Photorefractive crystal increases capacity

A potassium lithium tantalum niobate (KLTN) crystal for electroholography (EH) has been demonstrated by a multiuniversity research group headed by Aharon Agranat of Hebrew University of Jerusalem (Jerusalem, Israel). The EH process controls reconstruction of volume holograms by an electric field and can form the basis of devices for very-large-scale optical memory and artificial neural networks, interconnection networks for massively parallel supercomputers, and spatial light modulation. With KLTN crystal, stored holograms do not fade, even when being read, and are selected electrically, rather than electro-optically, by merely changing the voltage applied to the crystal.

Conventional photorefractive materials used for optical data storage have their index of refraction changed in response to incident illumination. The interference patterns between the reference and signal beams are transformed into a space charge. The electric field induced by the space charge modulates the index of refraction. Thus the hologram is stored as a space charge that induces a spatially correlated modulation in the refractive index.

Many inorganic photorefractive crystals are ferroelectrics and have low levels of symmetry. The electro-optic effect in the space-charge modulation of the refractive index in these crystals is linear. Agranat says that, with KLTN, "the idea was to work with symmetrical crystals in which the electro-optic effect is quadratic. Such crystals are bad photorefractives. However, if an external electric field is applied to them the symmetry is broken; the field creates an induced electric dipole in the direction in which it is applied." Thus with the electric field turned on, the electro-optic effect becomes essentially linear and the crystal behaves as a common photorefractive. When the field is not present, the holograms that are stored in the form of a space charge do not affect the index of refraction and thus are not visible to the reconstructing beam. "The fact that the electric field can be turned on and off and the symmetry can therefore be broken at will means that the holograms stored in the form of a space charge can be switched on and off," Agranat says.

Agranat developed the composition and growth procedure for KLTN in the late 1980s at the California Institute of Technology (Pasadena, CA) working under Ammon Yariv. The crystals are being grown there and at Hebrew University.

"The first KLTN crystals we operated at around 150 K, whereas today we can set the work point anywhere between 100 K and 350 K," Agranat notes. "It turns out, however, that the physics of the storage process of holograms in KLTN is much more complex than in normal photorefractive crystals due to its operation close to the phase-transition temperature. Thus, with a strong electric field, we can induce a larger change in the refractive index of KLTN than can be achieved in normal photo refractives." The researchers expect the storage capacity of KLTN to supersede that of lithium niobate, generally regarded as the most effective candidate material for volume holographic storage.

Also near the phase transition, the holograms can be stored as dipole clusters, which originate from local modulation of the transition temperature. This permits highly effective fixing of the holograms. A hologram with an efficiency of 66% reflection of incident light has been fixed in a KLTN crystal at a low phase-transition temperature. Effort is under way to reproduce this at room temperature.

In addition, Agranat is planning to demonstrate KLTN in an optical interconnect configuration in a massively parallel computing prototype. Each stored hologram will be a different arrangement for interconnecting optical channels. By changing the voltage applied to the KLTN crystal, the desired connections will be selected.