华中科技大学 博士学位论文 太赫兹 波在 亚波长金属结构 亚波长金属结构 中的传输特性 及其 传输特性及其 应用研究 姓名：袁英豪 申请学位级别：博士 专业：电子科学与技 指导教师：姚建铨;刘劲 2011-05-19 华华 中中 科科 技技 大大 学学 博博 士士 学学 位位 论论 文文 I 摘摘 要要 太赫兹波技术在传感检测和高速率无线通信领域有着重要的应用价值, 但目前太 赫兹波段与波束控制相关的功能器件非常匮乏, 这严重制约了太赫兹波应用技术的发 展. 本论文在研究了亚波长金属结构在太赫兹波段的表面等离子体激元(surface plasmon polariton, SPP)和伪表面等离子体(spoof surface plasmon, SSP)效应的基础上, 探索了亚波长金属结构在太赫兹波束控制中的应用, 提出了一系列的设计方案, 具体 的研究内容如下: (1) 分析了亚波长金属结构在太赫兹波段的透过特性并设计了一种基于双层金属 光栅填充液晶结构的电控太赫兹波开关器件. 其开关机制是利用外部偏置电压控制液 晶材料的折射率, 来实现太赫兹波谐振峰位置的移动. 该方案的创新之处在于: 结构中 的亚波长金属结构不仅仅作为支持高透射率的光栅来使用, 同时还用作金属电极. 该 设计方案可以解决以往液晶太赫兹波开关器件中由于采用氧化铟锡电极所带来的高插 入损耗问题. 此外, 我们还提出了一种基于多层金属光栅填充液晶结构的宽带太赫兹 波开关设计方案, 其开关状态是通过外部电场控制改变液晶的吸收系数来实现的. 文 中采用严格耦合波方法分析了器件的结构设计参数与太赫兹开关性能之间的关系. (2) 从理论上分析了亚波长金属表面光栅结构支持的 SSP 的色散关系并提出了一 种填充硅介质的金属表面光栅结构来实现动态局域太赫兹波的技术方案. 利用有限元 模拟证明了当光泵浦使金属凹槽中的硅折射率产生梯度线性变化分布时, 该结构可以 将不同频率的太赫兹波局域到不同的空间位置; 通过控制改变凹槽中硅折射率的分布 函数, 可以控制某一频率的太赫兹波局域位置沿水平方向在亚波长尺度控制移动. (3) 提出了一种亚波长金属狭缝-凹槽结构来控制太赫兹波透过亚波长金属狭缝的 传播特性. 该设计创新之处在于结构中含有两种不同深度的金属亚波长凹槽阵列. 其 中浅凹槽用来支持太赫兹表面波的传播, 而深凹槽作为衍射光栅, 将太赫兹表面波重 新衍射到自由空间. 理论分析和数值模拟结果表明: 通过控制结构中深凹槽阵列的分 布, 可以实现多种波束控制效果, 包括: 分束, 垂直准直和离轴准直效果. (4) 采用太赫兹时域光谱系统(THz-TDS)实验测量了环氧玻璃衬底的亚波长金属 华华 中中 科科 技技 大大 学学 博博 士士 学学 位位 论论 文文 II 狭缝光栅样品的透射谱, 详细的分析了透射谱各种类型的透射峰和吸收峰的形成机制 和特点. 分析结果表明: 透射谱中窄透射峰的形成依赖于 SPP 和导模的相互耦合作用. 此外, 由于太赫兹波在衬底中的多重反射作用也可以形成 Fabry-Perot (FP)类型的宽透 射峰. 如果满足 FP 谐振的同时激发了 SPP, 则会出现 FP 谐振峰的红移现象. 透射谱中 吸收峰的出现则是由于在金属光栅上表面或者下表面激发形成了纯粹的 SPP 模式, 或 者在衬底中形成了纯粹的导模现象. 关键词关键词: 太赫兹波 波束控制 金属光栅 亚波长结构 表面等离子体 伪表面波等离子体 华华 中中 科科 技技 大大 学学 博博 士士 学学 位位 论论 文文 III Abstract Terahertz technology has important application values in the field of sensing and high-speed wireless communications. However, its development is seriously hampered by the shortage of basic beam control devices in the terahertz frequency. In this thesis, based on the theoretical studies of surface plasmon polariton (SPP) and spoof surface plasmon (SSP) effects in the terahertz domain supported by the subwavelength metal structures, we investigated the terahertz beam steering technologies by the subwavelength metal structures, and proposed a series of schemes. The research details are listed in the following: (1) We analyzed the transmission properties of subwavelength metal structures in the terahertz regime, then proposed an electrically controlled terahertz switch based on the metallic grating-liquid crystal-metallic grating (MG-LC-MG) structures. The switching mechanism is based on the shift of the resonance peaks, which is realized by modifying the effective refractive of the LC using different bias electric fields. The innovation of this design is that the subwavelength metal structures not only support the super transmittance at certain frequencies, but also act as the electrodes. Therefore, the proposed device has the potential advantage of avoiding high insertion loss due to the tin indium oxide (ITO) electrodes used in previously reported LC-based terahertz switch devices. Moreover, we proposed an electrically controlled broadband terahertz switch based on the LC-filled multilayer metallic grating (MMG) structures. The on and off state of the switch is realized by the changing of the absorption coefficient of the LC in the presence of different electric fields. The relations among the device design parameters and switching performances are studied by the rigorous coupled-wave analysis (RCWA) method. (2) We analyzed the dispersion relation of the SSP, which is supported by the subwavelength metal surface grating structures, then proposed a method to realized dynamic trapping of terahertz waves using a silicon-filled metallic grating structure. Using the finite element method, we demonstrated that if a graded refractive index distribution in the grooves is optical induced, the device has the ability to dynamic trap terahertz waves of different frequencies at different positions. Moreover, we demonstrate that the trapped position of a certain frequency of the terahertz waves can be moved continuously along the grooves in 华华 中中 科科 技技 大大 学学 博博 士士 学学 位位 论论 文文 IV subwavelength scale by ingenious control of the distributions of the refractive indices of silicon filled in the grooves. (3) We proposed a subwavelength metal slit-groove structures to control the propagation characteristics of the terahertz waves passing through a subwavelength metal slit. The most significant feature of the design is that the surface gratings consist of two sets of grooves of different depths. The shallow grooves are designed to support the SSP, and the deep grooves are utilized to diffract the terahertz surface wave into the free space. Theoretical analysis and numerical simulations based on the finite element method confirm that various beaming effects including splitting, on-axis and off-axis beaming can be realized by controlling the distributions of the deep grooves. (4) We measured the transmission spectrum of a subwavelength metal slit grating fabricated on