Holographic memory

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This article refers to computer storage, for neuroscience, see Holonomic brain theory.

Holographic memory is a technology that can store information at high density inside crystals or photopolymers. Current storage techniques such as DVD use multiple layers to store information in the volume. Both techniques share an information density limit given by the used wavelength. In case of point-wise information retrieval this is called diffraction (of the writing beams). Note that many cheap holograms are stored in the surface only. Holographic storage has the potential to become the next generation of storage media^{[citation needed]}. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. This three-dimensional aspect allows for a phenomenon known as Bragg selectivity to be utilised, whereby many information laden holograms can be superimposed or multiplexed in the same volume of medium. It is necessary to Bragg detune each hologram recorded with respect to its neighbours. This can be achieved by a number of methods, e.g. rotation of the media with respect to the recording object and reference laser beams or changing the wavelength or phase of the recording laser beams for each hologram.

Like other media, holographic media is divided into write once (where the storage medium undergoes some irreversible change), and rewritable media (where the change is reversible). Rewritable holographic storage can be achieved via the photorefractive effect in crystals:

Mutually coherent light from two sources creates an interference pattern in the media. These two sources are called the reference beam and the signal beam.
Where there is constructive interference the light is bright and electrons can be promoted from the valence band to the conduction band of the material (since the light has given the electrons energy to jump the energy gap). The positively charged atoms they leave are called holes and they must be immobile in rewritable holographic materials. Where there is destructive interference, there is less light and few electrons are promoted.
Electrons in the conduction band are free to move in the material. They will experience two opposing forces that determine how they move. The first force is the coulomb force between the electrons and the positive holes that they have been promoted from. This force encourages the electrons to stay put or move back to where they came from. The second is the pseudo-force of diffusion that encourages them to move to areas where electrons are less dense. If the coulomb forces are not too strong, the electrons will move into the dark areas.
Beginning immediately after being promoted, there is a chance that a given electron will recombine with a hole and move back into the valence band. The faster the rate of recombination, the fewer the number of electrons that will have the chance to move into the dark areas. This rate will affect the strength of the hologram.
After some electrons have moved into the dark areas and recombined with holes there, there is a permanent space charge field between the electrons that moved to the dark spots and the holes in the bright spots. This leads to a change in the index of refraction due to the electro-optic effect.

When the information is to be retrieved or read out from the hologram, only the reference beam is necessary. The beam is sent into the material in exactly the same way as when the hologram was written. As a result of the index changes in the material that were created during writing, the beam splits into two parts. One of these parts recreates the signal beam where the information is stored. Something like a CCD camera can be used to convert this information into a more usable form.

Holograms can theoretically store equal to one bit per cubic block the size of the wavelength of light in writing. For example, light from a helium-neon laser is red, 632.8 nm wavelength light. Using light of this wavelength, perfect holographic storage could store 4 gigabits per cubic millimetre. In practice, the data density would be much lower, for at least four reasons:

The need to add error-correction
The need to accommodate imperfections or limitations in the optical system
Economic payoff (higher densities may cost disproportionately more to achieve)
Design technique limitations--a problem currently faced in magnetic Hard Drives wherein magnetic domain configuration prevents manufacture of disks that fully utilize the theoretical limits of the technology.

Unlike current storage technologies that record and read one data bit at a time, holographic memory writes and reads data in parallel in a single flash of light^[1].

At the National Association of Broadcasters 2005 (NAB) convention in Las Vegas, InPhase conducted the first public demonstrations of the world’s first prototype of a commercial storage device at the Maxell Corporation of America booth.