Single-Beam, Two-Photon Recording

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. 

The ultrashort-pulse lasers increase the cost of a recording device and make it difficult to produce a compact system, but a compact ultrashort pulse laser in which Er-doped fiber is used as a resonator has been developed. Semiconductor lasers with mode-locked operations are also under investigation,so the cost and size of ultrashort-pulse lasers will decrease in the future. 

For reading data, a similar two-photon process was employed. When isomer B molecules were excited by absorbing two 1064 nm photons, the excitation will cause only the written molecules to emit fluorescence. Although the fluorescence readout method is sensitive, the memory is erased during the reading process because the reverse reaction from the photoexcited isomer B to isomer A takes place, to some extent. Such a destructive readout 

In transmission confocal microscopes that are equipped with phasereading systems, the focus deviates from the pinhole because of the inhomogeneity of the refractive index and/or the thickness of the memory medium and substrate. As a result, the detected signal has a background owing to the local inhomogeneity of the medium and substrate. 

The use of RCM alleviates the problems encountered when the transmission microscope is used, because the beam that is reflected at the data bit arrives at the pinhole even in the presence of inhomogeneity. RCMs, however, have not been used for reading 3D-stored data, primarily because the 3D spatial-frequency band of the reading optics of this configuration does not necessarily pick up the 3D band of the writing optics involved. 

18 Band of the writing optics for two-photon processes, and the band of the reading optics for RCM. 

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