【作者】 张莹；

【导师】 韩家广；

【作者基本信息】 天津大学 ， 光学工程， 2018， 博士

【摘要】 表面等离激元是沿金属-介质界面传播的电磁波,因具有高度局域化和场增强特性而在众多领域都有重要的应用前景。在太赫兹波段,由于大多数金属表现为良导体,对表面等离激元的束缚性较差。为解决束缚性差的问题,我们采用了两种金属微结构实现了束缚态的太赫兹表面波,在此基础上,开展了太赫兹表面等离激元波导、表面等离激元隐身、表面等离激元透镜等相关功能器件的研究。主要研究内容有:1.基于周期性金属圆柱的太赫兹表面等离激元波导:我们设计并提出了一种基于周期性金属圆柱的波导结构,实现对太赫兹表面等离激元的直线、弯曲、直角及分束传输等。在水平面上,两端平行的周期排列的金属柱子在横向上约束了表面波,使表面波局域在小于波长的空间内,并在其中覆盖介质层以减小表面波在自由空间中的衰减长度。基于此结构设计了不同功能的表面波波导,如S形波导、Y形分束波导、以及直角波导。利用太赫兹近场扫描系统对表面波的传播进行了表征。2.基于周期性金属柱子的人工表面等离激元波导:我们以金属方块为基本结构单元,对金属表面做皱褶处理,实现对人工表面等离激元在太赫兹波段的传输控制。结构尺寸远小于波长的金属方柱周期性排列在金属基底上,这样的金属表面支持太赫兹人工表面等离激元的传输。设计和加工了不同种类的人工表面等离激元的传输器件,包括直波导、S形波导、Y形分束器和定向耦合器。利用光纤化太赫兹近场扫描系统,定量地研究了表面等离激元的传播属性。近场场强成像图表明这些传输器件能控制表面波的传播,使其能沿非直线路径传播,并具有分束和耦合的功能。3.太赫兹人工表面等离激元开关、分束器及波分复用器:通过控制人工表面等离激元传输器件中金属结构旋转的角度实现了对表面等离激元开关的控制、分束以及频率选择性输出的控制。(1)基于马赫-曾德干涉仪,通过控制传输路径中一段区域内柱子的旋向可控制表面波的相干或相消,最终实现了在特定频率处的光开关。(2)在相互平行的两直波导中,在其中一波导保持不变的前提下改变另一波导内金属结构旋转角度,从而控制两波导之间的耦合强弱,实现了可控分束比的1×2分束器件。(3)因马赫-曾德干涉仪对波长很敏感,设计了基于马赫-曾德干涉仪的表面波波分复用器,通过控制不同频率处的相位差,可使频率为0.57 THz和0.62THz的表面波分别从不同的端口输出。4.太赫兹人工表面等离激元的隐身及透镜:利用周期排列的矩形金属柱子实现太赫兹表面波反射式隐身和两种不同的等离激元透镜。随着转换光学理论的发展及优化,折射率均匀分布的各项异性材料能实现反射式隐身,基于此,通过调控矩形柱子的长宽实现了等离激元模式折射率的各向异性。根据理论推导和模拟结果验证了此结构的隐身器件能实现太赫兹表面波反射式隐身。除此之外,根据金属微结构尺寸的连续变化会导致模式有效折射率渐变这一特性,设计了折射率渐变的等离激元透镜:伦勃透镜和伊顿透镜。5.基于DSTMS和InGaAs/InAlAs的宽带太赫兹时域测试系统:搭建了有机晶体DSTMS作为太赫兹源与InGaAS/InAlAs光电导天线作为探测器的宽带太赫兹时域系统。频谱范围可覆盖0.2-8 THz,中心频率高至1.44 THz,动态范围达50dB。由于晶体和光电导天线皆工作在1.55μm波段,此系统有潜力发展成为光纤化、紧凑的、低成本的宽带太赫兹时域系统。更多还原

【Abstract】 Surface plasmon polaritons(SPPs)are electromagnetic waves propagating along metal-dielectric interfaces,SPPs with the nature of subwavelength confinement and strong field enhancement have found promising application in many field.At terahertz(THz)frequencies,since most metals behave as perfect electrical conductors,the surface does not support bound modes of THz surface waves.In order to realize highconfinement SPPs at THz frequencies,two different structures are proposed to support and propagate the SPPs.Based on these structures,different functional devices such as SPP waveguides,SPP cloak and SPP lens are designed and studied.Main content are listed below:1.THz surface plasmon polariton waveguiding with periodic metallic cylinders: We demonstrated a structure with periodic cylinders arranged bilaterally and a thin dielectric layer covered inside that supports bound modes of SPPs at THz frequencies.This structure can confine SPPs in the lateral direction,and at the same time reduce field expansion into space.We examined and explored the characteristics of several different structures including a straight waveguide,a Y-splitter and a sharp bend waveguide by using scanning near-field THz microscopy.Waveguide components based on this structure accomplished power splitting and wave bending.2.THz spoof SPP waveguides based on periodic pillars: We report the design,fabrication and characterization of THz spoof SPPs waveguide based on the corrugated metal surface.The various waveguide components including: straight waveguide,S-bend waveguide,Y-splitter and directional coupler were experimentally demonstrated using scanning near-field THz microscopy.The proposed waveguide indeed enables propagating,bending,splitting and coupling of THz SPPs.3.THz spoof SPP switch,splitter and demultiplexer: Based on THz spoof waveguides,different functional devices such as optical switch,splitter and demultiplexer can be realized by rotating pillars of the waveguide.(1)Based on Mach-Zehnder interferometer,the interference and destructive interference of SPPs can be controlled by rotating the pillars.Finally an optical switch is realized at a particular frequency.(2)The coupling strength between two parallel straight waveguides can be controlled by changing the rotation angle of pillars in one of the waveguides.Thus an active splitter is realized.(3)Because of Mach-Zehnder interferometer is sensitive to wavelengths,a demultiplexer based on Mach-Zehnder interferometer is designed.By controlling the phase difference at different frequencies,the SPPs at 0.57 THz and 0.62 THz are selected and transmitted from different output.4.THz spoof cloak and lens: The spoof SPP cloak and lens are realized based on periodic pillars.The cloak design was based on transformation optics theory.To optimize the width and length of periodic pillars,the anisotropy of SPP mode index was achieved.The simulation result demonstrates the effectiveness of our design.In addition,plasmonic lens such as luneberg lens and eaton lens with refractive index gradient are also designed in this way.5.A broadband THz-TDS based on DSTMS emitter and LTG InGaAs/InAlAs photoconductive antenna detector: We demonstrated a 4-f THz time domain spectroscopy(THz-TDS)system using an organic crystal DSTMS as the THz emitter and a low temperature grown(LTG)In GaAs/InAlAs photoconductive antenna as the sensor.The system covers a frequency range from 0.2 up to 8 THz at a central frequency 1.44 THz.The dynamic range is larger than 50 dB.This demonstration pays a way to realizing a compact,low cost and fiber technology friendly THz-TDS with a much broader frequency range in future.更多还原