Development of Easily Phase-Matchable Wavelength-Conversion Devices Due to Fabrication of Periodically Polarization-Inverted Structures Using Organic Nonlinear-Optical Crystals.

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The research mainly consists of two parts. One is to exploit nonlinear-optical organic materials transparent enough in the visible region and the other is to realize quasi-phase-matching (QPM) in second-harmonic generation (SHG) for organic crystals which possess higher figures of merit due to their constituent molecules oriented parallel to each other.In the former part, we could overcome the trade-off between the cutoff wavelength or the maximum-absorption wavelength and the figures of merit in the previous organic materials. Using the hyper Rayleigh scattering method, we studied aromatic ions with a viewpoint of their ionic polarizabilities in addition to the internal charge-transfer. We compared the ionic species with the corresponding neutral derivatives and found that the former molecules have the 1st hyperpolarizabilities several times larger than the latter ones for given maximum-absorption wavelengths. We expect that those molecules will be effectively used in crystals and/or polymers.In the latter part, we could successfully achieve the QPM guided-wave SHG using the organic 2-methyl-4-nitroaniline (MNA). Since the molecules of MNA are oriented nearly parallel to each other and aligned in the crystallographic X-axis, the largest coefficient d_<11> cannot be phase-matched by birefringence and cannot be effectively used by the Cerenkov-radiation type phase-matching. We proposed to use QPM for the organic crystals mentioned above. Generally speaking, however, the coherence lengths are too short to produce periodic structures for QPM.In the research, we sdopted planar waveguides, in which the coherence lengths can be elongated. We could grow ridge-shaped single crystals on glass substrates with a period of 5 micrometer. As a result, we could realize the QPM SHG by angle tuning to the coherence legth.