【作者】 刘燕；

【导师】 刘盛纲；

【作者基本信息】 电子科技大学 ， 物理电子学， 2019， 博士

【摘要】 表面等离子体激元(SPP)能够突破衍射极限,实现对电磁波在亚波长尺度内的局域特性,将光子器件的尺寸压缩到纳米量级。混合等离子体波导可以看作是介质光纤波导和SPP波导的结合,这种波导结构具有很强的模式局域能力和较低的传输损耗。混合等离子体波导为发展太赫兹到中红外波段的高性能、高集成度的SPP波导提供了新思路。电磁超材料是一种由亚波长单元结构构成的新型人工电磁材料,它具有自然材料所不具备的超常电磁特性。将局域表面等离子体激元与新型人工电磁超材料结构相结合的电磁器件应用涉及到光学天线、滤波器、探测器、调制器、光学透镜、热成像、太阳能电池、吸波材料、隐身斗篷等多个重要研究领域。传统基于金属SPP的波导器件利用结构变化的方法来调节波导的传输特性,不利于灵活实时调控。为了解决这一问题,人们将可以实现动态控制的一些材料,如液晶、液态金属、半导体等加入到SPP波导器件中,然而这些材料构成的可调谐器件通常不易于集成。近年来,石墨烯因其独特的电化学性质如高电子迁移率、灵活可调谐性、低损耗特性和强局域性等,成为各国科研工作人员关注的焦点。石墨烯表面等离子体激元工作在太赫兹和到中红外频段时,具有非常强的模式约束能力和较小的损耗,能够把工作在几十微米波长的波导器件缩小到半个微米尺度,通过调节外加电压或者化学掺杂,可以实现波导器件的电调谐,对太赫兹和红外波段可调谐型波导器件的小型化集成具有重要意义。正是基于以上考虑,本文通过将半导体、石墨烯、新型人工电磁超材料与表面等离子体激元相结合,取得了以下研究结果。1.本文提出了一种基于“不平坦”半导体基底的、具有高度模式局域性的混合等离子体波导,该波导可以看做是介质光纤波导和等离子体波导的结合。在太赫兹和中红外等低频段,光滑金属表面对SPP的局域性很弱,使得这种结构不适合在应用系统中紧凑的集成。为了解决这一问题,本文采用等离子体频率在太赫兹波段的半导体材料InSb代替传统贵金属,通过分析和尝试多种混合SPP波导的传输特性如等效模场面积、传播长度、品质因数、能量分布情况等,最终提出一种基于“不平坦”基底的混合SPP波导,实现了工作在1THz的具有低损耗、高局域性的混合等离子体波导,最小等效模式面积可达衍射极限模式面积的1/2000。此外还探索了利用增益材料实现无损传输的可行性条件。2.本文提出了一种低串扰传输的可调型双介质脊加载石墨烯混合等离子体波导。由Kubo公式出发,详细分析了石墨烯工作于3THz到300THz时表现出的不同材料特性。将石墨烯材料引入到介质波导结构中代替传统的贵金属材料构成混合等离子体波导,石墨烯SPP相比于SPP在贵金属表面的传输具有更好的模式约束性。通过对多种不同结构的石墨烯加载混合SPP波导传输特性的对比研究,最终提出了一种对称双介质脊加载石墨烯混合SPP波导结构,该波导工作于中红外波段,表现出良好的模式约束性,两相邻波导实现低串扰传输的最小距离为120nm,可以满足元器件高度密集的集成电路的需求。另外,研究了波导结构加工可能出现的形变如双脊不对称、长方形脊形变为梯形、长方形脊的尖角变圆角等情况,结果显示该波导具有较高的加工误差容限。通过改变石墨烯的外加电压或者化学掺杂,可以实现对波导传输模式特性的灵活调节。3.针对目前超材料滤波器存在的调制深度不够,结构复杂不易于灵活设计,不能实时调谐等问题,充分利用局域型SPP特性,提出了一种工作于中红外波段的、基于金属-石墨烯超材料结构的可调谐型双阻带滤波器。基于“明模”和“明模”之间的耦合作用,通过简单增减周期单元中刻蚀金属臂数量即可以调整带阻滤波器阻带个数,双阻带滤波器的调制深度可达-23.26dB。通过调整臂长和石墨烯层外加电压,可以改变结构的谐振频率,从而调节滤波器的工作频带。进一步优化石墨烯的载流子浓度,可获得一个非常深的调制深度。该金属-石墨烯周期性结构对周围环境介电常数变化的灵敏度高达2393nm/RIU,可用折射率传感器,最后,提出了发展多阻带滤波器等多频谱器件的思路。4.设计了一种基于多层金属-石墨烯超材料的动态独立可调型吸波器,实现了吸波器在中红外波段的多频带独立调谐和超宽带的吸收特性。通过对多层金属-石墨烯超材结构的层数、单元结构个数的调整,可以实现多频、宽频吸波器的任意定制。调整每层金属-石墨烯超材料中石墨烯所加载的门电压,可以实现对多个吸收频带的动态独立调谐。研究发现,该结构非常适合制作实现超宽带吸波器。通过叠加双层并联排列的金属-石墨烯超材料结构,可获得一个吸收率超过80%的7.5THz的宽带吸收,在该吸收宽带范围内的平均吸收峰值为88.5%。进一步堆叠超材料层数为三层超材料时,在27.5THz到38.4THz频带范围内,吸收峰最小值为60%,平均峰值吸收率为84.7%。对于获得结构简单、设计灵活、易于集成的微结构器件提供了新思路。更多还原

