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Organic photorefractive materials are materials that exhibit a temporary change in refractive index when exposed to light. The changing refractive index causes light to change speed throughout the material and produce light and dark regions in the crystal. The buildup can be controlled to produce holographic images for use in biomedical scans and optical computing. The ease with which the chemical composition can be changed in organic materials makes the photorefractive effect more controllable.

History

Although the physics behind the photorefractive effect were known for quite a while, the effect was first observed in 1967 in LiNbO₃. For more than thirty years, the effect was observed and studied exclusively in inorganic materials, until 1990, when a nonlinear organic crystal 2-(cyclooctylamino)-5-nitropyridine (COANP) doped with 7,7,8,8-tetracyanoquinodimethane (TCNQ) exhibited the photorefractive effect. Even though inorganic material-based electronics dominate the current market, organic PR materials have been improved greatly since then and are currently considered to be an equal alternative to inorganic crystals.

Theory

There are two phenomena that, when combined together, produce the photorefractive effect. These are photoconductivity, first observed in selenium by Willoughby Smith in 1873 , and the Pockels Effect, named after Friedrich Carl Alwin Pockels who studied it in 1893.

Photoconductivity is the property of a material that describes the capability of incident light of adequate wavelength to produce electric charge carriers. The Fermi level of an intrinsic semiconductor is exactly in the middle of the band gap. The densities of free electrons n in the conduction band and free holes h in the valence band can be found through equations :

$n=N_{c}{\frac {e^{-(E_{c}-E_{F})}}{k_{B}T}}$

and

$h=N_{v}{\frac {e^{-(E_{c}-E_{F})}}{k_{B}T}}$

where N_C and N_V are the densities of states at the bottom of the conduction band and the top of the valence band, respectively, E_C and E_V are the corresponding energies, E_F is the Fermi level energy, k_B is Boltzmann’s constant and T is the absolute temperature. Addition of impurities into the semiconductor, or doping, produces excess holes or electrons, which, with sufficient density, may pin the Fermi level to the impurities’ position .

Doped semiconductor: Fermi levels pinned at the position of the dopant in the band gap

A sufficiently energetic light can excite charge carriers so much that they will populate the initially empty localized levels. Then, the density of free carriers in the conduction and/or the valence band will increase. To account for these changes, steady-state Fermi levels are defined for electrons to be E_Fn and, for holes – E_Fp. The densities n and h are, then equal to

$n=N_{c}{\frac {e^{-(E_{c}-E_{Fn})}}{k_{B}T}}$

$h=N_{v}{\frac {e^{-(E_{Fp}-E_{v})}}{k_{B}T}}$

Doped semiconductor: Fermi E_f and quasi-stationary Fermi levels upon illumination

The localized states between E_Fn and E_Fp are known as ‘photoactive centers.’ The charge carriers remain in these states for a long time until they recombine with an oppositely charged carrier. The states outside the E_Fn – E_Fp energy, however, relax their charge carriers to the nearest extended states.

The effect of incident light on the conductivity of the material depends on the energy of light and material. Differently-doped materials may have several different types of photoactive centers, each of which requires a different mathematical treatment. However, it is not very difficult to show the relationship between incident light and conductivity in a material with only one type of charge carrier and one type of a photoactive center. The dark conductivity of such a material is given by

$\sigma _{d}=e\left(N_{d}-N_{d}^{+}\right)\beta \mu \tau$

where σ_d is the conductivity, e = electron charge, N_D and N_D⁺ are the densities of total photoactive centers and ionized empty electron acceptor states, respectively, β is the thermal photoelectron generation coefficient, μ is the mobility constant and τ is the photoelectron lifetime. The equation for photoconductivity substitutes the parameters of the incident light for β and is

$\sigma _{ph}=e\left(N_{d}-N_{d}^{+}\right){\frac {sI\mu \tau }{h\nu }}$

in which s is the effective cross-section for photoelectron generation, h is the Planck constant, ν is the frequency of incident light, and the term I = I₀e^-αz in which I₀ is the incident irradiance, z is the coordinate along the crystal thickness and α is the light intensity loss coefficient.

