고분자 복합체에서 내부 공간 전하장 형성이 광굴절 현상에 미치는 영향에 관한 연구

Abstract

Space charge field formation (Esc) is the crucial process for photorefractivity, since both the index contrast and the response time of photorefractive (PR) grating are closely related with the strength of Esc. The PR effect refers to spatial modulation of refractive index via space charge field formation and electro-optic (EO) nonlinearity, which are strongly influenced by the components of polymeric composites such as sensitizer, photoconducting polymer, and non-linear optic (NLO) chromophore. In particular, the spatially varying charge distribution responsible for Esc formation depends on photoconductivity, including the dynamics of charge generation, transport, trapping, and detrapping in the materials. Therefore, in this thesis, we investigate the influence of key parameters for PR performance on the space charge field formation and discuss the effect of space charge field on the dynamic and steady-state photorefractivity in polymeric composite.

In Part I, we reported the influence of sensitizers and photoconducting polymers on the photorefractivity and synthesized a new photoconducting polymers containing covalently attached Ru complex as a photosensitizer. The strength of space charge field was strongly dependent of the photo-charge generation efficiency of the charge transfer complex formed between sensitizer and photoconducting polymer. The refractive index modulation was enhanced as the space charge field increased and the PR grating buildup rate sped up with increasing the photoconductivity. New photoconducting polymers showed the high photoconductive properties through the presence of their lowest MLCT energy.

In Part II, we reported the effect of chromophore content on the space charge formation. The chromophore is expected to act as a trap for hole transport in the matrix when the chromophore has lower ionization potential as compared with the hole transporting material. The charge transport dynamics influenced by the presence of the trap molecules control the formation of Esc via charge trapping, detrapping, and recombination. All experimental data are in excellent agreement with the predictions calculated from our modified Schildkraut differential equation.

In Part III, we reported the temperature dependence of the grating formation. For polymeric PR materials, the temperature is one of the most important factors together with the external electric field, because it is closely related on charge generation, transport, EO effect, and so on. Above glass transition temperature, the space charge field decreased linearly with increasing temperature due to a decrease in the photo-charge generation efficiency and an increase in the hole detrapping by the dark conductivity. A decrease in diffraction efficiency was explained with the decrement of the strength of space charge field and birefringence.