Photochromic Materials Photochromics are real-time recyclable materials. Both organic and inorganic materials have been studied for photochromism. They require no processing for development and can be erased and reused. The holograms can be readout during or immediately after the recording. This property is useful for holographic interferometry. There is no inherent resolution limit since they are essentially grain free and operate on atomic or molecular scale. Their storage capacity of photochromic materials is high since the storage process occurs throughout the volume of the material.
Photochromic materials change their transmission spectrum in response to exposure of light of appropriate wavelength. They, in general, become dark under the action of shortwave visible or ultraviolet radiation. They are bleached by exposure to longwave visible or infrared radiation. This reversibility of the colour change distinguishes these materials from other photosensitive materials.
Holograms are recorded in photochromic materials generally by selective optical erasing or bleaching of material which has been darkened by uniform exposure of light. When the material is exposed to an interference pattern of erase light, the transmission increases at the bright portion of the pattern due to bleaching effect. This creates an absorption hologram. To erase the hologram, it is illuminated by the switching light which darkens the crystal uniformly.
n spite of several advantages, the photochromic materials have not been popular for hologram recording. This is because their sensitivity, efficiency and storage time are low. The reconstruction beam usually degrades the stored information. In alkali halide crystals the playback can be nondestructive if the temperature of the material is lowered after recording.
Photodichroic Materials Photodichroism can also be used to create switched and unswitched states in suitable crystals. This happens through selective alignment of anisotropic absorption centres indu ced by exposure to linearly polarized light . The material selectively absorbs light of a certain polarization only. Na-doped KCl crystal is a good example of photodichroism.
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