Development of thermally fixed photorefractive holograms without light

Holographic gratings are recorded and thermally fixed in iron-doped photore fractive lithium-niobate crystals. A spatially modulated concentration of f illed and empty electron traps (Fe2+ and Fe3+) yields a modulated dark cond uctivity. As a consequence, dark diffusion currents and dark drift currents arise. Space-charge fields and electro-optic refractive-index changes buil d up. An additional refractive-index grating, which may originate from a mo dulated proton concentration, as well as a pronounced absorption grating ar ising from the modulated Fe2+ concentration are also observed. The dark dev elopment has practical advantages: thermally fixed holograms can be used in devices without the need to develop them freshly from time to time by illu mination. Although with dark development diffraction efficiencies up to 50% , are demonstrated, in general the efficiencies are smaller compared to tho se achieved by development with light.