Space charge field

The movement of the fast electrons leads to the fonnation of a space-charge field that impedes the motion of the electrons and increases the velocity of the ions (ambipolar diffusion). The ambipolar diffusion of positive ions and negative electrons is described by the ambipolar diffusion coefficient... 

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

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. 

We note from Eq. (15) that the space-charge field is shifted in space by n/2 with respect to the intensity pattern [Eq. (11)], which corresponds to a distant shift of Ag/4 in the x direction (Fig. 3d). This space-charge field induces an index volume grating via the Pockels effect (Fig. 3e). The refractive index including the fundamental component of refractive index modulation with magnitude of An can be written as... 

Kukhtarev s model predicts the fundamental component of the steady-state space-charge field created by intensity distribution in Eq. (11) as... 

When the space-charge field can be constructed, the index modulation can also be built correspondingly. This mechanism can be a linear EO effect or an index modulation via alignment of the NLO chromophore in a low glass transition... 

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... 

It is worth pointing out that in low-7(, PR materials with molecular birefringence as the dominant contribution to the index modulation, the real response time and the PR sensitivity can also be limited by the diffusion time of angular rotation of the chromophores under the drive of the space-charge field, and even the time constant xe for the formation of the space-charge field may be smaller [27],... 

The presence of a trapping center is very important since Eq. (19) indicates that the steady-state strength of the photoinduced space-charge field depends on the number density of the deep traps. Nevertheless, the nature of the traps in organic PR materials is the least studied of all the elements for the PR effect. The main reason is the lack of structural information of the trapping centers. The amorphous nature of these materials warrants the existence of a variety of trapping centers, such as energy levels localized at impurities or structural defects. However, one can differentiate between deep traps, which are localized... 

The last requirement for a PR effect is the mechanism for index modulation in the response of the space-charge field. Two mechanisms have been found to lead to index change EO response and birefringence. 

The bipolar single-trap model assumes that both electrons and holes share identical trap centers. Since sequential trappings of the electrons and holes by the identical centers mean the neutralization of the electric charge, the effective space-charge field will depend on the relative power (i.e., the mobilities) of electron and hole transports. The expressions for the writing and erasing diffraction efficiency are [100] ... 

Although similar grating cancellation and revelation behaviors have been observed in other PR polymeric composites, they are attributed either to the trap s intercommunication or to the residual ionic motion [101-103]. However, the complementary gratings in the present study are formed by the space-charge field of two types of photogenerated charge carriers. 

Figure 1 Phase relationship between the optical interference pattern and the space-charge field. For liquid crystals, this example illustrates mobile anions migrating into the nulls of the interference pattern. The application of an applied electric field Ej is usually required to observe a phase-shifted photorefractive grating.

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