The photorefractive effect is a nonlinear optical

Nonlinear optics

Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...

 effect seen in certain crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s and other materials that respond to light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 by altering their refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

.
The effect can be used to store temporary, erasable holograms

Holography

Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...

 and is useful for holographic data storage

.
It can also be used to create a phase-conjugate mirror or an optical spatial soliton

Soliton (optics)

In optics, the term soliton is used to refer to any optical field that does not change during propagation because of a delicate balance between nonlinear and linear effects in the medium. There are two main kinds of solitons:...

.

Mechanism

The photorefractive effect occurs in several stages:

1. A photorefractive material is illuminated by coherent
   Coherence (physics)
   In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
   
    beams of light. (In holography, these would be the signal and reference beams). Interference between the beams results in a pattern of dark and light fringes throughout the crystal.
2. In regions where a bright fringe is present, electron
   Electron
   The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
   
   s can absorb the light and be photoexcited from an impurity level into the conduction band
   Conduction band
   In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
   
    of the material, leaving an electron hole
   Electron hole
   An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
   
    (a net positive charge). Impurity levels have an energy
   Energy
   In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
   
    intermediate between the energies of the valence band
   Valence band
   In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....
   
    and conduction band of the material.
3. Once in the conduction band, the electrons are free to move and diffuse
   Diffusion
   Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
   
    throughout the crystal. Since the electrons are being excited preferentially in the bright fringes, the net electron drift is towards the dark-fringe regions of the material.
4. While in the conduction band, the electrons may with some probability recombine with the holes and return to the impurity levels. The rate at which this recombination takes place determines how far the electrons diffuse, and thus the overall strength of the photorefractive effect in that material. Once back in the impurity level, the electrons are trapped and can no longer move unless re-excited back into the conduction band (by light).
5. With the net redistribution of electrons into the dark regions of the material, leaving holes in the bright areas, the resulting charge distribution causes an electric field
   Electric field
   In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
   
   , known as a space charge field to be set up in the crystal. Since the electrons and holes are trapped and immobile, the space charge field persists even when the illuminating beams are removed.
6. The internal space charge field, via the electro-optic effect
   Electro-optic effect
   An electro-optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light...
   
   , causes the refractive index of the crystal to change in the regions where the field is strongest. This causes a spatially varying refractive index grating
   Diffraction grating
   In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
   
    to occur throughout the crystal. The pattern of the grating that is formed follows the light interference pattern originally imposed on the crystal.
7. The refractive index grating can now diffract
   Diffraction
   Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
   
    light shone into the crystal, with the resulting diffraction pattern recreating the original pattern of light stored in the crystal.

Application

The photorefractive effect can be used for dynamic holography, and, in particular, for cleaning of coherent beams.
For example, in the case of a hologram, illuminating the grating with just the reference beam causes the reconstruction of the original signal beam. When two coherent laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 beams (usually obtained by splitting a laser beam by the use of a beamsplitter into two, and then suitably redirecting by mirrors) to cross inside a photorefractive crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

, the resultant refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 grating

Grating

A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicular to the first...

 diffracts the laser beams. As a result, one beam gains energy and becomes more intense at the expense of light intensity reduction of the other. This phenomenon is an example of two-wave mixing. It is interesting that in this configuration, Bragg diffraction condition

Bragg's law

In physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...

 is automatically satisfied.

The pattern stored inside the crystal persists until the pattern is erased; this can be done by flooding the crystal with uniform illumination which will excite the electrons back into the conduction band and allow them to be distributed more uniformly.

Photorefractive materials include barium titanate

Barium titanate

Barium titanate is the inorganic compound with the chemical formula BaTiO3. Barium titanate is a white powder and transparent as larger crystals...

 (BaTiO₃), lithium niobate

Lithium niobate

Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...

 (LiNbO₃), certain photopolymer

Photopolymer

A photopolymer is a polymer that changes its properties when exposed to light, often in the ultraviolet spectrum. These polymers are useful in dentistry for fillings and in rapid prototyping in the stereolithography and 3D printing processes. This material is also used in the creation of ADA...

s, and some multiple quantum well structures.

There were even claims that an amplifier based on photorefractive crystals can have less than the quantum limit of noise that is typical for optical amplifiers of any kind.