Strong permanent reversible diffraction gratings in copper-doped lithium niobate crystals caused by a zero-electric-field photorefractive effect

Permanent reversible diffraction gratings with refractive-index modulations of up to similar or equal to 10(-4) and with a grating vector perpendicular to the polar axis are realized in LiNbO3: Cu 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 narrow-band absorption gratings of Cu+/ Cu2+ ions via the Kramers - Kronig relations.