  We demonstrate how to achieve reversible nonreciprocal optical response in a
periodic photonic structure with a pair of defects, one of them being a
nonlinear liquid crystal defect layer. The twin defect layers structure is
symmetric at low intensity and becomes asymmetric above a power threshold
corresponding to the optical reordering of the liquid crystal. We show that
nonreciprocal effects can be reversed by changing the wavelength as a
consequence of the defect mode dependent light localization inside the
structure.
