| Organic nonlinear optical (NLO) crystals are important function material. Due to their big nonlinear coefficients, high laser damage threshold values, fast response rates, lower relative permittivity, good processability and designable etc. Materials of this kind show enormous potentiality in the applications of terahertz(THz) technology, modern laser technology, optical communication,data storage, optical information processing and so on. Among the organic NLO crystals, the most interesting crystal is organic stilbazolium-salt because it has very high second-order NLO and EO susceptibility and is ideal for THz generation and detection. Although there are many experimental studies about stilbazolium crystals recently, quantum chemical calculations have been rarely reported. To exploring the relationships between tile molecular stuctures and properties microscopic NLO origin. In this thesis, we used density functional theory based on the pseudo-potential plane wave basis set to investigate the linear and nonlinear optical properties of three series of organic stilbazolium-salt with strong SHG effect.And the relationship between the optical properties and the electronic structure. Moreover, the information from quantum chemical calculations may provide a valuable guideline for further design and synthesis of NLO materials.The results of organic stilbazolium-salts, DAST and DSTMS crystals with different anion indicated that they are semiconductors with direct band gap. Two compounds showed similar band structures and the top of the valence band and the bottom of the conductive band were mainly originated from N 2p states of dimethylamino and methylpyridine, respectively. For the linear optical properties, the birefringence index An of two compounds were very large (An> 0.5), and they exhibited good transmissions of light in the mid-and far-IR regions. Regarding to the second-order nonlinear optical characteristics, DAST and DSTMS crystals showed strong second harmonic generation (SHG) response and the corresponding SHG coefficients (d11) were about 150 pm/V. Generally speaking, the influence of different Anion on the optical properties was very small.Based on the studies of DAST, the configurations, electronic structure and optical properties of organic stilbazolium-salts MBST and MC-PTS with different donor substitution are also discussed. Our results show that they have similar band structure and density states. The linear optical properties of this series compound, including the average dielectric function, refractive index, and the reflectivity and adsorption spectrum in IR region are similar to DAST system, however they show relatively modest birefringence (△n=0.36-0.4). There are advantages over DAST on the aspect of infared band phase matching. For the nonlinear optical characters, the values of the static SHG coefficient are ranked in this order:DAST> MBST> MC-PTS, which correspond to the electron donor ability sequence of the three componds. The strongest is dimethylamino, followed by methoxy, hydroxy weakest. The second harmonic generation (SHG) increases along with the powerful ability to electron donor, which varies from 40 to 150 pm/V.In addition, the effect of conjugation bridge on the optical property of the system are further studied. The FIBTEQ and APPTS crystals which contain different conjugation bridges, are also discussed. The results exhibit that the SHG effect of FIBTEQ is stronger than that of APPTS, which indicate conjugation bridge has obvious effects on the second-order nonlinear optical organic materials. Overall, the larger molecules conjugated system, the higher degree of electron delocalization molecule, and the second-order NLO response of the molecules will increase. |
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