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

本研究主要由两部分组成。一方面是开发在可见光区足够透明的非线性光学有机材料，另一方面是实现有机晶体的准相位匹配，这类有机晶体由于其组成分子相互平行取向而具有较高的品质因数。我们可以克服截止波长或最大吸收波长与先前有机材料中的品质因数之间的折衷。利用超瑞利散射方法，从离子极化率和内部电荷转移的角度研究了芳香族离子。我们比较了离子物种与相应的中性衍生物，发现前者分子的第一超极化率比后者的几倍，对于给定的最大吸收波长。在后一部分中，我们利用有机2-甲基-4-硝基苯胺（MNA）成功实现了QPM导波倍频。由于MNA的分子几乎彼此平行地取向并且在晶体学X轴上对齐，因此最大系数d_<11>不能通过双折射进行相位匹配，并且不能被切伦科夫辐射型相位匹配有效地使用。我们建议对上述有机晶体采用准相位调制。然而，一般来说，相干长度太短，不能产生QPM的周期性结构。我们可以在玻璃衬底上生长周期为5微米的脊形单晶。结果表明，通过角度调谐到相干长度，可以实现准相位匹配倍频。

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.