| Summary: | 博士 === 國立清華大學 === 材料科學與工程學研究所 === 85 === A real-time poling method for preparing second-order nonlinear optical (NLO) polymer thin films is presented in this study. The main characteristics of the method is that the NLO-active molecule can be poled during its vapor deposition polymerization (VDP) process, without going through the traditional complicated synthesis and poling procedures.
The theoretical calculation regarding the feasibility of the real-time approach is first presented. The result of the calculation shows that poling can be accomplished even at a relatively low poling field.
Dye-doped NLO thin films were prepared by using the real-time poling approach. The temperature-dependent deposition rates of the NLO-active molecule and two polymer thin film forming monomers (dianhydride and diamine) were separately determined to acquire the optimal deposition conditions. The structure of the resultant films was characterized to be poly(amic acid) (PAA) by using Fourier transform infrared (FTIR) spectrometry. It had been converted into the polyimide (PI) counterpart after curing. Doping concentration of the NLO-active molecule in the films was determined. Degradation phenomenon of the resultant thin films was also discussed.
The prepared films were uniform and transparent when observed by naked eyes or by using an optical microscope, implying that these films were well polymerized and uniformly doped. Polarized optical microscope observation showed that there existed several different microstructures in the different films.
Both aluminum (Al) and gold (Au) electrodes can be used for the poling process. However, the Al electrodes used had suffered a severe electrode damage during the curing process. Whilst, the Au electrodes used did not encounter avoid such the same kind of damage and had achieved the curing purpose.
Second harmonic generation (SHG) was measured to investigate the poling effectiveness. The SHG coefficients increased with the increase of the poling field strength, indicating that the fluctuation of the NLO-active molecules had been better restrained in the presence of a stronger poling field. When the films were exposed to elevated temperatures, the SHG signals underwent a rapid decay initially and then gradually leveled off after a certain time period. Curing the specimens at a moderately high temperature had greatly improved the films integrity and likewise the NLO characteristics.
Mainchain NLO polyimide films were also prepared by using the real-time poling approach. This was done by co-depositing a dianhydride with a NLO-active diamine, while concurrently applying a poling field during the deposition process. The structure and curing condition of the resultant films were characterized using FTIR spectrometry. The film had a NLO coefficient of 1 pm/V. Its NLO effect remained stable up to 200℃.
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