3D optical memory

Figure 1.15 Principle of 3D optical memory and a model photochrome.

Information storage in polymeric media based on phenoxynaphthacenequinones was permanent at room temperature. This property suggests that these media may be suitable for use as materials for 3D optical memory.63 66 In this connection, photofluorochromic quinones providing large two-photon cross sections, high fluorescence quantum yields for the photoinduced form, and relatively low ones for the initial form are of chief interest.67,68... 

A. A. Angelutc, A. Yu. Chikisev, N. I. Koroteev, S. A. Magnitskii, Yu. P. Meshalkin, I. A. Ozheredov, S. Yu. Orzhancev, V. V. Shubin, and N. T. Sokolyuk, Photochemical and spectroscopic properties of naphthacenequinones. Candidates for 3D optical memory devices, in International Symposium on Optical Memory and Optical Data Storage, 1996 Technical Digest Series, 12, 178-180 (1996). 

Dvornikov, A. S., Malkin, J., and Rentzepis, P. M. Spectroscopy and kinetics of photo-chromic materials for 3D optical memory devices. J. Phys. Chem. 1994, 98, 6746-6752. 

Figure 16.1 shows a principle of bit-oriented three-dimensional (3D) optical memory. A laser beam is focused on a point in a recording medium. Chemical reactions of the medium should be induced at that spot because extremely high intensity is produced at the focus point. By 3D scanning of the focus... 

P. M. Rentzepis et al. first demonstrated a bit-oriented 3D optical memory using a photochromic spirobenzopyran shown in Figure 16.3. Isomer A has an absorption band shorter than 450 nm upon irradiation with ultraviolet (UV) light, it converts to isomer B, which has an absorption band around 600 nm, as shown in Figure 16.4. Isomer B gives fluorescence around 700 nm upon photoexcitation with 500-700 nm light. 

Since single-photon recording does not require ultrashort-pulse lasers, conventional semiconductor lasers can be used in the 3D memories. The optics in CD and DVD devices can be easily applied for a recording system of 3D optical memory. 

Two-photon excitation is preferable in 3D optical memory because the crosstalk between two adjacent layers is much reduced. Another advantage of two-photon excitation is reduction in multiple scattering. This reduction occurs because of the use of an illumination beam at infrared wavelength. 

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],... 

Multi-photon-induced modification of optical properties of materials including refractive index, absorbance, polarization and fluorescence appearance or wavelength shifting could be utihzed for 3D optical memory in various media, including polymers, inorganic crystals, or glasses. 

Fig. 1 Photochromic materials for 3D optical memory, a Absorption spectra of open-ring and closed-ring diarylethene derivative B1536. b Bit patterns written by femtosecond two-photon absorption and readout using a reflection confocal microscope. Refer to Fig. 60h for ring close-open reactions...

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