Photorefraction process

Fig. 1. Schematic of the photorefractive process. Note the phase shift, (j), between the incident intensity grating and the resulting photorefractive grating in the material. Depending upon the trapping depth and depth of modulation of the intensity of light incident on the material, the phase shift can vary between 0 and ti/2. The absence of this phase shift indicates that mechanisms of photoinduced refractive index change other than photorefraction are involved. Many photorefractive applications require phase shifts approaching jt/2.

Electro-Optic and Electric-Field-Induced Birefringent Materials. As can be seen from the analysis of The Photorefractive Process section, one material parameter to be optimized is the electro-optic and orientational response. This can be accomplished either by finding compatible materials with large linear electro-optic coefficients or by optimizing the orientational effects using materials with large birefringence (polarization anisotropy). One major difference in these approaches is the response speed of the photorefractive effect in that the orientational approach is expected to be slower, but submillisecond responses of low Tg materials have been demonstrated (93-96). 

Photoconductivity. As outlined in Introduction section, the first three steps of the photorefractive process are related to photoconduction, which controls the rate of grating formation and the amplitude of the charge grating, as well as the rate of erasure imder imiform illumination. Therefore, to imderstand and optimize the physical processes involved in photorefractive pol5miers, it is... 

Phase Materials. Phase holograms can be recorded in a large variety of materials, the most popular of which are dichromated gelatin, photopolymers, thermoplastic materials, and photorefractive crystals. Dichromated gelatin and some photopolymers require wet processing, and thermoplastic materials require heat processing. Photorefractive crystals are unique in that they are considered to be real-time materials and require no after-exposure processing. 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

These electrons are thermally ionized from the vacancy and may combine with an available acceptor, thus altering the charge state of the acceptor species. Experiments have shown that such a process can lead to a change in sign of the dominant photocarrier as well as modified gain and response time of the photorefractive effect. 

Charge transport is one of the important processes that control the speed of the PR index grating formation and the PR sensitivity. According to the standard theory of photorefraction [21], the response time for the formation and erasure of the space-charge field [xr in Eq. (21)] is proportional to the dielectric relaxation... 

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