Photorefractive effect

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The photorefractive effect describes the change in the refractive index of a material caused by light. It became known in 1966 as a disruptive effect ( English “ optical damage ” ) in lithium niobate and occurs generally in photoconductive , electro-optical crystals. Photorefractive materials are known from a whole series of material classes , of which inorganic crystals (e.g. iron-doped lithium niobate) are the most established. In addition, the photorefractive effect in organic crystals, organic polymers and glasses as well as liquid crystal cells could be demonstrated.

description

Photorefractive effect with a sinusoidal intensity pattern.

A combination of different material properties is necessary for the photorefractive effect to occur. The material must be transparent to light. It must have a photoelectric effect . This means that light must be able to generate moving charge carriers . An electric field has to change the refractive index due to the electro-optical effect .

The photorefractive effect can be broken down into several steps:

  1. Uneven illumination of the substrate and generation of mobile charge carriers in the bright zones
  2. Redistribution of the charge carriers in dark zones through various transport processes ( drift , diffusion , photogalvanic effect )
  3. Formation of an inhomogeneous space charge distribution and a corresponding internal electric field ( space charge field ). The relationship between the charge distribution and the field is determined by Gaussian law .
  4. Influence of the macroscopic refractive index through the resulting space charge field via the Pockels effect .

These steps take place almost simultaneously without any noticeable delay.

Possible applications

Photorefractive materials have achieved a certain prominence as potential fully reversible optical-holographic data storage media. For this purpose, an interference pattern consisting of an information-carrying laser beam and a reference beam is used for non-uniform illumination. As information comes z. B. a bit matrix in question. This can then be mapped as a hologram in the material and possibly saved. Analog image processing is also conceivable, as is a number of other applications that are correlated with it, such as B. optical coherence tomography for non-invasive diagnostics.

However, the materials were only partially able to meet the initially high expectations. The constant expansion of the superparamagnetic limit and the resulting unrivaled economic advantages of magnetic storage media have prevented this future technology as a data storage medium from exceeding the threshold from physical phenomena to commercial use.

Optical coherence tomography is currently considered to be the most promising potential application of photorefractive materials.

Individual evidence

  1. Jump up ↑ A. Ashkin, GD Boyd, JM Dziedzic, RG Smith, AA Ballman, JJ Levinstein, K. Nassau: Optically induced refractive index inhomogeneities in LiNbO 3 and LiTaO 3 . In: Applied Physics Letters . tape 9 , no. 1 , 1966, p. 72-74 , doi : 10.1063 / 1.1754607 .