Martti Kauranen, Thierry Verbiest, Carlo Boutton, Stephan Houbrechts, Andre Persoons, and Celest Samyn, University of Leuven, Heverlee, Belgium
Only noncentrosymmetric molecules can possess a second-order nonlinear response, i.e., they have a nonvanishing first molecular hyperpolarizability. Polar molecules with donor and acceptor groups connected by a conjugated π-electron system are traditional organic second-order materials (Fig. 1). For macroscopic noncentrosymmetry, such molecules are poled in a host material using a static electric field. The nonlinear coefficients of poled materials are proportional to μβ where μ is the permanent dipole moment of the molecules and β is the vectorial part of the first hyperpolarizability.
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