In this thesis, I describe several studies based on organic nonlinear optical crystals for highly efficient THz photonics. In order to help the better understanding for the THz photonics utilizing organic single crystals, the several explanations, including definition, application and generation/ detection method of THz wave are presented. In addition, conventional requirements for nonlinear optical molecular structures and several examples of crystal engineering technics for non-centrosymmetric crystal packing with high nonlinear optical response and THz wave generation are also denoted. We have investigated the new ionic organic π-conjugated crystals with various anion for highly efficient THz wave generation. Moreover, we have developed various crystal engineering techniques utilizing different solvents, additives, tailor-made auxiliaries and substrates to achieve highly favorable material properties and improved optical performances including THz wave generation.
Using a strong benzothiazolium electron-acceptor group, we have successfully investigated new HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) crystals showing large macroscopic optical nonlinearity and excellent crystal characteristic for THz wave generation, which lead to very intense and broadband THz wave generation efficiency (one order of magnitude higher than standard inorganic ZnTe crystal) by optical rectification (OR).
We have demonstrated the growth of organic ionic nonlinear optical crystals with large area and suitable thickness for highly efficient THz generation, in which nucleation is induced by local supersaturation on a template and crystals grow following the surface of the template based on strong hydrogen bonds. We have developed the new morphological crystal engineering technique introducing in-situ tailor made additives for organic phenolic nonlinear optical crystals. The morphology of phenolic crystals grown with in situ tailor-made additives are changed to be more optimal for photonic applications, which allows the two optical configuration within a one single crystal towards THz photonics. We have observed favorable crystal growth characteristics including large area and desired thickness, optimal for THz wave photonics, by new crystal engineering technique using confinement geometry. By resulting crystal, the correlation between optical-to-THz conversion characteristics and crystal characteristics are successfully investigated, where grown crystal exhibits higher THz amplitudes compared to crystals grown by typical solution growth method. We have examined the origin of solubility behavior of polar π-conjugated crystals in mixed solvent system of polar non-aromatic and non-polar aromatic solvents are experimentally provided. The aromatic characteristics of solvents lead to the variation of crystal characteristics including morphology and polymorphism of single crystals.
