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1、 Molecular Design for Third-Order Nonlinear OpticsUlrich Gubler, Christian BosshardNonlinear Optics Laboratory, Institute of Quantum Electronics,-ETH Hnggerberg, CH-8093 Zrich, Switzerland present adress: Centre Suisse dElectronique et de Microtechnique (CSEM), Untere Grndlistrasse 1, CH-6055 Alpnac
2、h, Switzerlande-mail: ulrich.gublercsem.ch, christian.bossardcsem.chWe discuss recent developments in the design of molecules for applications in third-order nonlinear optics with emphasis on all-optical signal processing and two-photon absorp- tion. We especially concentrate on functional substitut
3、ion patterns and conjugation length expansion. We show that low molecular symmetry with regard to the conjugation path of the delocalized electrons was found to be a good guideline towards linearly conjugated mol- ecules with large second-order hyperpolarizabilities . We show that this guideline is
4、also valid for two-dimensionally conjugated systems and that the observed effects can be ex- plained by the symmetry of the electronic wavefunctions. We present scaling laws and crit- ical conjugation lengths of rod-like molecules with electrons delocalized over a one-dimen- sional path and show tha
5、t the exponent tends to be constant for various polymers in the transparency range and that the values presented here are of similar magnitude for various organic materials systems. Finally, we discuss different materials systems with regard to thefigures of merit relevant for all-optical signal pro
6、cessing.Keywords.Third-order nonlinear optics, Second-order hyperpolarizability, Molecular de- sign1Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1252Third-Order Nonlinear Optics. . . . . . . . . . . . . . . . . . . . . 1262.1Definitions and Conventions . . . . . . . .
7、. . . . . . . . . . . . . . . 126 2.1.1Definition of the Nonlinear Optical Susceptibilities and Molecular Hyperpolarizabilities . . . . . . . . . . . . . . . . . . . . . 126 2.1.2Electric Field Definition and Prefactors of Third-Order Effects . . . 128 2.1.3Permutation and Kleinman Symmetry . . . .
8、. . . . . . . . . . . . . 129 2.1.4Nonlinear Refractive Index and Two-Photon Absorption . . . . . . 130 2.2Relations Between Microscopic and Macroscopic Coefficients . . . 132 2.3Reference Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 2.4Material Requirements for All-Optical
9、 Signal Processing . . . . . . 136 2.4.1Figures of Merit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 2.4.2Nonlinearity and Intensity Requirements . . . . . . . . . . . . . . . 139 2.4.3Further Critical Factors . . . . . . . . . . . . . . . . . . . . . . . . . . 140Advances in Pol
10、ymer Science, Vol. 158 Springer-Verlag Berlin Heidelberg 2002124U. Gubler, C. Bosshard3Measurement Techniques. . . . . . . . . . . . . . . . . . . . . . . 1413.1Third-Harmonic Generation . . . . . . . . . . . . . . . . . . . . . . 142 3.2Degenerate Four-Wave Mixing . . . . . . . . . . . . . . . . .
11、. . . . 145 3.3Z-Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 3.4Two-Photon Absorption Spectroscopy . . . . . . . . . . . . . . . . 151 3.5Fluorescence Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 1544Discussion of Molecular Properties. . . . . . . . .
12、 . . . . . . . . . 1554.1Basic Molecular Units . . . . . . . . . . . . . . . . . . . . . . . . . . 155 4.1.1Electronic States and Symmetry Considerations . . . . . . . . . . 159 4.1.2Bond-Length Alternation . . . . . . . . . . . . . . . . . . . . . . . . 163 4.2Influence of Molecular Substitution .
13、. . . . . . . . . . . . . . . . . 164 4.31-D vs 2-D Conjugation . . . . . . . . . . . . . . . . . . . . . . . . . 168 4.4Conjugation Length Expansion . . . . . . . . . . . . . . . . . . . . 173 4.5Two-Photon Absorption . . . . . . . . . . . . . . . . . . . . . . . . 1775Concluding Remarks. . . . . .
14、 . . . . . . . . . . . . . . . . . . . . 182References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187List of Symbolsalinear absorption coefficienta2two-photon absorption coefficienta3three-photon absorption coefficientaijmolecular polarizabilitybijkmolecular first-order hyperpolarizability (or molecular second-order polarizability)Ew, E3wFourier components of the electric field strengthdtwo-photon cross-sectionemolar extinction coefficientflocal fiel
