## 2 3D Optical Memory Systems by Photochromic Molecules Using Two-Photon Absorption  
Several photochromic compounds have previously been investigated as two-photon photochromic data storage media. The pioneering studies for 3D optical memory using photochromic compounds have been reported by [PERSON] and co-workers [1,34]. They have used photochromic spiropyrans (SPs). SP molecules exist in two isomeric forms, A and B, which are a colorless cyclic form and a colored open form, respectively (Figure 2). Irradiation of UV (ultraviolet) light to the colorless form A converts to the colored from B, which exhibits fluoresce at around 700 nm upon photoexcitation. Orthogonal two beams system was employed for the reading and writing, where the molecule was excited at the intersection of the two beams by simultaneous 2 PA. For writing information, the excitation was performed by two-photon irradiation of either a 1064 nm photon and a 532 nm photon corresponding to the excitation at 355 nm, or two photons at 532 nm corresponding to one 266 nm photon. Then, isomer A at the intersection photoisomerized to isomer B. Figure 1 illustrates the energy level diagram along with the molecular structures. For reading data, only the 1064 nm beam was used for irradiating the media. As isomer A does not absorb 532 nm photons, isomer A cannot be excited by 2 PA with a 1064 nm beam. Therefore, only isomer B can be excited and emit fluorescence. [PERSON] _et al_. have proposed the use of a reflection confocal microscope (RCM) as a reading system since 1998 [2,39]. They employed PMMA (polymethylmethacrylate) films, doped with photochromic diarylethene B1536 (Scheme 7), as an optical memory medium [39]. The 3D data were written using femtosecond pulses at 760 nm with an NA (numerical aperture) 1.4 objective lens. At the focus point, B1536 molecules were photoconverted by 2 PA. The written data could be erased by 543 nm irradiation. The reading was performed using an RCM, where a He-Ne laser beam at 633 nm was used with an NA 1.4 objective lens. At this wavelength, both isomers have no absorption. Different refractive indices were used for discrimination of information. Figure 2: Energy level diagram of two-photon writing and reading system. **Scheme 7. ** Structure of B1536. In 2002, [PERSON] _et al._ reported two-photon-induced photoisomerization of indolyfulgide [40], shown in Scheme 8. Two-photon photochromic reactions photoisomerization was performed using a pump-probe experimental setup, with femtosecond pulses at 775 nm. To support a two-photon-induced process, a kinetic study was performed to investigate the dependence of the photoisomerization rate on incident intensity. Plots of absorbance at 585 nm originated from ring-closed photoisomer versus time showed linear dependence being supportive of a two-photon-induced process. The 2 PA cross-section was determined using an open aperture Z-scan as 1030 GM. Further, two-dimensional interferometric recording in a polymeric film was performed. The interference pattern was monitored at the sample plane obtained by the spatial and temporal overlap of two pulses. The two-photon induced changes were observed in the high-intensity region in a thin film of a poly(methyl methacrylate)/phosphorylated styrene copolymer composite, doped with the indolyfulgide, on a glass substrate. The results demonstrated the possibility of using two-photon-induced photoisomerization of a fulgide for holographic recording in a polymeric medium. **Scheme 8. ** Photoisomerization of indolyfulgide. In 2005, [PERSON] _et al._ also reported 2 PA recording using diarylethene B1536, which allowed thermally stable photon-mode rewritable recording with a large refractive index change, containing PMMA films [41]. They demonstrated 2 PA recording using a laser diode operated with a 2 ns pulse duration without thermal effect. Comparison with recording sensitivity of femtosecond fiber laser revealed that the peak power could be reduced by decreasing pulse width.