Holographic 448 INDEX

With reference to Eq. (7), in most situations the holographic index contrast increases quadratically with the dipole moment of the chromophore. The dipole moment thus provides a parameter for material optimization, albeit at the expense of increased disorder slowing down charge transport. Transport rate appears not to be critical, however. 

Holographic information can be recorded by spatially modulating either (or a combination of) the absorption or phase (index or thickness change). 

Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

The gratings can also be made in situ by holographic irradiation as was demonstrated for low molecular stilbenes in a polystyrene matrix [197]. Here, the spatial modulation of gain dominates over the refractive index modulation in its contribution to optical feedback. The principles of holographic irradiation will be described in Section VIII, which discusses photosensitive materials. 

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).

Burggraf, N., Krattiger, B., de Rooij, N.F., Manz, A., de Mello, A.J., Holographic refractive index detector for application in microchip-based separation systems. Analyst 1998, 123(7), 1443-1447. 

To the best of our knowledge, most of the work on refractive-index recording in organic photochromic materials has been directed toward systems for holographic recording. Furthermore, this work has been concerned with reading out the stored... 

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