Organic optical memory

Fig. 11 shows the photoswitching of the injection current. Upon UV irradiation, the hole injection current increased, while decreasing to zero on irradiation with visible light. Very thin amorphous diarylethene film as thin as 0.2 pm could also control the hole injection to the organic hole transport layer (Fig. 9b). These results are potentially applicable to optical memory-type organic photoconductors.

Organic photochromic systems have actual applications in variable transmission optical materials, authentication systems and novelty items. In addition, they offer great potential in erasable optical memories and many other fields where reversible changes of physical properties other than color are wanted. The domain is interdisciplinary and expanding. 

There is no limit to the number of photochromic systems possible. The systems discussed are excellent candidates for integration into solid-state devices because nearly all retain their photochromic properties in the absence of solvent. The organization of these systems in tandem with other molecular systems is being pursued. For the switching applications many of these systems have much too slow a turnover rate to be explored as working devices. That is unless the connectivity in these systems can be increased. In the meantime, photochromic systems will probably be explored as possible optical memory devices. The most promising switches are those based on the much faster processes of electron and energy transfer. We will now examine research in these areas. 

In principle, any kinds of organic dyes which undergo photochemical reactions by irradiation with visible laser light (635-830 nm) can be used as the memory media. The photochemical reactions can be detected by changes in refractive index as well as in absorption properties. Although most of photochemical reactions are irreversible, some organic dyes undergo reversible photochemical reactions [photo-chromism) and the photochromic dyes are potentially applicable to erasable optical memory media. 

In this chapter the properties of organic molecules which are potentially useful as archival digital optical memory media, mainly erasable media, will be described. The memory media which require wet development/fixing processes will not be mentioned. Photopolymers and photoresists are also beyond the scope of this chapter. 

As described in Section 7.1, a modern digital optical memory system, the CD-R, uses organic dyes as the memory layer. The dyes used are shown in Figure 1. All these dyes have absorption tails around 780 nm, where the recording laser has the emitting line. At the absorption tails the dyes show the maximum refractive index... 

When we intend to apply organic molecular materials, especially photochromic dyes, to optical memory media, the indispensable condition is stability, both thermal and photochemical. The photogenerated isomers are required never to return to the initial isomers in the dark, even at elevated temperatures, e.g., 80 °C. In addition, the coloration/decoloration can be cycled many times while the photochromic performance is maintained, and the memory media are provided with nondestructive readout capability. Although several molecules which fulfill the former condition have been developed, some problems still remain to gain access to molecules and systems which fully satisfy the latter condition. 

It is clear that this volume is truly different from the preceding accounts. Photochemists will appreciate Volume 2 as a nice complement to Volume 1, although it can be read independently. Organic photochromic systems are known for their applications in variable-transmission optical materials, ophthalmic lenses, authentification devices (photochromic inks), and novelty items, but they also have great potential in any domain where reversible physical properties are desired (optical memories, gradation masking, optoelectronic systems, nonlinear optical devices, etc.). This book is thus strongly recommended to anyone interested in materials science. 

Dalton LR, Harper AW (1996) Photoactive organic materials for electro-optic modulator and high density optical memory applications. In Kajzar E, Agranovich VM, Lee CYC (eds) Photoactive organic materials science and application. NATO ASI Series, vol 9, Kluwer Academic, Dordrecht, p 183... 

V.A. Barachevsky, Organic Storage Media for Holographic Optical Memory State of the Art and Future, Optical Memory and Neural Networks 9 (2000) 251, and Proc. SPIE 4149 (2000) 205. 

Treves, D. and Bloomberg, D. 1986. Signal, noise, and codes in optical memories. Optical Eng. 25 881. Wrobel, J., Marchant, A., and Howe, D. 1982. Laser marking of thin organic films. Appl. Phys. Lett. 40 928. 

The present time is characterized by the rapid development of studies aimed at the constmction of nanostmctured materials with external-stimuli-controlled physicochemical properties/° ° In this respect, the multifunctional comb-shaped LC polymers offer a vivid example of self-organized thermocontrolled, photocontrolled, and electrically controlled nanostmctured materials (field-responsive smart materials) that show promise for various applications optics, optoelectronics, photonics, display technology, optical memory devices, telecommunications systems, and so on (Chapters 8.08, 8.09 and 8.15). 

Dalton L. R. and Harper A. W., Photoactive Organic Materials for Electro-Optic Modulator and High Density Optical Memory Applications in Photoactive Organic Materials. Science and Application , F. Kajzar, V. M. Agranovich and C. Y.-C. Lee Eds, NATO ASI Series, vol. 9, Kluwer Academic Publishers, Dordrecht 1996,... 

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