US researchers believe they have solved a major problem with holographic storage systems which could provide much greater storage capacity than current devices available today. Holographic storage crystals work by illuminating the material with a series of pulses to create an optical interference pattern. This pattern alters the electric fields in the crystal and hence the refractive index of the material. Once the pattern has been set, data in the crystal can be read by shining light into the material and observing the diffraction pattern. However, reading data stored in a memory crystal erases the information. Now, Demetri Psaltis, Karsten Buse and Ali Adibi at the Californian Institute of Technology have developed a new doped crystal which can save and read data for as long as required (Nature 393 665).
Their technique works by using a process called two-colour holography. One wavelength of light records information on the crystal and another is used to read the data. Their method fires two light sources at the same time at the target.
The researchers doped a lithium niobate crystal with iron and manganese to create two different ‘deep electron traps’ between the valence and conduction bands in the crystal. An ultraviolet lamp was used to excite electrons from a low energy state (the manganese ion), to an intermediate state (on the iron ion) via the conductance band. Energy from a visible red laser can then excite the electrons in the intermediate state back up to the conduction band where it drops to the manganese low energy state. The ions left behind then change the refractive index of the material and record the diffraction pattern. Without the ultraviolet light, then the red light does not affect the refractive index of the crystal, so the material can be read without erasing the data. If more ultraviolet light is played on the crystal, the diffraction pattern is erased and new data can be written.
