Quantum Lattice Fluctuations and the Linear and Nonlinear Optical Susceptibility of Degenerate Ground State Conjugated Polymers
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概要
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The contributions of soliton-antisoliton (SS^^-) configurations to the linear and third-order nonlinear optical response are investigated for conjugated polymers having a degenerate ground state. We treat the direct photoproduction of charged solitons as a nonlinear Franck-Condon problem and obtain analytical expressions for the linear optical susceptibility, χ^<(1)>(ω) With the help of the oscillator strength sum rule, we decompose the π-π^* oscillator strength into two parts; a contribution where the final states are SS^^- pairs and a contribution where the final states are free electron-hole pairs (as in the noninteracting rigid-lattice). The linear optical coefficients calculated from χ^<(1)>(ω) are in general agreement with optical data obtained from trans-polyacetylene. The results imply that approximately 25% of the integrated oscillator strength of trans-polyacetylene arises from the direct photoproduction of solitons. A parallel treatment of the generalized third-order nonlinear optical susceptibility, χ^<(3)>(ω_σ) is presented, demonstrating that, for any third-order process, contributions arising from neutral SS^^- pair configurations as intermediate states are one to two orders of magnitude larger than the corresponding rigid-lattice contribution. This mechanism for χ^<(3)> is enabled by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and SS^^- excited state lattice wavefunctions. The large contribution to χ^<(3)> from the SS^^- intermediate states results from the large transition dipole moment between the free electron-hole pair excited states of B_u symmetry and the A_g symmetric neutral SS^^- excited state. This enhanced transition dipole moment is a consequence of the large virtual shifts of oscillator strength associated with the localized SS^^- electron-lattice configuration. The third-harmonic conversion effciency χ^<(3)>(3ω) is further enhanced by a condition unique to degenerate ground state systems, simultaneous two and three-photon resonance.
- 理論物理学刊行会の論文
- 1993-10-20
著者
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Heeger A.
Department Of Physics And Institute For Polymers And Organic Solids University Of California
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Hagler T.
Department Of Physics And Institute For Polymers And Organic Solids University Of California
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Heeger A.J.
Department of Physics and Institute for Polymers and Organic Solids University of California
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Hagler T.W.
Department of Physics and Institute for Polymers and Organic Solids University of California