User:Pei123/sandbox2

Organic photorefractive materials are organic materials that exhibit a temporary change in refractive index when exposed to light. The photorefractive effect was first observed by Bell Laboratories in 1966 in LiNbO₃ and LaTaO₃, but was not found in organic materials until 1991 when IBM discovered it in bisphenol-3-diglycidylether 4-nitro-1, 2-phenylenediamine doped with diethylamino-benzaldehyde diphenylhydrazone.(citation?) Changing the refractive index of a material changes the speed at which light travels through that material.( Organic?) photorefractive materials are able to change their refractive index due to a combination of photoconductivity(link?), the production of an electric field due to illumination, and the Pockels effect(link?), the change in refractive index due to an electric field. Electrons and holes in (organic?) photorefractive materials are excited by light, diffuse through the material, and recombine leading to variations in the refractive index of the material. (These) variations 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. Organic photorefractive materials offer a number of possible advantages over inorganic photorefractive materials including reusability, lower cost, and easier processing. The ease with which the chemical composition can be changed in organic materials makes the photorefractive effect more controllable.

History

Photorefractive effect, is the ability of material to change its refractive index due to light. This effect was first observed in inorganic crystals in 1966 by the Bell Laboratories, and for some time, investigations into the photorefractive effect was carried out with inorganic semiconductors. However, the effect was subsequently observed in other kinds of materials, in particular – organic, in 1991 by IBM (This information is repeated)and, as of today, photorefractive materials can be classified into following categories(The following are not the classifications of organic photorefractive materials. Are they really important?):

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

Refraction is the property of material that changes the direction of photon wave by changing the speed of the wave, while keeping the frequency constant. The entering wave disturbs the charges on the atoms of the materials and the atoms then radiate their own electromagnetic wave. The apparent light passing through materials is actually the macroscopic superposition of all the electromagnetic waves in the material. Photorefractive materials owe their unique property to an electro-optic phenomenon known as Pockels effect, and photoconductivity. Pockels effect is the change of the refractive index of materials due to an applied electric field: an applied electric field changes the polarity within the material and the electromagnetic waves radiated by the atoms within the materials vary under the influence of the external electric field. photoconductivity is the property of a material in which incident light of adequate wavelength is capable of producing electric charge carriers. Photorefractive materials have charge carriers in the localized states in the forbidden energy band gap which get excited under incident light and carried over to the conduction or valence bands. The carriers are then retrapped and excited again. The charge carriers accumulate in different regions of the material, depending on their sign, resulting in a spatial modulation of the electric field within the material. That modulation produces a modulation of the index of refraction by Pockels effect. The change of the refractive index can be undone by incident light of a different wavelength, or by relaxing the material in the dark.

Pockels effect can only occur in noncentrosymmetric media such as LiNbO₃ and GaAs crystals or electric field-poled organic polymers and glasses. Recently, research into organic photorefractive materials has been drawing a lot of attention: synthesis of organic photorefractive materials is much easier and cost-effective than that of inorganic materials. Organic photorefractive materials also have unique tunable properties that can be controlled through chemical compositional changes. Polymer and 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 photorefractive materials.

The photorefractive effect has long been used in holographic display applications. Reusability is the primary feature of organic photorefractive materials over traditional inorganic photorefractive materials. The appeal of reusability has inspired a great deal of research in photorefractive materials and expanded the usefulness of the photorefractive effect.

Traditional holographic applications, like optical computing, that rely on the high information density of holograms, highlight direct competition between inorganic and organic photorefractive materials. As is the case with many technologies where organic devices try to compete against established inorganic devices (e.g. photovoltaic), organic photorefractive materials face a number of challenges before they reach the success of their inorganic counterparts.

Biomedical imaging is perhaps the most exciting application of organic photorefractive materials due both to its scale and usefulness. High quality images have been produced using near infrared light sources as shown in figure &&(???). Parallel development in fMRI and other useful biomedical technologies will likely speed development in photorefractive materials as the technology becomes more useful.

