【作者】 王磊；

【导师】 秦亦强； 陆延青； 胡伟；

【作者基本信息】 南京大学 ， 材料物理与化学（专业学位）， 2014， 博士

【摘要】 由于太赫兹波的独特性,太赫兹技术在安全检查、高速通信、生物传感等领域有着广泛的应用前景。相对于太赫兹源和探测器的飞速发展,高性能太赫兹功能器件的研发仍处于初级阶段,尤其是可调太赫兹器件。由于液晶具有宽带可调特性且具有成熟的工业技术基础,使得基于液晶的太赫兹可调器件的研究越来越引起人们的关注。但目前仍面临三大挑战：一、大多数液晶材料的双折射在太赫兹频段比较小,因此可调控量也较小,在一般的设计中很难达到实用的要求。二、通常采用的透明导电薄膜氧化铟锡将不再适用,而单纯的金属层也无法满足要求,这也导致通过叠层结构增加调制量时在透明电极缺乏的条件下,损耗会严重增加。三、液晶盒厚较大,使得取向效果,响应速度等性能严重下降。本论文针对太赫兹电控液晶领域上述三大问题,设计并实现了具有广泛应用前景的性能优异的太赫兹液晶光子学材料与器件,为上述问题的解决,提供了一整套可行的方案。论文的主要研究内容包括以下三个部分：1.设计和合成了一种在太赫兹频段具有大双折射的液晶混晶材料NJU-LDn-4,由76%的具有氟代二苯乙炔苯骨架结构的系列衍生物R1-PPT (2,6-F)P-R2作为a组分、12%的具有氟代二苯吡啶骨架结构的系列衍生物R1-P’ (3-F)PP-F作为b组分、5%的具有二苯乙炔骨架结构的系列衍生物R1-PTP-R2作为c组分、7%的具有氟代二苯乙炔骨架结构的系列衍生物R1-PT(2,6-F)P-R2作为d组分按总量100%熔融混合形成的液晶混晶。该液晶材料太赫兹频段内的双折射率平均在0.3以上,兼具宽温液晶相(-15～150℃)和低粘度的特点,能够实现低工作电压、快速响应、紧凑型太赫兹调制器件的制备,适用于材料科学、生物医学、无损检测等广阔的应用领域。并讨论了分子结构与双折射之间的关系,为进一步设计新型太赫兹波段大双折射液晶材料提供了理论参考。2.设计和制备了一种基于多孔石墨烯透明电极的宽带可调液晶太赫兹波片,具体为利用亚波长金属线栅作为太赫兹入射面透明电极和集成起偏器,多孔石墨烯作为太赫兹出射面透明电极,沿金属线栅方面斜45°进行大双折射率液晶的光控取向,制成平行取向盒,利用液晶的电控双折射特性,通过电压调节寻常光与非常光的相位延迟来实现对应不同频率的特定波片。我们首先找到了太赫兹波段的透明电极——少层多孔石墨烯,其太赫兹透过率高达98%以上。最终实现了工作电压只有50V的单液晶盒宽带可调波片。通过双盒设计,在整个测试频段0.5-2.5THz都可达到可调四分之一波片的功能。该器件具有超宽频段、自偏振、透过率高、调制量大、响应快速等特性；另外,制备成本低、效率高、可批量生产,器件的稳定性和重复性都满足实用要求,应用前景十分广阔,能在蓬勃发展的太赫兹领域发挥广泛应用。3.设计和实现了一种新型太赫兹超材料吸收器。该结构可以解决液晶太赫兹器件盒厚大,响应速度慢等问题。首先介绍了传统太赫兹吸收器的原理与研究现状,利用CST和Comsol软件进行了大量理论模拟,对太赫兹超材料吸收器各参数进行了优化设计。增加的上层衬底层,为吸收频率的调节提供了一个新的自由度。实验制备了一种液晶可调太赫兹超材料吸收器并测试了在不同偏振态下吸收器的特性。TM模式,可调范围在0.843 THz-0.870 THz之间；TE模式,可调范围在0.655 THz-0.670 THz,并详细分析了中间液晶层对吸波特性的影响。初步设计了一种集成太赫兹波片的超材料吸收器,能实现更大的可调范围。最后,对该吸收器可作为太赫兹传感器应用进行了初步探讨。更多还原

【Abstract】 Terahertz technology has great potentials in security screening, biological sensing and high-speed wireless communication. Compared with the rapid development of terahertz source and detector, the research and development of high performance key terahertz photonics device is still in its infancy, especially tunable terahertz devices. Liquid crystal, based on broadband tunability and mature industrial technology, has been proven a perfect enabler to fulfill these needs in visible and telecom bands. It is also a very promising candidate in the new platform of terahertz devices. However, corresponding researches still face significant challenges:Firstly, the dispersion of liquid crystal refractive indices induces a comparatively low birefringence in terahertz regime, so the quantity of modulation is small, which is difficult to meet the practical requirements. Secondly, the commonly used conductive indium tin oxide films will be no longer applied, and the pure metal layer is unable to meet the requirements, which has led to a seriously increasing loss when stacked structure is used to increase modulation quantity under the condition of lack of transparent electrode.Thirdly, since thick liquid crystal cell gap is larger, the orientation effect and response time seriously decline in performance. In order to solve all the problems above, this dissertation provides a feasible strategy that designs and implements terahertz liquid crystal photonics materials and devices which have broad application prospects and excellent performance.The main content of the dissertation is divided into three parts:1. We develop a fluorinated phenyl-tolane based nematic mixture NJU-LDn-4 and evaluate its frequency-dependent birefringence utilizing terahertz time domain spectroscopy. A large mean birefringence of 0.306 is obtained in a broad range from 0.4 to 1.6 THz, with a maximum of 0.314 at 1.6 THz. It has wide liquid crystal phase temperature (-15-150℃) and the characteristics of low viscosity. It can achieve compact terahertz devices with low working voltage and fast response, which is applicable to broad application fields such as material science, biological medicine, nondestructive testing. Furthermore, the relation between molecular structures and birefringence property is discussed, which provides a theoretical reference for further design of new large birefringence liquid crystal material in terahertz band.2. We design a broadband tunable liquid crystal terahertz waveplate driven with porous graphene. Sub-wavelength metal wire grid is utilized as integrated high efficient electrode and polarizer. The few-layer porous graphene is employed as electrode with transmittance over 98%. Homogeneous alignment of a large birefringence liquid crystal is implemented on both frail electrodes via non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Broadband tunable wave plate of single liquid crystal cell is achieved with only 50 V. Its polarization evolution process is directly demonstrated. Furthermore, electrically controllable quarter waveplates over the whole testing range are achieved via stacking two cells. This device has the characteristics of broadband, high transmittance, fast response which meet the practical requirements. And the proposed total solution may pave a simple and bright road towards various liquid crystal terahertz apparatuses.3. We design and implement a new type of terahertz liquid crystal tunable metamaterial absorber. This structure can solve the problems of large cell and slow response. Firstly the theory and research status of traditional terahertz absorber is introduced. Then lots of theoretical simulations through using CST and Comsol software are made to optimize the parameters. The added upper substrate layer can provide a freedom degree for absorption frequency. Liquid crystal tunable terahertz metamaterial absorbers are tested under different polarizations. The tunable range of TM mode is between 0.843 THz and 0.870 THz; the range of TE mode is between 0.655 THz and 0.670 THz, and the impact of middle liquid crystal layer of absorber is analyzed in detail. The preliminary design of integrated terahertz waveplate metamaterial absorber can realize larger tunable range. Finally, the discussion of the absorber used as a terahertz sensor has been carried on.更多还原