Full-color recording is technologically important not only for the storage and reproduction of color information but also for the enhancement of recording density without a reduction in the size of one unit area by utilizing the spectrum of light for recording and reading memories, as in photochemical hole burning. Although full-color "writeonce" recording media (e.g., photographs and photocopiers) or rewritable "on-and-off" recording media (e.g., magnetic, opto-magnetic, and thermo-optic recordings) are well developed, rewritable full-color recording has still not been realized. This can be attributed mainly to the difficulty of finding thermally stable materials that change color reversibly under the action of external stimuli. Here we present a cholesteric low-molecular-weight compound that, in the form of a thin solid film, shows stable iridescent colors in the visible region. The color of the compound is totally, partially, or in an imagewise manner changed into selected colors by thermal treatment above 87°C. Compounds with this property should find applications as highdensity memory media that use the spectrum of light or as thermo-sensitive full-color recording materials.Cholesteric color has fascinated people since the discovery of liquid crystals.''] The ability to change the cholesteric colors over the whole visible region by stimuli such as temperature, pressure, and electric field is important from not only a scientific but also a practical point of view. On the other hand, fixing of specific colors, namely colors insensitive to stimuli, is also attracting much attention in connection with the iridescent colors of a jewel beetle''] and for applications such as optical filters for selective reflection or large optical rotation. Polymeric materials have already been used to obtain thermally insensitive solid films with cholesteric str~cture.[~-~] High viscosity or three-dimensional networks of polymer chains practically fixes the pitch of the helical configuration of polymers, which determines the colors of cholesteric liquid crystalline compounds, providing a thermally insensitive cholesteric color. Although it is also possible to tune the color, by changing the monomer ratio in copolymer^,[^'^] by changing the temperature during photopolymerization of cholesteric monom e r~, '~] or by changing the ratio between polymerizing solution and lyotropic reversible and fast switching of the cholesteric color, which is desired for applica-tions as rewritable memory or recording media, is impossible to achieve with these polymeric materials. A low-molecular-weight cholesteric compound that we recently synthesized shows, in the solid state, changeable iridescent colors that can be selected from the whole visible region by thermal treatment.The cholesteryl derivative 1 (Scheme 1) was synthesized by condensation of cholesterol and the corresponding diacid in the presence of dicyclohexyl carbodiimide and 4-dimethylamiriopyridine in dichloromethane. This compound, 0 Scheme 1. Chemical structure of cholesteryl deriv...

New results are presented on optical recording to the films of polymers substituted with azo dyes. Holographic gratings are formed in the polyurethane films by exposure to laser light without any post-treatment as in previous works using polyacrylates, epoxy-based polymers and polyesters substituted with azo dyes. It is shown for the first time that the principal mechanism of optical recording could be the mass transfer of polymers during the cis-trans isomerization of azo dye units on the polymers from illuminated areas to non-illuminated areas. Besides this transport, some irreversible photochemical reactions involving atmospheric oxygen are suggested as another possible origin for the holographic recording.

The electro-optic effect in helical ferroelectric liquid crystal deformed by transverse electric field is exploited to realize a diffraction-based light modulator. Experiments show that a contrast ratio better than 100:1 is easily achievable for the short-pitch liquid crystal aligned homeotropically with oblique readout breaking the initial symmetrical geometry. The modulation obtains low polarization sensitivity for selected directions of the readout.

A polymer film surface modified by laser illumination was used as a command surface for nematic liquid crystal (LC). Azobenzene-containing polyurethanes were taken for the material of the film. A relief grating inscribed by laser illumination onto the surface of these polymers induced a reorientation of the LC director from the initial homeotropic alignment. Possible applications of the described processes include command-surface creation for liquid-crystal displays, reversible and adaptable liquid-crystal structures for laser-beam targeting and aiming, etc.

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