Theoretical Description

Photorefractive effect

      i.Pockels Effect : change of refractive index of material due to an applied electric field 
      ii.Photoconductivity: light of adequate wavelength is able to produce electric charge carriers that are free to move by diffusion
  b.Photorefractive effect: combination of Pockel’s effect and photoconductivity
      i.Photoactgive centers: localized states in the forbidden band gap 
      ii.Light excites charge carriers at photoactive centers from the forbidden band gap to conduction or valance bands, the carriers are retrapped and excited again. 
      iii.Charge carriers accumulate in the darker regions of the sample – charges of one sign accumulate in the darker regions, opposite sign – brighter regions.
      iv.Essentially light produces photoconduction-based electric field spatial modulation that produces an index of refraction modulation by the electro-optic effect.  
  c.Reversal: different light wavelength, or relaxation in the dark 

1.Refraction: change of direction of a wave due to a change in its speed.

  a.Alteration of phase velocity causes a change in direction
  b.Change of wavelength, but constant frequency 

Classifications

liquid-crystalcontaining photorefractives

or - Organic crystalline materials - Polymer dispersed Liquid crystalline materials (PDLC) - Organic amorphous materials  

References

A. Ashkin, G.D. Boyd, J.M. Dziedzic, R.G. Smith, A.A. Ballman, J.J. Levinstein, and K. Nassau, “Optically-induced refractive index homogeneities in LiNbO3 and LiTaO3,” Appl. Phys. Lett. 9 [1] 72-4 (1966).

J. Frejlich, Photorefractive Materials, Fundamental Concepts, Holographic Recording and Materials Characterization, 2007.

O. Kwon, M. Jazbinsek, S. Kwon, and P. Günter, “Organic Photorefractive Materials Based on Mesophase Photoconductive Polymers,” The Optical Society (2007).

O. Ostroverkhova, W.E. Moerner, Z. Chen, M. Asaro, M. Sheldon, M. He, and R.J. Twieg, “Recent Advances in Photorefractive Organic Materials,” Photorefractive Eff., Mater., and Dev. (2005).

P.M. Lundquist, R. Wortmann, C. Geletneky, R.J. Twieg, M. Jurich, V.Y. Lee, C.R. Moylan, and D.M. Burland, “Organic Glasses: A New Class of Photorefractive Materials,” Science 274 [5290] 1182-5 (1996).

S. Ducharme, J.C. Scott, R.J. Twieg, and W.E. Moerner, “Observation of the Photorefractive Effect in a Polymer,” Phys Rev. Lett. 66 [14] 1846-9 (1991).

S. Köber, M. Salvador, and K. Meerholz, “Organic Photorefractive Materials and Applications,” Adv. Mater. (2011).

W.E. Moerner and S.M. Silence, “Polymeric Photorefractive Materials,” Chem. Rev. 94 127-55 (1994).

Link to original sandbox: http://en.wikipedia.org/wiki/User:Sketchc89/sandbox

Peer Review

1. I suggest you text “Organic photorefractive materials” in bold in the first sentence of “Organic photorefractive materials are materials that exhibit a temporary change in refractive index when exposed to light.” so that the audience could know your topic. However, it’s pretty good that you have linked all the related sites.

2. The general public summary is easily to read, but some phases are confusing. Do you want to talk about organic photorefractive materials or photorefractive materials? Make it clear when you’re trying to narrow down the topic from photorefractive materials to organic photorefractive materials.

3. I didn’t see the Background and Significance, I assume History is the one I’ m looking for. I think it’s very good and clear, but some of the sentences are repeating. Also, it’s too detailed when you’re trying to explain the concepts such as “photorefractive effect,” “refraction ” as you have linked them.

4. I suggest you create content (outline) in a box so it’s more straightforward for the audience. The outline you have is so confusing. I think you might want to modify the outline a little bit.

a. Create a section called “classifications” with subsections of different types of organic PR material.

b. Add a section called “Theoretical descriptions” with subsections of “Photorefractive effect”, and “mechanism.”

c. Add a section called “experimental techniques” with subsections of “standard 2BC method and “FWM methods”
d. Add a section called " material design"

5. Please add citation numbers at the end of the sentences.

6. Lack of figures. I guess you may want to add some images of the organic photorefractive materials or some schemes to explain the mechanism.

7. It's good that all the reviews are referenced since 2008. However, there is a very good reference you might want to cite although it was published in 2004:

O. Ostroverkhova , W. E. Moerner , “Organic Photorefractives: Mechanisms, Materials, and Applications.”Chem. Rev. 2004 , 104 ,3267–3314.

8. I also made some comments in brackets.
9. You may want to modify the History into the Theoretical Description.