Development of thermally fixed photorefractive holograms without light

Abstract.

Holographic gratings are recorded and thermally fixed in iron-doped photorefractive lithium-niobate crystals. A spatially modulated concentration of filled and empty electron traps (Fe²⁺ and Fe³⁺) yields a modulated dark conductivity. As a consequence, dark diffusion currents and dark drift currents arise. Space–charge fields and electro-optic refractive-index changes build up. An additional refractive-index grating, which may originate from a modulated proton concentration, as well as a pronounced absorption grating arising from the modulated Fe²⁺ concentration are also observed. The dark development has practical advantages: thermally fixed holograms can be used in devices without the need to develop them freshly from time to time by illumination. Although with dark development diffraction efficiencies up to 50% are demonstrated, in general the efficiencies are smaller compared to those achieved by development with light.