The electro-optic effect is a change of the optical properties of a given material in response to an electric field. There are many different occurrences, all of which are in the subgroup of the electro-optic effect, and Pockels effect is one of these occurrences. Essentially, the Pockels effect is the change of the material’s refractive index induced by an applied electric field. The refractive index of a material is the factor by which the phase velocity is decreased relative to the velocity of light in vacuum. At a microscale, such a decrease occurs because of a disturbance in the charges of each atom after being subjected to the electromagnetic field of the incident light. As the electrons move around energy levels, some energy is released as an electromagnetic wave at the same frequency but with a phase delay. The apparent light in a medium is a superposition of all of the waves released in such way, and so the resulting light wave has shorter wavelength but the same frequency and the light wave’s phase speed is slowed down.

Whether or not the material will exhibit Pockels effect depends on its symmetry. Both centrosymmetric and non-centrosymmetric media will exhibit an effect similar to Pockels, the Kerr effect. The refractive index change will be proportional to the square of the electric field strength and will therefore be much weaker than the Pockels effect. It is only the non-centrosymmetric materials that can exhibit the Pockels effect: for instance, lithium tantalite (trigonal crystal) or gallium arsenide (zinc-blende crystal); as well as poled polymers with specifically designed organic molecules.

It is possible to describe the Pockels effect mathematically by first introducing the index ellipsoid – a concept relating the orientation and relative magnitude of the material’s refractive indices. The ellipsoid is defined by

${\frac {R_{1}^{2}}{\varepsilon _{1}}}+{\frac {R_{2}^{2}}{\varepsilon _{2}}}+{\frac {R_{3}^{2}}{\varepsilon _{3}}}=1$

in which ε_i is the relative permittivity along the x, y, or z axis, and R is the reduced displacement vector defined as D_i/√8πW in which D_i is the electric displacement vector and W is the field energy. The electric field will induce a deformation in R_i as according to:

$\Delta R_{i}^{2}=\sum _{j=1}^{3}r_{ij}E_{j}$

in which E is the applied electric field, and r_ij is a coefficient that depends on the crystal symmetry and the orientation of the coordinate system with respect to the crystal axes. Some of these coefficients will usually be equal to zero.

Organic Photorefractive Materials

Chemical structure of PVK

In general,photorefractive materials can be classified into following categories, the border between categories may not be sharp in each case

• Inorganic crystal and compound semiconductor
• Multiple quantum well structures
• Organic crystalline materials
• Polymer dispersed Liquid crystalline materials (PDLC)
• Organic amorphous materials

Electronic states of PATPD (a) and PVK (b)-based composite with DBDC and 7-DCST as chromophores and C60 as sensitizer

In the field of this research, initial investigations were mainly carried out with inorganic semiconductors. There have been huge varieties of inorganic crystals such as BaTiO3, KNbO3, LiNbO3 and inorganic compound semiconductors such as GaAs, InP, CdTe are reported in literature. First photorefractive(PR) effect in organic materials was reported in 1991 and then, research of organic photorefractive materials has drawn major attention in recent years compare to inorganic PR semiconductors. This is due to mainly cost effectiveness, relatively easy synthetic procedure, and tunable properties through modifications of chemical or compositional changes.

Polymer or polymer composite materials have shown excellent photorefractive properties of 100% diffraction efficiency. Most recently, amorphous composites of low glass transition temperature have emerged as highly efficient PR materials. These two classes of organic PR materials are also mostly investigated field. These composite materials have four components -conducting materials, sensitizer, chromophore, and other dopant molecules to be discussed in terms of PR effect. According to the literature,design strategy of hole conductors is mainly p-type based and the issues on the sensitizing are accentuated on n-type electron accepting materials, which are usually of very low content in the blends and thus do not provide a complementary path for electron conduction. In recent publications on organic PR materials, it is common to incorporate a polymeric material with charge transport units in its main or side chain. In this way, the polymer also serves as a host matrix to provide the resultant composite material with a sufficient viscosity for reasons of processing. Most guest-host composites demonstrated in the literature so far are based on hole conducting polymeric materials.

