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All optical poling technique allows orientation of dye molecules in a polymer matrix by purely optical means. The coherent superposition of two beams at fundamental (FUND)and second harmonic (SH)frequencies results in the presence of the polar field E(t) inside the material, able to break the centrosymmetry of the medium. The temporal average cube 3>t of this field E(t) = Eω(t)+E2ω(t), is non-zero, which leads to an orientation-selective excitation of molecules and the second order χ(2)-susceptibility grating is encoded, with a period satisfying the phase matching condition for SH generation. The physical origin of the effect lies in the orientational hole-burning in the initially isotropic distribution of dye molecules (via trans-cis trans photoisomerisation). We investigate other phenomena responsible for the SHG in polymer films like the electron transfer between a polymer matrix (donor of electrons) and a dye molecule (electron acceptor) and processes which accompany polar orientation mechanism like the one-direction photoinduced molecular migration. The stability of the induced polar order after orientation has been also studied and the crosslinkable polymer system has been developed D side chain molecules with ends can thermally react with epoxy groups were randomly inserted in the side groups of the polymer backbone, which leads to the reticulation and enhancement of the rotational stability. After poling the second order nonlinear response exhibits a much longer relaxation time. In the set-up configuration used we monitor non-perturbatively the all optical poling and there is no necessity of taking into account the phase difference between writing beams (fundamental and SH). The periodical modulation of the relative phase provides the desired control on the polar order at the molecular level as well as the possibility of encoding surface relief gratings with a period equal to the half of the period of the χ(2) grating.
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