In two-dimensional (2D) holographic storage, a reference beam and a signal (or object) beam containing the pattern to be stored overlap in a photorefractive material to form a grating. The hologram is retrieved by shining the reading beam through the material at the recording angle. Using angle multiplexing, many pages of data can be stored in the same volume of the material by changing the angle of the reference beam for each recording. Specific pages can be accessed by selecting the corresponding angle for the reference beam. In addition, wavelength multiplexing can be used by changing the wavelength of the reference beam for each recording.O. Ostroverkhova and W. E. Moerner, Chem. Rev. 104, 3267 (2004), S. Tay et al., Nature 451, 694 (2008)