液晶填充太赫兹微结构光纤及其热/电调谐特性研究

Terahertz science and technology has unique superiority and broad application prospect in fields such as characterization of materials, broadband communications, environmental monitoring, medical imaging and nondestructive testing. Terahertz systems were hard to be light, compact and functional on account of the lack of flexible low-loss terahertz waveguide and functional devices. This project will prepare a kind of terahertz bandgap guiding microstructured optical fiber based on the novel terahertz low loss polymer material and infiltrated with high refractive index liquid crystal. The intensity, phase and polarization characteristics of terahertz wave can be controlled effectively take advantage of the temperature and electric field sensitivity of molecular orientation and dielectric anisotropy of liquid crystal. This flexible fiber waveguide is capable of guiding terahertz wave with low loss and continuous thermal/electrical tenability. Structural dependence and thermal tunability of the bandgaps and transmission properties for the liquid crystal infiltrated terahertz microstructured optical fiber will be studied by using the finite element method. Frequency/temperature/electric field dependence of optical properties of the liquid crystal will be studied by using the elastic free energy theory. By comparing theoretical results with experiment research, establish a theory model which is able to analyze the thermal and electrical tunability of both the designed ideal fibers and the prepared ones. And then the low loss and high tuning sensitivity fiber transmission for terahertz wave can be achieved. This project is aimed to provide theoretical and experimental guidance for fabrication of tunable all-in-fiber terahertz functional devices and integrated terahertz systems.

太赫兹科技在材料表征、宽带通信、环境监测、医学成像和无损检测等领域具有独特优越性和广阔应用前景。目前太赫兹系统不能实现轻量化、集成化和功能化的最大障碍是缺乏柔性、低损耗的太赫兹波导及功能器件。本项目设计并制备一种以太赫兹低损耗聚合物为基材的微结构光纤，其包层空气孔中填充高折射率的液晶。该光纤基于光子带隙效应引导太赫兹波沿柔性波导传输的同时，利用液晶分子取向和介电各向异性对温度和外加电场的敏感性来有效控制太赫兹波的强度、相位和偏振等特性，实现光纤传输特性的热/电连续可调。分别基于有限元方法和液晶弹性自由能理论分析光纤传输特性随结构参数的变化规律和液晶光学特性随频率/温度/电场的变化规律，理论与实验对比研究，构建一套可以模拟实际光纤的热/电调谐特性的理论模型。最终实现太赫兹波的低损耗、高调谐灵敏度光纤传输，为制造全光纤可调谐的太赫兹功能器件和集成化、功能化的太赫兹系统提供理论参考和实验指导。

太赫兹（THz）科技在诸多领域有着重大的科学研究价值和广阔的应用前景，是国内外的研究热点之一。目前，随着THz波的产生、探测及其应用技术的日趋成熟，柔性、低损耗太赫兹波导及可调谐功能器件已成为太赫兹系统实用化的瓶颈问题之一。.本项目以新型THz低损耗聚合物材料Topas为基材，设计了基于微结构光纤（MOF）的低损耗太赫兹波导，对其损耗、色散和双折射等传输特性以及传输特性的热/电可调谐性进行了系统和深入的研究。具体研究如下：（1）设计了填充E7和5CB液晶（LC）的太赫兹微结构光纤（THz LC-MOF），总结了其结构设计原则和带隙随光纤结构参数的变化规律，研究了THz LC-MOF的导模特性、带隙和限制损耗的热可调特性、电可调的偏振相关带隙结构和传输特性。研究结果表明：随着温度由25°C增加到34°C，所设计的THz LC-MOF限制损耗最小值位置（对应于透射谱的峰值位置）发生红移，具有-30GHz/°C的平均热调谐灵敏度；施加x方向外电场后，THz LC-MOF显示出了与偏振相关的带隙分裂特性，总传输带宽得以展宽。产生了一些有意义的特性，比如电可调的高双折射特性，平坦近零色散特性，THz波的单模单偏振传输等。在研制电可调THz滤波器、降低宽带皮秒THz脉冲的色散导致的信号劣化等领域具有潜在应用。（2）通过引入椭圆形包层孔，研究了E7液晶填充的THz LC-MOF的带隙随结构参数的变化规律、双折射和损耗等传输特性；（3）设计了一种具有Kagome晶格包层的THz MOF，最大限度降低传输损耗的同时引入了高双折射。在1THz频率处获得了0.089的超高双折射和0.055cm-1的有效材料吸收损耗，y偏振基模在0.5THz-1.5THz频率范围内具有0±0.45 ps/THz/cm的超平坦近零色散。该Kagome MOF在THz通信领域和保偏系统中具有潜在的应用。（4）对THz LC-MOF实现了初步的实验制备和表征。实验结果表明即使是制备简单结构（仅包含少量圆形空气孔道）的THz MOF预制棒，在成品效率和质量方面“模具挤出法”都优于“钻孔法”。采用“热挤出-拉伸法”成功制备了所设计的THz MOF并进行了液晶的填充和封装，采用THz TDS系统对其热调谐特性进行了实验表征。