Photorefractive Materials, Crystals (Electro-Optic Materials)
Photorefractive crystals are excellent materials for recording volume phase holograms in real-time. These materials have excellent resolution, efficiency, storage capacity, sensitivity and reversibility. The application of these materials for hologram recording was first considered by Chen et al., who suggested that "optical damage" effect in these crystals could be exploited to record a thick hologram.
Materials involving Photorefractive Effect
The important crystals are lithium niobate LiNbO3, lithium tantalate LiTa03, barium titanate BaTiO3, potassium tantalate niobate KTN, barium sodium niobate (SBN), Bismuth silicon oxide (BSO) , Bismuth germanium oxide (BGO) , GaAs, InP and PLZT ceramics. The crystals are grown using Czochralski method. The grown crystals are cut and polished. The optic axis (cxis) of the crystal is kept parallel to the plane of polarization of the hologram recording beams. Mechanism of Recording
The photorefractive materials contain localized centres with trapped electrons that can be excited into the conduction band by the action of light . These materials have dark conductivity which allows charges to freeze in place. The dark storage time varies from crystal to crystal. For example SBN has a dark storage time of a few seconds, while for lithium niobate it is a few years.
When the material is exposed to an interference pattern the electric charges from interference maxima drift and/or diffuse and are (trapped) collected at the interference minima. The space charge pattern creates a strong spatially periodic field. This field deforms the crystal by the Pockels effect and causes a refractive index modulation producing a hologram.
Materials which lack inversion symmetry exhibits a strong photorefractive effect. The change in the index of refraction is independent of the total light intensity. Thus even weak beams can induce strong optical nonlinearities in photorefractive crystals. In crystals like BaTiOnd SBN though the Pockels coefficient is large, the speed of charge migration is slow, while opposite is the case for BSO, GaAs and InP. LiNb03 on the other hand exhibits weak Pockels effect and slow speed of charge migration while KTN is strong and fast.
Fixing and Erasure of Photorefractive Materials
The recorded hologram is immediately visible. It can be erased by exposure to a uniform beam of light which releases the trapped electrons. These released electrons redistribute evenly throughout the volume. This erases the hologram as the crystal returns back to its original condition having a uniform distribution of trapped electrons. Continued readout of a hologram will erase it. This is a undesirable feature of photorefractive crystals.
The hologram in these crystals can be fixed by converting the charge patterns to the patterns of ions which are not sensitive to light. The hologram in Fe:LiNb0 an be fixed via thermally activated ionic conductivity. The LiNb0 crystal is heated during or after storage to a temperature above 100 C. The resulting ionic pattern is frozen upon cooling to the room temperature. The fixing process neutralizes the space charge field but the ionic pattern is not erased. Hence the hologram can be readout without erasure.
The fixed holograms in LiNb03 can be erased by heating them to the fixing temperature and exposing them to uniform light.
It is possible to record a hologram with 100% diffraction efficiency in a 1 cm thick crystal of LiNb0 . It is also possible to record 1000 holograms with usable levels of diffraction efficiency in the reconstructed image. An efficiency of 95% has been achieved in BGO crystal.
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