The photorefractive effect is a
nonlinear optical
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...
effect seen in certain
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s and other materials that respond to
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
by altering their
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
.
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The effect can be used to store temporary, erasable
holograms
Holography is a technique that enables a wavefront to be recorded and later re-constructed. Holography is best known as a method of generating real three-dimensional images, but it also has a wide range of other applications. In principle, it ...
and is useful for
holographic data storage
Holographic data storage is a potential technology in the area of high-capacity data storage. While magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface of the recor ...
.
[
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It can also be used to create a
phase-conjugate mirror
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...
or an
optical spatial soliton.
Mechanism
The photorefractive effect occurs in several stages:
#A photorefractive material is illuminated by
coherent
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a deri ...
beams of light. (In holography, these would be the signal and reference beams).
Interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extr ...
between the beams results in a pattern of dark and light fringes throughout the crystal.
#In regions where a bright fringe is present,
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s can absorb the light and be photoexcited from an
impurity level into the
conduction band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
of the material, leaving an
electron hole
In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle which is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or ...
(a net positive charge). Impurity levels have an
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
intermediate between the energies of the
valence band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
and conduction band of the material.
#Once in the conduction band, the electrons are free to move and
diffuse
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
throughout the crystal. Since the electrons are being excited preferentially in the bright fringes, the net electron diffusion current is towards the dark-fringe regions of the material.
#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).
#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
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
, 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.
#The internal space charge field, via the
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. The term encompasses a number of distinct phenomena, which can be subdivided i ...
, 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
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 perpendicul ...
to occur throughout the crystal. The pattern of the grating that is formed follows the light interference pattern originally imposed on the crystal.
#The refractive index grating can now
diffract
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
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
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
beams (usually obtained by splitting a laser beam by the use of a
beamsplitter
A beam splitter or ''beamsplitter'' is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding wide ...
into two, and then suitably redirecting by
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s) cross inside a photorefractive
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
, the resultant
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
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 perpendicul ...
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. In this configuration,
Bragg diffraction condition 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 (BTO) is an inorganic compound with chemical formula BaTiO3. Barium titanate appears white as a powder and is transparent when prepared as large crystals. It is a Ferroelectricity, ferroelectric, Pyroelectricity, pyroelectric, and ...
(BaTiO
3),
lithium niobate
Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...
(LiNbO
3),
vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...
doped
zinc telluride
Zinc telluride is a binary chemical compound with the formula ZnTe. This solid is a semiconductor material with a direct band gap of 2.26 eV. It is usually a p-type semiconductor. Its crystal structure is cubic, like that for sphalerite and di ...
(ZnTe:V),
organic photorefractive materials, certain
photopolymer
A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardeni ...
s, and some
multiple quantum well
A quantum well is a potential well with only discrete energy values.
The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...
structures.
References
{{DEFAULTSORT:Photorefractive Effect
Optical materials
Nonlinear optics
Holography