Photorefractive effect

Nolte, D. D. Melloch, M. R., Photorefractive quantum wells and thin films, In Photorefractive Effects and Materials Nolte, D. D., Ed. Kluwer Academic Publishers, Dordrecht, 1995... 

Several methods can be used for waveguide fabrication in LiNbOs. Among them, titanium in-diffusion and proton exchange (PE) are the most popular ones since they lead to the formation of well-confined and low-loss layers. PE is mainly applied because it results in a considerable decrease of the photorefractive effect in LiNbOj. However, waveguides obtained by pure PE have reduced EO and NL coefficients and usually a post exchange aimealing (APE) is required for restoration of the EO activity. 

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.

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 classified here as a special third order effect for several reasons. First, it is perhaps the least well understood, mechanistically. Second, it represents the area of greatest current... 

Defect Properties and the Photorefractive Effect in Barium Titanate... 

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. 

The molecular PR materials based on the methine dyes are composed of single types of molecules and exhibit the best PR performances. They are chemically pure, structurally well defined, and morphologically stable. They may serve as model materials for detailed photochemical and photophysical studies. Through these model materials, clearer pictures and a deep understanding of the photorefractive effect will emerge. Better mechanistic understanding will surely assist in the search for new PR materials with improved macroscopic properties. 

In the pursuit of improved photorefractive materials, seminal research by a group at IBM led by W. E. Moemer discovered the photorefractive effect in polymers in 1990 [4,40], Photorefractive polymers are generally composite materials... 

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... 

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