New polymer materials for erasable holographic storage
P. S. Ramanujam; S. Hvilsted* and R. H. Berg*
*Solid State Physics Department, Ris¿ National Laboratory

The ideal material for erasable holographic storage should possess the following properties: high diffraction efficiency, high resolution, low-intensity response, high temporal sensitivity, no high fields in order to promote recording, no prealignment of the molecules, no wet processing, permanent storage until erasure, and fast erasure on the application of an external stimulus. A side-chain liquid crystalline azobenzene polyester almost fulfilling all the above criteria has been developed by us recently.

1 The polyester backbone provides the necessary mechanical rigidity; fabrication of free standing films and fibers poses no difficulties. The films can be stretched by 1000% before breaking. The problem of high diffraction efficiency can be solved through the use of photoanisotropic materials and polarization holography which promises almost 100% diffraction efficiency even in thin films.

2 The low efficiency of a dye doped polymer system is overcome in our case through the attachment of the azobenzene chromophores to the main polyester chain, which amplifies the effect of the response of the chromophore to irradiation. Furthermore, a high resolution is also guaranteed when using materials which are addressed at molecular level. Our polyester design is modular, allowing a variation of four structural parameters. These are

(1) length of the methylene main chain,

(2) length of the flexible spacer in the side chain,

(3) the substituent on the azobenzene and

(4) the molecular mass of the polyester. Only a very narrow range of these parameters has been found to be acceptable for permanent, yet erasable holographic storage. For example, 14 methylene groups in the main chain make the polymers crystalline, while 10 methylene spacers or less result in a hitherto unfathomed biphotonic process. The polyesters have glass transition temperature in the range 20-30 oC, which results in large anisotropy at room temperature. Through atomic force and near-field optical microscopic investigations as well as infrared spectroscopic studies,3 an aggregation process encompassing both side and main chains after irradiation is presumed to be the reason for permanent optical storage.

The polyesters are produced by melt transesterification of the selected precursors. About 5 mg of the resulting powder is dissolved and cast on a glass substrate 20 mm in diameter. Spin coating results in a more homogeneous, but thinner film. The films are ready for recording as soon as the solvent has evaporated. No prealignment of the molecules is undertaken prior to exposure. Non-destructive read-out is performed by a HeNe laser. Diffraction efficiency greater than 50%, resolution greater than 5000 lines/mm and storage times greater than four years have been achieved so far. The holograms can be globally erased by heating the film to about 80 oC or through UV irradiation. In the latter case, 10000 cycles of write, erase and read have been performed. We have constructed a portable holographic system which fills exactly an A4 page size breadboard (Fig. 2). This set-up includes the writing laser, the read-out laser, all the optics and a CCD camera to collect the diffracted image. Only the power supply for the green laser and for the Peltier supply together with the screen are outside.

1. S. Hvilsted, F. Andruzzi, C. Kulinna, H. W. Siesler and P. S. Ramanujam, Macromolecules 28, 2172 (1995).

2. L. Nikolova and T. Todorov, Opt. Acta 31, 579 (1984).

3. P. S. Ramanujam, N. C. R. Holme and S. Hvilsted, Appl. Phys. Lett. 68, 1329 (1996).

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