The vast majority of the polymers are based on carbazole containing polymers like poly-(N-vinyl carbazole) (PVK) and polysiloxanes (PSX). PVK is the well studied system for huge varieties of applications . In polymers,charge is transported through the HOMO and the mobility is influenced by the nature of the dopant mixed into the polymer, also it depends on the amount of dopant which may exceed 50 weight percent of the composite for guest-host materials. The mobility decreases as the concentration of charge-transport moieties decreases, and the dopant’s polarity and concentration increases.

Besides the mobility, the ionization potential of the polymer and the respective dopant has also significant importance. The relative position of the polymer HOMO with respect to the ionization potential of the other components of the blends determines the extent of extrinsic hole traps in the material. TPD (tetraphenyldiaminophenyl) based materials are known to exhibit higher charge carrier mobilities and lower ionization potentials compare to carbazole based (PVK) materials. The low ionization potentials of the TPD based materials greatly enhance the photoconductivity of the materials. This is partly due to the enhanced complexation of the hole conductor, which is an electron donor, with the sensitizing agents, which is an electron acceptor. It was reported a dramatic increase of the photogeneration efficiency from 0.3% to 100% by lowering the ionization potential from 5.90 eV (PVK) to 5.39 eV ( TPD derivative PATPD). This is schematically explained in the diagram using the electronic states of PVK and PATPD.

Applications

As of 2011, no commercial products utilizing organic photorefractive materials exist. All applications described are speculative or performed in research laboratories. Large DC fields required to produce holograms lead to dielectric breakdown not suitable outside the laboratory.

Reusable Holographic Displays

Recording of Holographic Image

Many materials exist for recording static, permanent holograms including photopolymers, silver halide films, photoresists, dichromated gelatin, and photorefractives. Materials vary in their maximum diffraction efficiency, required power consumption, and resolution. Photorefractives have a high diffraction efficiency, an average-low power consumption, and a high resolution.

Updatable holograms that do not require glasses are attractive for medical and military imaging. The materials properties required to produce updatable holograms are 100% diffraction efficiency, fast writing time, long image persistence, fast erasing time, and large area. Inorganic materials capable of rapid updating exist but are difficult to grow larger than a cubic centimeter. Liquid crystal 3D displays exist but require complex computation to produce images which limits their refresh rate and size.

Blanche et al. demonstrated in 2008 a 4 in. x 4 in. display that refreshed every few minutes and lasted several hours. Organic photorefractive materials are capable of kHz refresh rates though it is limited by material sensitivity and laser power. Material sensitivity demonstrated in 2010 require kW pulsed lasers.

Tunable Color Filter

White light passed through an organic photorefractive diffraction grating, leads to the absorption of wavelengths generated by surface plasmon resonance and the reflection of complementary wavelengths. The period of the diffraction grating may be adjusted by modifying to control the wavelengths of the reflected light. This could be used for filter channels, optical attenuators, and optical color filters

Optical communications

Free-space optical communications(FSO) can be used for high-bandwidth communication of data by utilizing high frequency lasers. Phase distortions created by the atmosphere can be corrected by a four-wave mixing process utilizing organic photorefractive holograms. The nature of FSO allows images to be transmitted at near original quality in real-time. The correction also corrects for moving images.

Image and Signal Processing

Organic photorefractive materials are a nonlinear medium in which large amounts of information can be recorded and read. Holograms due to the inherent parallel nature of optical recording are able to quickly process large amounts of data. Holograms that can be quickly produced and read can be used to verify the authenticity of documents similar to a watermark Organic photorefractive correlators use matched filter and Joint Fourier Transform configurations.

Logical functions (AND, OR, NOR, XOR, NOT) were carried out using two-wave signal processing. High diffraction efficiency allowed a CCD detector to distinguish between light pixels (1 bits) and dark pixels (0 bits).

