Defect modes supported by optical lattices in photovoltaic-photorefractive crystals

We study defect modes in optically induced one-dimensional lattices in photovoltaic-photorefractive crystals. These defect modes exist in different bandgaps due to the change of defect intensity. For a positive defect, defect mode branches exist not only in the semi-infinite bandgap, but also in the first and second bandgaps. When the defect mode branch is fixed, the confinement of defect modes increases with the defect strength parameter. For a negative defect, defect mode branches exist only in the first and second bandgaps. For a given defect mode branch, the strongest confinement of the defect modes appears when the lattice intensity at the defect site is not the smallest in its branch. On the other hand, when the defect strength parameter is fixed, the most localized defect modes arise in the semi-infinite bandgap for the positive defect and in the first bandgap for the negative defect.