Albert in lights
Using this technique, Kachru's team stored a picture of Albert Einstein in a 7-mm-thick Eu3+ doped Y2SiO5 crystal. The ions, once excited by a laser pulse, store information as a population modulation of their ground state. The population modulation is the result of resonant excitation, he said, which is instantaneous and works well with low laser powers. Kachru's group stored information in the crystal for up to several days by keeping the material at cryogenic temperatures.
A third read pulse retrieves the data from the crystal. This reconstructed image, typically known as a photon echo, appears from the crystal as an emitted laser pulse, which is separate in time from the read pulse. A charge-coupled device camera captures the emitted data, and a frame grabber digitizes it. Kachru's group used a modulated ring dye laser from Coherent Laser Group of Santa Clara, Calif., to create the read, data and write pulses; read and write pulses required peak powers of only 200 mW.
Using this setup, the crystal stored a 356-kb high-resolution black-andwhite photograph at recording speeds of 300 Mb/s. The reconstructed photograph looked as good as the original image.
The size of this stored image represents 1 percent of the storage capacity of the crystal, Kachru said. This means that in principle 100 such photographs would fit on a single crystal, which translates to a storage capacity of about 6 x 108 bits/cm3.
Future developments could concentrate on eliminating the cryogenic requirement for this type of time-domain holographic storage system.