Abstract
The cover picture shows the underlying mechanism and one of the applications of the photorefractive effect, which produces a spatial modulation of the refractive index of a material under nonuniform illumination. As illustrated on the right side, the photorefractive effect begins with light and dark fringes produced by intersecting laser beams, which, in the presence of an applied electric field E0, produce charge separation. The mobile charges (primarily holes) move and eventually trap in dark regions, leading to a space–charge electric field (green). This field produces a refractive index change, that is, a hologram that can diffract light. A key feature of the effect is that it leads to asymmetric energy transfer between the two beams incident on the material. An application of this effect is image amplification, which is demonstrated on the top left side of the picture where the image of the number 5, carried by a weak beam, is amplified in the presence of a strong beam. Among the best photorefractive materials developed thus far are organic, amorphous glasses, the properties of which depend critically on the glass transition temperature (Tg) and photoconductivity (σph), as well as polarizability anisotropy (Δα) and hyperpolarizability (β) of the molecules. The picture shows the structure of the photoconductive, nonlinear optical chromophore DCDHF-6 that forms a high-performance low-molecular weight photorefractive glass. The detailed properties of several photorefractive glasses containing DCDHF derivatives are described in the article by Ostroverkhova et al. on page 732.
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