References

sketchc89 response to edits - for help rewriting section in future

• • Peer Review"
    

This article covers theories, classification and applications of photorefractive materials while some parts are not completed with insufficient informationi'd rather have all parts not completed with insufficient information then our article would be perfect . Some parts of the article correspond to general photorefractive materials and others correspond to organic ones, making the whole article not so focused. Specific comments are as follows:

1 The title is appropriate since it captures a thematic area of materials for a Wikipedia site. The authors provided sufficient links to other topics in Wikipedia for readers to find relevant information. However, the contents of the article should be adjusted in accordance with the title so that all parts focus on organic photorefractive materials. For example, the title of one section is also “Organic Photorefractive Materials”, same as the overall title, which is improper. This section should focus on certain specific examples of organic photorefractive materials.
good point will focus the article more on organic

2 The first sentence provides a concept. However, the concept only explains photorefractive materials instead of organic photorefractive materials. Further explanation followed is somewhat abstract. A brief description of some specific applications along with references and pictures will be helpful.
my understanding is that they have the same theory, different properties - need to look into

3 In Section “History”, the authors addressed some discoveries concerning photorefractive materials as a background. This description need be focused more on the importance of the series of discoveries other than just stating the facts. What problems have been solved through researches in this area and why are they significant? Both development of theories and applications could be added. Also, why organic photorefractive materials are developed as alternatives to inorganic ones and why they are important need be explained.
i disagree with this point. discovery of photorefractive materials is relevant to the article despite the focus on organic. the importance of discovering the material is self-evident and needs no further explanation. the further explanation would be dealt with in its applications or significance not history. it is explained, but not in this section. we'll see if it makes sense to explain it here

4 The first two figures help explain the theories of the materials. The following two figures are two specific examples, but corresponding explanations are not completed yet. The last figure is the scheme of holographic image recording. This figure is unnecessary since it is unrelated to the topic and should be explained in detail under the term “holography” which has already been linked. In addition, a figure to explain the concept is needed to help people have a quick idea of the structure and properties of these materials.

5 In Section “Organic Photorefractive Materials”, which actually provides specific examples, corresponding references to the examples are missing. This is probably due to the incompleteness of this part. The last paragraph of this section is rather long and lacks logic, for it contains some history and some examples, both of which are not clearly explained. After completing all subsections with references, only a brief summary is enough to wrap up this section.

6 In Section “Applications”, the authors only introduces some optical and imaging techniques. The connections between the techniques and the materials are missing. Why organic photorefractive materials are unique for these applications should be explained relating their properties. Moreover some subsections seem overlapping, for example image processing includes holography.

7 References are not complete. For example, ref 3, 13 and 14 lack pages and ref 15, 16 and 17 lack publishing years. All references should be complete with authors, journal names, years and pages.

--Lengmartin (talk) 16:06, 23 November 2011 (UTC)

• Peer Review

1) The topic is very similar to the Wikipedia page on the “photorefractive effect”, but gives more detail on the theory of the photorefractive effect that the actual Wikipedia page on that topic. More detail and clarity on how these materials exhibit the photorefractive effect would have helped. The authors linked all appropriate sites.
we would be more than happy to receive extra credit for fixing all of wikipedia. will explain how these material exhibit it better

2) The general public summary was written well, but is a bit short. More detail would have been helpful, like a one-sentence summary of how organic and inorganic photorefractive materials differ. Also, there are other inorganic materials that should be mentioned to be complete and thorough such as potassium tantalate niobate, bismuth silicon oxide, barium sodium niobate, bismuth germanium oxide, barium titanate, and lithium tantalite. Also, there could be better connections between theory and the applications of these materials.
3) There is only a “history” section which is very short and does not give the proper background. The importance of this topic is implied in the applications section, but is not explicitly stated or discussed. By contrasting the importance of organic versus inorganic photorefractive materials, this could be addressed.
4) The figures that are here do add to the article, but a simple figure of light changing differently when entering a photorefractive material in the first section would be useful. Also, a figure of the structure of psk would be helpful.
5) The entire section “Organic Photorefractive Materials” had no references. These need to be added in. The following references of reviews should be added:

