Development of thermally fixed holograms in photorefractive lithium-niobate crystals without light

Summary form only given. Holographic gratings are recorded at room temperature in photorefractive iron-doped congruently-melting LiNbO/sub 3/ crystals. Afterwards the diffraction efficiency of these gratings is measured from time to time to monitor the dark decay. Samples with different iron contents are investigated, and the Fe/sup 2+//Fe/sup 3+/ concentration ratio is varied. Time constants /spl tau/ of the decay range from minutes to years, and the dark conductivities /spl sigma//sub d/=/spl epsi//spl epsi//sub 0///spl tau/ are deduced, where /spl epsi/=28 is the dielectric constant. The major outcomes are: (1) There is a small, iron independent background conductivity (/spl tau/=1 year) which arises from mobile ions. (2) For Fe concentrations in excess of about 20/spl times/10/sup 18/ cm/sup -3/ (0.05 wt.% Fe/sub 2/O/sub 3/): the dark conductivity is proportional to the effective trap density N/sub eff/=(1/C(Fe/sup 2+/)+1/C(Fe/sup 3+/)/sup 9/-1) and the normalized dark conductivity /spl sigma//sub d//N/sub eff/ rises exponentially with (C/sub Fe/)/sup 1/3/.