Holography is a method for recording and then reproducing a complete image of a Three-dimensional object. It means the recording of an entire picture. Dennis Gabor developed it in 1948. Unlike an ordinary photograph, a hologram has the three-dimensional quality of depth and can recreate parallax- an apparent change on perspective that occurs when an object is viewed from different positions. It is required that the light must be coherent and the waves must be the same wavelength and in phase for the hologram to appear. To make a hologram, a beam of coherent light, generally from a laser, is spit into two beams. One beam–the object beam–illuminates the object and is reflected onto the photographic plate, the other beam, called the reference beam, is aimed directly onto the photographic plate. Since the two beams superimposed on the plate are coherent, they form an interference pattern. In the phenomenon of interference, two or more waves are superimposed at each point. The record of this interference pattern on photographic film is the hologram. It looks nothing like the original object, but it contains the information needed to reproduce the light that originally came from an object. The development of holography had to await the perfection of a laser. This form of lensless photography has many uses such as credit cards, jewelry, magazines, and book covers. Many artists work with holograms and researchers are developing small computer-generated images that move.

Holography can be applied to a variety of uses that don't include taking "photographs" of things. For instance, some people believe that holographic storage (ala Hal) will be the next generation storage technique for data (DVD is close to diffraction limited and so you can't go much higher in terms of data density with that technology without doing strange stuff with evanescent waves.)

Just like any other media, holographic media is divided into write once and rewritable media. I'll describe how rewritable holographic storage works. Writing a hologram basically consists of 5 stages:

  • 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. 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 electric 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 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 typical laser pointer is red, 532nm light. Using light of this wavelength, one square inch of holographic storage would be able to hold 1.63E22 bits which is about 2,048,383,000 terabytes.

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