(1) A beam of light with a specific frequency ω (or wavelength λ) is incident on the material. (2) The light is absorbed by an acceptor (NA), and a photoexcited electron is separated from a hole, creating a free hole and an ionized acceptor. (3) The hole moves under electric field E0 along the transport sites with mobility μ. (4) The hole can be trapped with a rate γT at a trapping site (T) and then thermally released (detrapped) with a rate β. More than one kind of a trap can be present in the material. (5) Finally, a hole can recombine with another ionized acceptor with a rate γ.O. Ostroverkhova, "Organic and Polymeric Photorefractive Materials and Devices", in "Introduction to Organic Electronic and Optoelectronic Materials and Devices" (Eds. S. Sun and L. Dalton, CRC Press, 2008) .

For more information regarding modeling of the space-charge field formation in photorefractive materials, see References O. Ostroverkhova and K. D. Singer, J.Appl. Phys. 92, 1727 (2002), L. Kulikovski et al., Phys. Rev. B 69, 125216 (2004), J. Schildkraut and A. Buettner, J. Appl. Phys. 72, 1888 (1992), J. Schildkraut and Y. Cui, J. Appl. Phys. 72, 5055 (1992), C. Lee et al., Chem. Phys. Lett. 422, 106 (2006).

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