Search for Organic Nonlinear Optical Materisls and Estimation of Figure of Merit

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1. As compact and modified method of electric-field induced second-harmonic generation has been developed for measuring the secon-order hyperpolarizability of organic moleucules. The method can also remove the inevitable effect of interference to the prevoius method between the second-harmonic waves henerated at the cell windows and the second-harmonic wave to be mesaured. Theoretical expressions have newly been derived to desctibe the mactoscopic nonlinear polatixzations associated with this new configuration. It has been applied to organic molecules, p-NA and MNA, whose hyperploarizabilities were already known and applied to unkown MBA-NP, PNP and DAN. The results of measurement coincide well with the thoretical calcultaions using a P-P-P and a CNDO/S method.2. It has been concluded that the powder method for evaluating a figure of merit of second-order nonlinearity of crystals should not be utilized for the organic crystals, since the coherence lengths of such crystals with well-aligned molecules are too short. Only ceystals with molecules nearly parallel to each other could surpass d33 of LN, which was used for and efficient frequency doubler of a semiconductor laser in the form of a waveguide devices. The new of or improved method instead of the powder method is needed in order to seek easily useful organic materials.3. For flaky crystals, whose hyperpolarizabilities large enough, two procedures have been proposed to estimate htier figure of merit. One, for crystals with an optical grade, is to measure the refractive indices by means of a spectrometer or an ellipsometry, and the other, for crystals with an optically deteriorated grade, is to analyze and to know the orientation of molecules in the crystal by an X-ray method. Both procedures have been found to be comlimentary, from the results of measurement of MBA-NP and DAN.