| Nonlinear optical (NLO) responses, including the linear polarizability (α), first hyperpolarizability (β) and second hyperpolarizability (γ), of conjugated linear chains such as polyene and polythiophene are investigated at the ab initio levels. Both the finite field (F.F.) method and a recently proposed method based on the local atomic charge derivatives, are employed with the aim to identify the origins and the length dependence of NLO responses. The growth of α, β, and γ for regular polyenes shows saturation behavior at long chain-length. The local charge derivative picture gives a qualitative account for the saturation behavior, and identifies the terminal parts of the chain as the main contributor for the nonlinearity. Using the (hyper)polarizability of the regular polyene as a starting point, we have also investigated the effect of substitutions of donor and acceptor that are grafted onto the chain. We found that the effect of the substituted group is localized, and that the increment of (hyper)polarizability due to the substituted groups rapidly increases initially with increasing length, but the increase becomes slower and slower, and finally reaches a constant value. The variation of the (hyper)polarizability per unit cell with the chain length becomes the same as that for the unsubstituted one. The localized defects induced by addition or removal of electrons) into or out of the chain play an important role in modulating the NLO response. It was revealed that the different combinations of substituents and charged defects result in profoundly different NLO polarizabilities for short chain-length cases. However, the effects by the presence of substituents and charged solitons become additive in the long chain-length limit. Comparisons among the NLO polarizabilities for the different conformations of polyene demonstrate that both α and γ are sensitive to the molecular conformations, and their difference cannot simply be interpreted by the difference in molecular physical length. Calculations of doped systems show that NLO responses are sensitive to the distance between the dopant atom and the polyenic chain, and enhancement of the responses can be resulted by appropriately positioning the dopant atom. In this thesis, we also shed some light on the structure-property relation based the local view picture. |
