A group of Stanford University researchers may be a step closer to the practical reality of holographic data storage systems. The team limited the volatility of the storage material by using two beams of light for recording and erasing, and only one for readout. Although holographic storage has been studied since the 1960s, it still faces hurdles; namely, finding a suitable storage material. One material that has shown promise is lithium niobate, but problems of sensitivity and volatility have to be addressed. The Stanford group, led by Lambertus Hesselink, decided to alter the ratio of lithium and niobate in the material. It also doped the material with iron and manganese to increase dark storage time and gating efficiency. Iron is commonly used to dope the congruent form of lithium niobate. It acts as a donor ion, producing an electron when it absorbs a photon. Manganese acts as a trap site for charges liberated and moved from the iron sites. "The basic photorefractive process involves liberating charges in bright areas of the illuminating beams from iron and moving them under the influence of an electric field, and then trapping them at manganese sites in the dark areas," Hesselink said. "The host material is electro-optic, which means that if charges are present, the speed of light changes locally and this gives rise to changes in the index of refraction." The net effect of adding these dopants was an increase in sensitivity by two orders of magnitude over previous work, according to the group. But a nagging problem remained: how to retrieve the stored information without erasing it, which occurs because lithium niobate remains photosensitive. As a remedy, Hesselink's group used a continuous-wave Ti:sapphire laser at 800 nm for writing and an incoherent blue light source at 476 nm for gating. Then they used only one wavelength in the near-IR for readout, eliminating erasure. Although these modifications mark a breakthrough in sensitivity, Hesselink said the group also is investigating variations of two other materials that could be even more sensitive: lead barium niobate and strontium barium niobate.