Abstract
We show that defect modes (DMs) in optical lattices are possible in biased centrosymmetric photorefractive crystals. These DMs exist in different bandgaps due to the change of the defect strength. For a positive defect, DMs exist in each bandgap. The strongest confinement of DMs appears in the semi-infinite bandgap when the positive defect strength is fixed. For a negative defect, DMs exist in each bandgap except the semi-infinite bandgap. When the negative defect strength is fixed, the strongest confinement of DMs appears in the first bandgap. For a given bandgap, the strongest confinement of DMs appears when the negative defect strength is equal to a certain value. The strong negative defects do not favor the creation of DMs in the first bandgap.
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