a. http://www. hololight .net/photorefractive-materials.html

b. R. A. Norwood, S. Tay, P. Wang, P.-A. Blanche, D. Flores, R. Voorakaranam, W. Lin, J. Thomas, T. Gu, P. St. Hilaire, C. Christenson, M. Yamamoto, N. Peyghambarian. 2010, ACS Syposium Series: Organic Thin Films for Photonic Applications; Chapter 6 “Photorefractive Polymers for Updatable Holographic Displays”; pp 85-95. doi: 10.1021/bk-2010-1039.ch006

c. Jayan Thomas, Cory W. Christenson, Pierre-Alexandre Blanche, Michiharu Yamamoto, Robert A. Norwood, and Nasser Peyghambarian; Chem. Mater., 2011, 23 (3), pp 416–429. doi: 10.1021/cm102144h

6) Language could use some revision to make the overall page more easily understood. In the section “Reusable Holographic Displays”, an explanation of what refresh and refresh rate are, why organic derivative are sensitive, and to what they are sensitive. In the section “Tunable Color Filter”, the required properties of the materials to be able to use for this are missing, as well as what current industry standards are lacking and what can be improved upon and how.
-Akcook8 (talk) 23:33, 28 November 2011 (UTC)

• Peer Review

1). It is unclear that this area of materials science is seemingly appropriate for an encyclopedic Wikipedia site. Although the page is for organic photorefractive materials, only one organic material is mentioned in the article, in the history section. Multiple inorganic materials are mentioned in the article. The topic is likely a thematic area of materials science appropriate for an encyclopedic Wikipedia site, but probably as a comprehensive page on photorefractive materials.

2) The general summary is written at an appropriate technical level, but fails to accurately capture the subject of the review. The amount of information present is insufficient to give the reader an idea of the subject topic. For example “The changing refractive index causes light to change speed throughout the material and produce light and dark regions in the crystal. The buildup can be controlled to produce holographic images for use in biomedical scans and optical computing.” This sentence begins with “The buildup”, but it is unclear as to what this actually refers to.

3) The background section is present and provides a sparse review of the history of these materials. The significance section is not present. As a result, no context is given as to where these materials fit into the field of materials science. Looking into the literature, it appears that many of these organic photorefractive materials are polymers, but this is not discussed

4) The figures present within the theory section contribute to the understanding of the topic being discussed. Two figures are planned in the section on organic photorefractive materials, but are not present. One figure I would like to see is the interaction of the incident light wave within the crystal with the electric field, providing a clear visual explanation of the effect.

5) Since the main section on organic photorefractive materials was not cited, important references are likely missing. In addition, no specific materials were addressed. One specific missing reference is by Marder, Kippelen, and Jen, Nature, 388 (1997). It has over 600 references and is more recent than some of the articles referenced on the site. Another important paper is by Ducharme, Scott, and Twieg, et al. Physical Review Letters, 66 (1991)

6) Some sections, such as theory, are worded correctly and easier to understand. The organic photorefractive materials section is less clear. This sentence: “First PR effect in organic materials was reported in 1991 and then, research of organic photorefractive materials has drawn major attention in recent years compare to inorganic PR semiconductors.”, needs to be rewritten to conform to Wikipedia level written English. In addition, “PR” effect is used many times on this site, but it is never explicitly stated that this refers to the photorefractive effect and may confuse readers. The “Optical communications” section uses correct grammar, but does not easily communicate the application. In “Image and Signal Processing”, the verb tense changes within the paragraph. The first part of the section is fine, but a seemingly random past tense section, “Logical functions (AND, OR, NOR, XOR, NOT) were carried out using two-wave signal processing.[17] High diffraction efficiency allowed a CCD detector to distinguish between light pixels (1 bits) and dark pixels (0 bits).[17] “ was inserted.

Na9234 (talk) 21:25, 1 December 2011 (UTC)