Recent advances in optical data storage have led to the development of a five-dimensional device that could hold up to 2,000 times more data than a conventional DVD. Bit-by-bit optical data storage uses photons to introduce a localized physical or chemical property change such as photoinduced fluorescence or reflectance modulation as information storing processes. When an optical disc is scanned, pre-stored information can be retrieved back by detecting the intensity variation of the reading beam. Researchers have pursued further research to explore the feasibility of three-dimensional (3-D) optical data storage, such as double-layer DVDs and double-layer Blu-ray discs. They have also introduced the 2P excitation technique into 3-D optical data storage with two orthogonal beams intersecting inside the focuses of two objectives. Since 2P absorption is a highly nonlinear process, a high-power pulsed laser is generally required to facilitate the 2P process. The urgent demand for more capacity compels the development of ultra-high-density storage devices.
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