Confocal readout of three-dimensional data bits recorded by the photorefractive effect under single-photon and two-photon excitation

In three-dimensional optical bit data storage, data bits can be recorded as a result of the change in refractive index in photorefractive and photochromic materials under single-photon and two-photon excitation by a high numerical-aperture objective. In principle, these recorded data bits can be read out using reflection confocal microscopy because such a microscope exhibits an optical sectioning property for three-dimensional imaging. In this article, the conditions are described under which three-dimensional data bits recorded by the photorefractive effect under single-photon and two-photon excitation in these materials can be read in reflection confocal microscopy. The effect of the refractive-index mismatch between a volume recording medium and its immersion medium on these conditions is also considered.