Crystallization and phase separation are severe degradation mechanisms of photorefractive (PR) guest-host systems. Additionally, the response time of PR guest-host systems is in some cases limited by a large contribution of slow chromophore reorientation to the PR effect. The synthesis of fully functionalized systems with glass transition temperatures well above room temperature is a way to restrict these degradation mechanisms and to reduce the PR effect to a pure charge generation and transport phenomenon. We present novel, fully functionalized PR polymers based on polyester. Permanent poling of the systems is demonstrated and quantitatively investigated. The PR properties are measured by degenerate four-wave mixing and two-beam coupling experiments as a function of the external electric field and the degree of permanent poling. One of the systems shows response times of approximately 200 ms with ⌬n Ϸ 1 ϫ 10 Ϫ3. To our knowledge, this work presents the first characterization of PR efficiencies for fully functionalized polymers as a function of the permanent Pockels coefficient (2) (Ϫ;, 0).
Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcarbazol) (PVK) or polysiloxane (PSX) generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.1In an effort to improve the charge carrier transport in organic (PR) systems, we introduced monomeric 1,3,5-tris(diaryl amino) benzene (TDAB) and 3,3'-dimethyltriphenyldiamines (TPD) as the photoconducting moiety. These triphenylamine derivatives are known for their excellent hole transport properties with high charge transport mobilities. An azo and a stilbene type of nonlinear optical chromophore were used in combination with trinitrofluorenone (TNF) as the sensitizing agent.Two-beam coupling measurements yield a photorefractive gain coefficient of up to I' = 142 cm1 at an applied electric field of Eext 76 V/pm. Diffraction efficiencies ij ofabout 23 % have been achieved in degenerate four-wave mixing experiments with sample thicknesses of only 25 tim. The fastest response times observed were about 300 ms.Time-of-flight measurements showed mobilities of the order of 2 • 1O_6 cm2/Vs, which is at least a factor of 10 higher than in doped PVK-based systems. As a consequence, charge carrier transport is not the limiting factor for the temporal buildup of the photorefractive grating in our systems. The dynamic behavior of the grating buildup is discussed in detail for the system TDAB:DMNPAA:TNF. We compare our results with a theory of the transient behavior of linear and nonlinear optical processes in polymer films during poling.
We present novel, fully functionalized, photorefractive (PR) polyesters. The chemical design and the material's glass transition temperature well above room temperature allows for permanent poling of these systems. The photorefractive properties were determined via degenerate four-wave mixing (DFWM) and two-beam coupling experiments as a function of the applied external electric field and the degree of permanent poling. Due to the restricted orientational mobility of the chromophores, the PR effect is reduced to an almost pure charge-generation and transport phenomenon. Consequently, the temporal response of the systems is limited by charge-generation or transport processes only. Degradation of the materials is substantially limited because of the covalent attachment of the photoconducting as well as the nonlinear optical moieties to the polymer backbone. No crystallization or phase seperation was observed. The systems show response times of about 200 ms with Ln 1 . i03. To our knowledge, this work presents the first characterization of PR efficiencies for fully functionalized polymers as a function of the permanent Pockels coefficient x2(-w;w , 0).
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