Crystal Growth And Properties Of Novel Imidazole-based Organic Crystals For Quadratic Optical Nonlinearities

Since the first second harmonic generation (SHG) phenomenon was discovered in1961on quartz (α-SiO2) by Franken, nonlinear optical (NLO)) materials have successfully become the material base of optical information processing such as modulation, storage and amplitude limiting due to its advantages in recording, storing last transmitting and processing high capacity information. Organic nonlinear optical crystals have the potential applications in telecommunications, optical data storage, medical diagnosis, optical information processing and optical computing. Especially in recent years. THz technologies play important roles in industry, biology, military affairs and scientific research, encouraging people to seek second-order nonlinear optical crystals with small absorption coefficient in THz band to obtain THz wave generation. Therefore, searching and synthesizing new type of second-order nonlinear optical crystals with excellent properties is still a focus for many scientists.So far, reports about second-order nonlinear optical materials such as DAST. organic salts and metal-organic complex are endless and there are some crystals with excellent properties among them. DAST is one kind of the most important materials in generating and detecting THz radiation for the reason that it has large optical nonlinear and electrooptic coefficients, low dielectric dispersion, large coherence length and ultra fast response time. However, the instability in growing process of DAST makes it difficult to obtain large size and high quality crystals. Therefore, designing and synthesizing crystals with large size, high quality and large second-order nonlinear optical coefficients is very important. At present, second-order nonlinear optical materials with imidozalium as electron drawing group are rarely reported. In this thesis a new cation core based on vinylimidazole as the electron-acceptor in the π-conjugated chromophores has been designed. A series of new second-order nonlinear optical crystals have been designed, synthesized and obtained through balancing the new cation core with different anions considering the important influence of the structures and sizes of anions on crystal packing. In addition,other neutral compounds were designed through changing electron-donor and acceptor. The growth technology, crystal structure and the relationship between physical and second-order nonlinear optical properties of crystals with high nonlinear optical effects of DITMS were discussed in detail. The main contents are as follows:1. A series of imidozalium salts with the same cation and neutral compounds with analogous structures were synthesized through Heck, Knoevenagel and ion exchange reactions based on trans-1-[p-(N,N-dimethylamino)styryl]-3-methylimidozalium and5-methyl-4-nitroimidazole. UV-visible absorption spectra and TGA/DSC curve of the compounds were measured and analyzed in detail. The absorption peak of the imidozalium salts is located at350nm. DSC shows that these compounds have excellent thermostability with high decomposition temperature. Moreover, The large differences between melting temperature and decomposition temperature of some crystals will offer a potential for the melt-based crystal growth.2. The single crystals of organic salts suitable for X-ray diffraction analysis were obtained by slow evaporation of the solvents at room temperature. Among the thirteen crystals, there are three crystals with acentric structure:trans-1-[p-(N,N-dimethylamino) styryl]-3-methylimidozalium mesitylenesulfonate (DITMS) belongs to Pn space group, trans-1-[p-(N,N-dimethylamino)styryl]-3-methylimidozalium1-naphthalenesulfonate (DINS-1(11)) belongs to Pna21space group and trans-5-[(4-methoxy)styry1]-4-nitroimidazole (DAN) belongs to C2221space group. Anions with different structures and dimensions can cause the same cation to twist in varying degrees, thus leading to different polarities and symmetries of crystals. With similar first-order hyperpolarizabilities (β), the differences in molecular arrangements become the main factor to affect the second-order nonlinear optical properties of crystals.3. Growth technology of DITMS crystal was explored. We chose three techniques to grow DITMS crystal, they are solvent evaporation method, vapor diffusion method and slow cooling technique, respectively. Bulk single crystals of dimensions in millimeters were obtained, some of the crystals have high optical quality.4. Second-order nonlinear optical effects of DITMS were studied preliminarily. Optical transmission spectrum of the crystal was measured and it indicates that DITMS crystal has excellent transparency in the range of665nm-1554nm. Kurtz and Perry powder test was carried out to estimate the macroscopic nonlinearity of DTTMS crystal. SHG test reveals that the SHG response of DITMS is about0.1times of DAST at2100nm and the DTTMS crvstal is phase-matchable.