  Generation and control of the reverse saturable absorption (RSA) and optical
limiting (OL) are investigated in a four-level Y-type quantum system. It is
demonstrated that the applied laser fields induce the RSA and it can be
coherently controlled by either intensity or frequency of the applied laser
fields. The effect of the static magnetic field on the induced RSA is studied
and we obtain that it has a constructive role in determining the intensity
range in which the OL is established in the system. In addition, we find that
the OL threshold can be decreased either by increasing the length of the medium
or by getting the atomic system denser. Finally, Z-scan technique is presented
to confirm our theoretical results. The proposed scheme can be used in
designing the coherent optical limiters with controllable threshold and
intensity range of OL.
