Photorefraction refractive index modulation

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.

In order for the space charge field to produce a refractive index modulation and hence a grating or a hologram within the organic photorefractive material, the... 

Other interesting applications for organic photorefractives result from the phase shift between the original optical interference pattern and the holographic refractive index modulation. The phase shift is unique to photorefractive materials and allows... 

We discovered this phenomenon by chance when attempting to compare the efficiency of the photoinduced birefringence process with the efficiency of the refractive index modulation under electric field in photorefractive polymers. 

The material has to simultaneously possess photoconductivity and electro-optical effect to have photorefractive properties. Typical candidate materials have low glass transition temperature, frequently reduced by the plasticizer. Diffraction efficiency is improved by addition of the plasticizer because chromophore groups have higher rotational mobility and increase their contribution of birefringence to the total refractive index modulation. ... 

Refractive Index Modulation by Esc- Photorefractive behavior in polymers can be very roughly modeled by assuming a one-dimensional index of refraction change caused by the vector siun of the applied bias field and a space-charge field resulting from separated, trapped charges within the material... 

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