There have been many exciting results since the first demonstration of the Photorefractive Effect (PRE) in a polymer in 1990. (1) Photorefracive polymers have exhibited low-beam energy coupling gain coefficients exceeding 200 cm^-1 and diffraction efficiency approaching 100%. (2,3)
A photorefractive polymer contains molecular components which serve four functions (see the Lego Approachª below): 1) Structural polymer "glue" to hold everything together; 2) Electro-optic (nonlinear optical) chromophores; 3) Photosensitizers, photosensitive dyes which must also serve charge traps; 4) Charge (usually holes) transport molecules which shuttle the photoionized charges among the traps.
Moerner and Silence have written a comprehensive review of photorefractive polymer research current to mid 1993. (4) Examples of organic photorefractive materials now include several kinds of polymers, (4) organic crystals, (5) organic glasses, (6) and liquid crystals. (7) Organic materials have large electro-optic response yet a small dielectric constant, leading to an inherent performance advantage over inorganic materials where large electro-optic response is always accompanied by a large dielectric constant. However, organic materials have significantly lower carrier mobility than most inorganic materials.
The grating formation speed is proportional to the photoconductivity and hence to the absorption coefficient, the quantum efficiency, the mobility, and the free carrier lifetime. Of these four quantities, increases in the quantum efficiency and mobility will have the most beneficial effects; (8) increased absorption means increased attenuation (9) and increased free carrier lifetime means decreased gain. The usual band conduction analysis developed for crystals has proven effective in describing polymers provided the electric-field dependence of photoconductivity is included. (10) This follows from experience with organic Xerographic photoreceptors (11) and influences the direction of our research.
Photorefractive polymers are readily altered to conform to complex device requirements imposed by the environment, size, shape, physical flexibility, reliability, and other constraints. polymers are amenable to mass production in a variety of forms and shapes, having a particular advantage over crystals in the production of thin films (by spin-coating, drawing, melt-forming, solvent casting, etc.) that can be patterned into waveguides for integrated optic-electronic systems. Given comparable performance, polymers should prevail due to their lower cost and ease of fabrication.
Sample Dripping |
Assembled Sample |
 |
 |
Sample Unpoled |
Sample Poled |
 |
 |
Beam amplification, or two-beam coupling is the transfer of optical power from one beam to another mediated by the nonlinear photorefractive grating. Two-beam coupling gain coefficient measurements determine the strength of the PRE and are essential to establish the presence of the PRE, to evaluate materials for potential application, and to explore models and phenomenology of photorefractive charge dynamics.
= Two-Beam Coupling
Gain CoefficientPhotorefractive polymers have very high Two-Beam Coupling Gain Coefficients, the highest (2,3) over 200/cm. The Gain Coefficient depends strongly on the applied electric field and is therefore easily controlled.
The Photorefractive Effect (PRE) is a persistent but reversible change in the refractive index of an electro-optic material caused by non-uniform illumination as first reported nearly thirty years ago. Kukhtarev et al. (12) developed a complete band conduction model of photorefraction in semiconducting materials taking into account charge transport, static electric fields, electro-optic response, and light diffraction, all in a self-consistent manner. The PRE is reviewed in several good books (13, 14, 15, and my favorite 18), and an outstanding Scientific American article. (17) .
The PRE depends on the photoconductivity, the number and type(s) of charge traps, and the electro-optic (Pockels) response in the material. Typical photorefractive experiments involve holographic recording, the production of nonuniform illumination by interference of mutually coherent beams. The two most important photorefractive performance characteristics are: 1) the speed of grating formation, which is proportional to the photoconductivity of the material and 2) the strength of two-beam coupling or of gratng diffraction, which increase with the electro-optic response.
(Hi
Marko)
Proposed applications of the PRE include optical image processing, high density optical data storage, optical computing, communications, image processing, neutral networks, associative memories, phase conjugation, laser resonators, and many others. Image processing applications include image correlation, image amplification, and dynamic novelty filtering. Data can be stored in photorefractive "data cubes" in the form of the three-dimensional phase holograms which have very high density and fast parallel optical access (16). The PRE has been used to correct image distortions suffered by optical beams in inhomogeneous or turbulent media. Photorefractive crystals have been used to demonstrate many of the proposed applications and to study the physics of the photorefractive effect. However, there are currently no widespread commercial applications of photorefractive phenomena, partly due to the high cost of quality crystals and partly because of performance limitations, particularly the low hologram formation speed.
| Intensity |  |
 |
| Charge |  |
 |
| Field |  |
 |
 Spatial Frequency |
 Modulation depth |
1. S. Ducharme, R. W. Twieg, J.C. Scott, and W. E. Moerner, Phys. Rev. Lett. 66, 1846 (1991).
2. M. Liphardt, A. Goonsekera, B. E. Jones, S. Ducharme, J. M. Takacs, and L. Zhang, Science 263, 367 (1994).
3. K. Meerholz, B. L. Volodin, Sandalphon, B. Kippelen, and N. Peyghambarian, Nature 371, 497 (1994).
4. W. E. Moerner and S. Silence, Chem. Rev. 94, 127 (1994).
5. K. Sutter, J. Hulliger, R. Schlesser, and P. Günter, Opt. Lett. 18, 778 (1993).
6. P. M. Lundquist, R. Wortmann, C. Geletneky, R. J. Twieg, M. Jurich, V. Y. Lee, C. R. Moylan, and D. M. Burland, Science 275, 1182 (1996).
7. I.C. Khoo, H. Li, and Y. Liang, Opt. Lett. 19, 1723 (1994).
8. A. Goonsekera, S. Ducharme, J. M. Takacs, and L. Zhang, in Proceedings of the Organic Photorefractive Materials and Xerographic Photoreceptors, 7-8 August 1996, S. Ducharme and J. M. Stasiak, eds. (SPIE, Denver, 1996), pp. 41.
9. M. Liphardt, A. Goonsekera, S. Ducharme, J. M. Takacs, and L. Zhang, J. Opt. Soc. Am. B 13, 2252 (1996).
10. S. Ducharme in Proceedings of the Xerographic Photoreceptors and Photorefractive Polymers, 10-11 July 1995, S. Ducharme and P. M. Borsenberger, eds. (SPIE, Bellingham, WA, 1995), pp. 144.
11. P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for Imaging Systems (Dekker, New York, 1993), 447 pages.
12. N. V. Kukhtarev, V. B. Markov, S. G. Odulov, M. S. Soskin, and V. L. Vinetskii, Ferro. 22, 949 (1979).
13. P. Günter and J. P. Huignard, eds., Photorefracitve Materials and Their Applications I: Fundamental Phenomena, V. 61 (Springer Verlag, Berlin, 1988), 295 pages.
14. P. Yeh, Intorduction to Photorefractive Nonlinear Optics (Wiley, New York, 1993), 410 pages.
15. B. I. Sturman and V. M. Fridkin, The Photovoltaic and Photorefractive Effects in Noncentrosymmetric Materials (Gordon and Breach, Philadelphia, 1992), 238 pages.
16. J. F. Heanue, M. C. Bashaw, and L. Hesselink, Science 265, 749 (1994).
17. D. M. Pepper, J. Feinberg, and N. V. Kukhtarev, "The Photorefractive Effect," in Scientific American, October 1990, p. 62.
18. L. Solymar, D. J. Webb, and A. Grunnet-Jepsen, The Physics and Applications of Photorefractive Materials (Clarendon Press, Oxford, 1996).