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 e − ( E c − E F ) k B T {\displaystyle n=N_{\text{c}}e^{\frac {-(E_{\text{c}}-E_{\text{F}})}{k_{\text{B}}T}}}
and
h = N v e − ( E c − E F ) k B T {\displaystyle h=N_{\text{v}}e^{\frac {-(E_{\text{c}}-E_{\text{F}})}{k_{\text{B}}T}}}
where Nc and Nv are the densities of states at the bottom of the conduction band and the top of the valence band, respectively, Ec and Ev are the corresponding energies, EF is the Fermi level, kB is the Boltzmann 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.
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, EFp. The densities n and h are, then equal to
n = N c e − ( E c − E Fn ) k B T {\displaystyle n=N_{\text{c}}e^{\frac {-(E_{\text{c}}-E_{\text{Fn}})}{k_{\text{B}}T}}}
h = N v e − ( E Fp − E v ) k B T {\displaystyle h=N_{\text{v}}e^{\frac {-(E_{\text{Fp}}-E_{\text{v}})}{k_{\text{B}}T}}}
The localized states between EFn 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 EFn − EFp 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
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