干渉法による有機単結晶の電気光学係数評価(有機材料・一般)  [in Japanese] Evaluation of Electro-Optic Coefficients of Organic Single Crystal by Interferometry  [in Japanese]

電気光学効果を利用した光変調器などのデバイス実現を目指して有機非線形光学材料が探索されている。本研究では、材料の正確な評価を行うため、電気光学係数の測定・評価法に改良を加えた。測定方法としては、電気光学テンソル成分を独立に測定できるマッハ・ツェンダー干渉計を用い、変調強度を検出する実験配置を採用した。サンプルヘの変調電圧印加には、電極取り付けの容易な面内電極がしばしば用いられる。面内電極により形成される変調電場分布を有限要素法により計算し、変調位相シフトを正しく補正・評価した。さらに、この補正位相シフトを含むガウシアンビーム2光束の干渉縞強度プロファイルおよび変調強度曲線の理論式を導出して、補正された電気光学係数を求めた。実際の測定は2次の有機非線形光学材料としてよく知られているDAST単結晶を用いた。厚さの異なる結晶について一定の電気光学係数が求められたことから、本測定法の有効性を確認した。

Organic nonlinear-optical materials have ever been studied and are being studied for realizing electro-optic devices. In order to accurately evaluate the materials, we improved the interferometric method of measuring electro-optic coefficients in consideration of adequate corrections in analyzing the experimental results. The Mach-Zehnder interferometry was used in order to seperately evaluate components of an electro-optic coefficient tensor. The modulating electric-field was applied by the coplanar electrodes and the modulated intensity was detected through the pin-hole in the interference-fringe pattern. The spatial distribution of the modulating electric-field was calculated by the finite element method, and the modulated phase-shift was corrected. In addition, the interference-fringe pattern formed by two Gaussian beams and the modulated intensity considering the calculated modulated-phase-shift were theoretically derived, and the electro-optic coefficients were evaluated by comparing the experimental results to the theoretical ones. The organic materials evaluated were single crystals of DAST, which was well known as the second-order nonlinear-optical material. The measured electro-optic coefficients did not depend on thickness of the crystals, indicating validity of our corrections.