Refractive indices photorefraction

The relatively simple study of fluorescence and phosphorescence (based on the action of colour centres) has nowadays extended to nonlinear optical crystals, in which the refractive index is sensitive to the light intensity or (in the photorefractive variety (Agullo-Lopez 1994) also to its spatial variation) a range of crystals, the stereotype of which is lithium niobate, is now used. 

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. 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

Figure 3.39. Holographic setup for photorefractive molecular glasses. The sample is tilted toward the grating, allowing an applied external field to support the motion of the mobile charges. The phase shift of the refractive index grating can be determined by measuring the transmitted writing beam intensities (two-beam coupling).

The photorefractive effect is the term used for the changes induced in the refractive index of a material by a redistribution of photogenerated charges. 

The photorefractive effect is usually probed by two beam-coupling experiments, in which one beam gains intensity at the expense of the other. This coupling is a characteristic property of the photorefractive effect. Such an asymmetric coupling requires an asymmetric shifting of the refractive index grating... 

Fig. 10.6 Outline of the steps involved in the generation of a refractive index grating in a photorefractive material. Reprinted with permission from Yu et al. (1996). Copyright 1996 American Chemical Society.

The initial sensitizer anion presence makes recombination of mobile holes possible in the dark regions. Which are the compensator sites Here, there exist different explanations. One possibility is that some of the electro-optic dye molecules present in photorefractive composites to provide refractive index change may become charged positively. An alternative theory in the case of amorphous materials is that the amorphous disorder leads to defect sites forming local potential minima at which positive charge may be immobilized (Figure 5). 

Figure 6. The photorefractive effect. Top in an idealized hole transport material, the net charge density is ti radians out of phase with the intensity pattern. Middle the electric field, E, due to this net charge density, p, is given by Gauss law, dEjdx = p/e, and is shifted in phase by njl radians relative to the charge density distribution. Bottom the refractive index will then follow the phase of the electric field. In real materials the charge distribution is not always n radians out of phase relative to the intensity pattern, as competition between drift and diffusion currents leads to a reduced phase shift. The refractive index contrast might therefore be shifted by only n/lO radians relative to the intensity pattern in some polymers.

The electro-optic effect leads to the modification of the refractive index of a suitable material when an electric field is applied (Figure 8). The electro-optic effect must be present in a photorefractive material, so that the space charge electric field pattern due the relocated charges will lead to a patterned refractive index in the material this is a hologram. 

Figure 9. Conventional model of photorefraction in crystals iron impurity forms defect states of variable valence within the forbidden band gap of a lithium niobate crystal. Optical excitation of the divalent state leads to creation of a mobile electron in the conduction band. This is able to move and recombines with a trivalent iron impurity at another location which becomes divalent. The displacement of charge leads to an electric field and the Pockels electro-optic effect leads to local modification of the refractive index.

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