DEVEROPEMENT OF NONLINEAR RELAXATION SPECTROMETER IN A FREQUENCY DOMAIN

由于聚合物和液晶等软凝聚体对外场的响应容易变为非线性，因此非线性弛豫光谱是研究这些物质动力学的一种有效方法。我们研制了以正弦电场为输入的频域非线性弛豫光谱仪。在我们的系统中，通过低通滤波消除了外加电场的谐波失真，并用数字平衡法从消除基波分量后的响应信号中提取了非线性响应。数字平衡法使高精度测量非线性光谱成为可能，不受光谱仪动态范围的限制。我们将所研制的系统应用于SMC^*相铁电液晶，并验证了该系统的实用性。实验测得的线性谱和三阶非线性谱与理论计算值吻合较好。通过对这两个光谱的比较，我们成功地估计了在显示应用中重要的材料常数，如自发极化和转动粘度。我们还将该系统应用于非线性电光测量中，其中通过填充铁电液晶的盒的透射光强度被检测为输出信号。发现奇谐振谱提供了与非线性介电谱相似的信息，而偶谐振谱提供了液晶电学和光学各向异性的信息。综上所述，我们研制了新的非线性弛豫谱，并展示了该系统在液晶动力学研究中的应用。

As a response to external field easily becomes nonlinear in soft condensed matters such as polymers and liquid crystals, the nonlinear relaxation spectroscopy is an useful method to investigate the dynamics of these matters. We have developed the nonlinear relaxation spectrometer in a frequency domain using a sinusoidal electric field as an input. In our system, harmonic distortion of the applied electric field has been removed by low-pass filter and the nonlinear responses have been extracted from response signals after cancel of the fundamental component by the digital balancing method. The digital balancing method makes it possible to measure the nonlinear spectra with high accuracy free from the dynamic range of the spectrometer.We have applied the developed system to ferroelectric liquid crystals in the SmC^* phase and checked the usefulness of the system. The measured linear and third-order nonlinear spectra are found to make good agreement with those calculated theoretically. We have succeeded in the evaluation of the material constants which are important in the display application such as spontaneous polarization and rotational viscosity by the comparison between those two spectra.We have also applied that system to the nonlinear electrooptic measurement, in which the intensity of the transmitted intensity through cells filled with ferroelectric liquid crystals is detected as an output signal. It is found the odd harmonic spectra offer the similar information to that obtained from the nonlinear dielectric spectra and the even harmonic spectra offer the information on the electric and optical anisotropy of liquid crystals.In conclusion, we have developed new nonlinear relaxation spectrometer and showed the usefulness of the system in the study of dynamics of liquid crystals.