Optical storage techniques

The application of nonlinear optical recording techniques for reversible optical data storage based on the excitation of photochromic molecules by two-photon processes also has been described (154). 

Despite minor drawbacks, optical data techniques are the technology of the future for data storage. The potential for great strides forward in performance clearly exists. 

D. Day, M. Gu, and A. Smallridge, Two-Photon Optical Storage in Photorefractive Polymers in the Near-Infrared Spectral Range. In Infrared Holography for Optical Communications, Techniques, Materials, and Devices, Vol. 86 P. Boffi, D. Piccini, and M. C. Ubaldi, Eds., Springer-Verlag, Berlin, 2003, p. 75. 

The past decade has seen an explosion in investigations of molecular ions using a variety of optical spectroscopic techniques in conjunction with trapping mass spectrometers. The mass selection and ion storage capabilities of instruments such as 3-D QITs and FT-ICR mass spectrometers provide valuable control over the ion population under investigation. Moreover, thanks to modem ion sources, the size of molecules is no longer a limitation for gas-phase ion spectroscopic studies. A number of spectroscopic techniques have been developed to probe gas-phase molecules that will be fruitful when applied to the spectroscopy of trapped ions. 

To overcome volatility problems, two-photon absorption (TPA) is the most promising long-term data storage technique to date. TPA can be observed in all materials and lacks the noncentrosymmetric symmetry requirements of even-order nonlinear optical effects. Nevertheless, in centrosymmetric polymers two-photon absorption is allowed between states that have the same parity according to parity selection rules (112). Such a system has been demonstrated with nondestructive readout (113). Additionally, use of two-photon absorption on photorefractive polymer dispersed liquid crystals has been used to record a high 3D data density of 204.8 Gbits/cm (158). 

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