Rewritable optical memory

The acceptance of optical data storage iato the mass storage market, which is as yet exclusively dominated by magnetic systems, will be fundamentally boosted if optical drives and media are subject to uniform standards and become fully compatible, and multiuser drives are offered which enable the user to employ alternatively CD-ROM and EOD disks, and maybe WORM disks as well (and CD-R disks, respectively). A prerequisite, however, will be whether rewritable optical memories will use the MOR or the PCR process. This accord especially will be hard to reach. 

Fulgides have been extensively investigated as erasable and rewritable optical memory systems (see Chapter 4) and these photochromic molecules are attractive candidates for chiroptical switch development.1471 Their bistability is based on the... 

Samoylova, E., DaUari, W., Allione, M., Pignatelh, F., Marini, L., Cingolani, R., Diaspro, A., Athanassiou, A., 2013. Characterization of fatigue resistance in photochromic composite materials for 3D rewritable optical memory appheations. Materials Science and Engineering B-Advanced Functional Solid-State Materials 178 (10), 730—735. 

Stellacci, F., Bertarelli, C., Xoscano, F., Gallazzi, M.C., and Zerbi, G., Diarylethene-based photochromic rewritable optical memories on the possibility of reading in the mid-infrared, Chem. Phys. Lett., 302, 563,1999. 

A direct use of the fluorescence approach is the photoreduction of noble [75] (for example Au and Ag+) and or rare-earth (for example Sm and Eu +) [76, 77] metals. For instance, Sm + in glass can be space-selectively photoreduced with an IR femtosecond laser. It is found that after photoreduction from Sm + to Sm +, a pronounced fluorescence band appears at 650-775 nm, which has been used as readout signal of 3D memory [77]. Another outstanding characteristic is that photoreduced Sm +, stable at room temperature, can be converted back to Sm by photo-oxidation with a CW laser, such as an argon-ion laser or a semiconductor laser. This enables rewritable optical memory with two stable statuses. 

Figure 4.11 Example of rewritable optical memory written image (left) and negative photomask used to record this image (right) [10], Reproduced from V. Chigrinov, H.-S. Kwok, H. Takada, and H. Takatsu, Photoaligning by azo-dyes physics and applications. Liquid Crystals Today 4, 1 (2005), International Liquid Crystal Society...

A photochromic compound is characterized by its ability to alternate between two different chemical forms having different absorption spectra in response to light of appropriate wavelengths. Photochromic materials are promising as recording media for optical memory, because the media store erasable/ rewritable data in photon mode. Because the data-recording mechanism is based on the photochemical reaction of each molecule, extremely high spatial resolution is expected. 

A 3D optical memory device can be based on different principles. Among them are the write once and read many times principle, based on TP initiated photopolymerization resulting in a photopolymer structure [242] or photo-bleaching of a fluorescent material [250]. The rewritable principle is based mainly on TP induced molecular change. This TP induced molecular change can be an isomerization reaction in the writing process and OP induced fluorescence in the reading process [119, 144, 247, 258],... 

As well as for Fe complexes spin-crossover effects are observed for Fe complexes Co° complexes (3d ) and, to a lesser extent, for Cr complexes (3d ) and for Mn complexes (3d ). Spin-crossover materials could find applications for the fabrication of rewritable optical, thermal or pressure memories at a nanometre scale. 

Chalcogenide alloys such as GeSbTe-based compounds are of interest for applications in rewritable optical media (Digital Versatile Disc, Blu-ray disc) and in non-volatile electronic memory devices [1-5]. Both applications rest on a fast and reversible transformation between the crystalline and amorphous phases which represent the two states of the memory. The phase change is induced by heating, either... 

In this section we present review of the physical parameter dependence of the optical memory characteristics. For memory devices, the most important characteristics can be listed as recording density, writing speed, reading speed, sensitivity, signal-to-noise (S/N) ratio, inforowtion retaining property, rewritability. etc. 

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