Rapid increases in the computer processing power available per unit cost, as well as a growing hunger for multimedia (text, audio, and image) entertainment and processing has led to heightened interest in multilayer and other volume optical storage technologies.1 One such technology is 2-photon absorption-based storage.2,3,4 In this approach, writing or erasure takes place only at the intersection of 2 beams of different wavelengths, thus allowing independent addressing of any point within a 3-dimensional volume. As a bit oriented (rather than holographic) memory, a unique physical location is allocated for each data bit. If individual bits are stored, volumetric densities on the order of terabits/cm3 may be achievable, and parallel reading of the bits (e.g.-in planes or pages) can enable 100 Gb/s data transfer rates. The key principle behind 2-photon memories is the creation of a molecular change in the media by the simultaneous absorption of 2 photons from 2 different beams by the dopant molecule, as shown in Fig. 1(a). The localized change can store bits by changing the index of refraction, absorption, fluorescence, or the material’s electrical properties. In our system experiments so far we have utilized materials in which 2-photon excitation creates a new absorption peak and subsequent 1 or 2-photon illumination results in efficient fluorescence for readout. The broad readout absorption profile, and the high fluorescence efficiency enable the use of inexpensive, low power replay sources, including LEDs and filtered lamps. In addition to non-destructive memory recall, we have also demonstrated optically induced erasure, thus enabling random accessibility and erasure of bits. These two-photon 3-D memories are similar to the multilayer optical disk systems with the potential for simple media fabrication, many layers, parallel access for high data transfer rates, and low raw bit error rates (BER). As shown in Fig. 1(b), orthogonal intersection of the writing beams may be used, or if ultra-short (e.g.-<100 femtosecond) pulses are used, a counter-propagating arrangement10,5 is feasible. While single bits may be stored and recalled, parallel access of lines or planes of data is naturally accommodated in two-photon 3-D memories, to provide increased data transfer rates. Due to diffraction of the addressing beam as it propagates across the data image, there exists a tradeoff between the volumetric density of the memory and the parallelism, or data transfer rate.9
