Electronic origin, nonlinear optical structures

However, its was found possible to infer all four microscopic tensor coefficients from macroscopic crystalline values and this impossibility could be related to the molecular unit anisotropy. It can be shown that the molecular unit anisotropy imposes structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from crystal symmetry. Such additional relations appear for crystal point group 2,ra and 3. For the monoclinic point group 2, this relation has been tested in the case of MAP crystals, and excellent agreement has been found, triten taking into account crystal structure data (24), and nonlinear optical measurements on single crystal (19). This approach has been extended to the electrooptic tensor (4) and should lead to similar relations, trtten the electrooptic effect is primarily of electronic origin. 

We have shown the molecular orbital theory origin of structure - function relationships for electronic hyperpolarizability. Yet, much of the common language of nonlinear optics is phrased in terms of anharmonic oscillators. How are the molecular orbital and oscillator models reconciled with one another The potential energy function of a spring maps the distortion energy as a function of its displacement. A connection can indeed be drawn between the molecular orbitals of a molecule and its corresponding effective oscillator . 

ELECTRONIC ORIGIN OF SECOND ORDER NONLINEAR OPTICAL RESPONSES IN CONJUGATED LINEAR CHAIN STRUCTURES AND HIGHLY CHARGE CORRELATED tt-ELECTRON STATES... 

So, optimizing the nonlinearity of a material begins at the molecular structural level, which requires a detailed understanding of the origin of an induced nonlinear polarization and its relationship to the molecular electronic structure. For this reason, much attention has been paid to the theoretical calculations of the nonlinear optical response by ab initio and semi-empirical methods, providing the chemist with the information that indicates which synthetic strategy should be followed [16-24],... 

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