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Organic photorefractive materials are materials that exhibit a temporary change in refractive index when exposed to light. The photorefractive effect was first observed by Bell Labs 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 Changing the refractive index of a material changes the speed at which light travels through that material. Photorefractive materials are able to change their refractive index due to a combination of photoconductivity, the production of an electric field due to illumination, and the Pockels effect, the change in refractive index due to an electric field. Electrons and holes in photorefractive materials are excited by light, diffuse through the material, and recombine leading to variations in the refractive index of the material. 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. Tthe effect was subsequently observed in other kinds of materials, in particular – organic, in 1991 by IBM, and, as of today, photorefractive materials can be classified into following categories:

- 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.

Theory

(still in progress, will do formatting and Pockels effect later)

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 = NcExp[-(Ec - EF) / kB T] and h = NVExp[-(EF – EV) / kB T]

where Nc and NV are the densities of states at the bottom and the top of the conduction and the valence band, respectively, E¬c and EV are the corresponding energies, EF is the Fermi level energy, kB 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. [insert pic 1]

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 Efn and, for holes – E¬fp. The densities n and h are, then equal to

n = NcExp[-(Ec - EFn) / kB T] and n = NVExp[-(EFp – EV) / kB T]

[insert pic 2]

The localized states between Efm and Efp 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 Efm¬ – Efp energy, however, relax their charge carriers to the nearest extended states.

A general quantification of the effect of incident light on the conductivity of the material is beyond the scope of this article since it depends not only on the energy of light but on the type of material itself. 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 the equation

σd = e(ND – ND+)∙βμτ

where σd is the conductivity, e = electron charge, ND and ND+ 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 σph = e(ND – ND+)∙sI/(hν)∙μτ

in which s is the effective cross-section for photoelectron generation, h is the Planck’s 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.

OUTLINES:

I. Photorefractive effect: 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 

2.Photorefractive materials:

  a.Electro-optic and photoconductive materials:
      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 

  II. Organic Photorefractive Materials 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 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 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 and will be discussed in some details. 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 were based on hole-conducting polymeric materials

Applications

Reversible holographic applications

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.

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.

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.

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

Drawbacks

As of 2011, no commercial products utilizing organic photorefractive materials exist. Large DC fields required to produce holograms lead to dielectric breakdown not suitable outside the laboratory.

References

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. 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).