Permanent reversible diffraction gratings with refractive-index modulations of up to 10<sup>-4</sup> and with a grating vector perpendicular to the polar axis are realized in doped LiNbO<sub>3</sub> crystals by making use of high-temperature recording and charge compensation. The index changes do not result from light-induced space-charge fields and the linear electro-optic effect. They are linked to fixed high-contrast absorption gratings of filled and empty electron traps, e.g. Cu<sup>+</sup> and Cu<sup>2+</sup> ions, via the Kramers-Kronig relations.

© 2005 Optical Society of America

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