Examples of dynamic holograms
The discussion above describes static holography, in which recording, developing and reconstructing occur sequentially and a permanent hologram is produced.
There exist also holographic materials which don't need the developing process and can record a hologram in a very short time. This allows to use holography to perform some simple operations in an all–optical way. Examples of applications of such real–time holograms include phase–conjugate mirrors (“time–reversal” of light), optical cache memories, image processing (pattern recognition of time–varying images), and optical computing.
The amount of processed information can be very high (terabyte/s), since the operation is performed in parallel on a whole image. This compensates the fact that the recording time, which is in the order of a μs, is still very long compared to the processing time of an electronic computer. The optical processing performed by a dynamic hologram is also much less flexible than electronic processing. On one side one has to perform the operation always on the whole image and on the other side the operation a hologram can perform is basically either a multiplication or a phase conjugation. But remember that in optics, addition and Fourier transforms are already easily performed in linear materials, the second simply by a lens. This enables some applications like a device that compares images in an optical way.
The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photo refractive crystals, but also in semiconductors or semiconductor hetero structures (such as quantum wells), atomic vapors and gases, plasma and even liquids it was possible to generate holograms.
A particularly promising application is optical phase conjugation. It allows the removal of the wavefront distortions a light beam receives when passing through an aberration medium, by sending it back through the same operating medium with a conjugated phase. This is useful, for example, in free–space optical communications to compensate for atmospheric turbulence (the phenomenon that gives rise to the twinkling of starlight).