simulation agrees very well with
the experimental results and dressed state analysis is presented to explain the
experimental results.
