The last step in the photorefractive grating formation in organic materials is a change in the refractive index under total electric field E, which is a vectorial sum of the applied electric field E0 (or, more precisely, its projection on the grating wave vector)O. Ostroverkhova and W. E. Moerner, Chem. Rev. 104, 3267 (2004) and space-charge field Esc.

Orientational enhancement effectW. E. Moerner et al., J. Opt. Soc. Am. 11, 320 (1994) is schematically illustrated on the right. In the absence of applied electric field, polar molecules (such as nonlinear optical chromophores, a part of any organic photorefractive material) are randomly oriented. When the electric field E0 is applied, the molecules align. When the space-charge field is formed, the molecules reorient in this field.

where ΔnBR (nEO) is the change in the refractive index due to electric field-induced birefringence (electro-optic effect), Δ&chi(1) (Δ&chi(2)) is a change in first- (second-) order susceptibility, N is the concentration of the polar molecules (dipoles), Δα is the polarizability anisotropy, β is the hyperpolarizability, μg is the ground-state dipole moment, n is the refractive index, and f0 and f are local field factors given by f0 = ε(n2+2)/(2ε+n2) and finf=(n2+2)/3, respectively.

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