【Abstract】 Surface plasmon polariton(SPP)offers promising strategies to realize highly compact circuits in nanotechnology because of their capability of confining electromagnetic waves at a scale beyond the diffraction limit.Hybrid plasmonic waveguide combines the advantages of the dielectric tube waveguide with the plasmonic polariton,which provide strong mode confinement with low propagation loss.Hybrid plasmonic waveguides provide a new idea for developing SPP waveguides with perfected performance and high integration in terahertz to mid-infrared bands.New artificial electromagnetic metamaterials,a kind of artificially structured EM materials,are usually composed of periodically arranged subwavelength microstructures or ?meta-molecules?,and have attracted great attention for their unique EM properties that the natural materials cannot realize.The application of electromagnetic devices,which combine the characteristics of the metallic metamaterials and the properties of localized SPP,involves many important research fields such as optical antenna,filter,detector,modulator,optical lens,thermal imaging,solar cells,absorbing materials and so on.However,traditional SPP waveguide and relative devices based on metallic structures cannot be changed once the structure is fabricated.To improve above situation,some actively controlled elements,such as liquid crystals,liquid metals and semiconductors,are applied in SPP waveguide and devices.Unfortunately,these tunable structures are unsuitable for compact integration.Recently,graphene,a flat monolayer of carbon atoms packed into a dense 2D honeycomb crystal lattice,has become a very promising material for its unique electrical properties,such as high electron mobility,flexible tunability,relatively low loss and tight field confinement.Graphene surface plasmons can work on the spectrum from THz to near infrared,with very strong mode confinement and low losses.The most attractive property is that the conductivity of graphene can be dynamically tuned by changing the Fermi energy through chemical doping or electrostatic gating.Therefore,the combination of graphene and SPP can provide an effective approach to achieve tunable and integrated waveguide and devices.To overcome the problems aforementioned,we investigated the proposed waveguide and relative devices by combing the semiconductor,graphene and new artificial electromagnetic metamaterials with surface plasmon polaritons.The main contents are divided into four parts shown as follows.1.A kind of new hybrid surface plasmonic waveguide working in the terahertz frequency range and based on uneven substrate structure was proposed.The proposed waveguide combines the advantages of the dielectric tube waveguide and the plasmonic polariton.SPP supported by the noble metals are weakly confined in the mid-infrared and terahertz regions,owing to the near-zero skin depth.To overcome this problem,InSb is used to replace the traditional noble metal in the hybrid waveguide for its plasma frequency loading in terahertz band.By comparing the mode properties of the multiple structured waveguide,such as the effective mode area,propagation length,figure of merit and energy ratio,we proposed a hybrid SPP waveguide based on uneven InSb substrate working at 1 THz.The results show that the proposed waveguide can reach the same propagation length with the traditional hybrid waveguide,while the mode area of the proposed structure is about 1/2000 of the diffraction-limited mode area in the free space,which indicates stronger mode confinement with similar propagation length.In addition,the possibility of lossless transmission is investigated by introducing gain materials into the hybrid.2.A single-mode graphene loaded double ridge plasmon waveguide(GDRW)with long propagation length and strong mode confinement was proposed.Based on Kubo formula,the different material properties of graphene working from terahertz to visible band are analyzed in detail.Graphene is introduced into the dielectric waveguide to replace the traditional noble metal and forms a hybrid plasmonic waveguide.Graphene SPP shows tighter mode confinement and lower propagation losses Compared with SPP modes on metal surface.Due to the investigations of the graphene loaded slab waveguide with different structures,we finally proposed a single-mode graphene loaded double ridge plasmon waveguide(GDRW)with long propagation length and strong mode confinement.High figures of merit and much lower crosstalk are obtained due to the dramatically suppressed interference between two parallel placed GDRWs with the nearest distance 120 nm,enabling thereby more tightly stacking in terahertz integrated circuits.Further investigation on fabrication errors,such as the horizontal misalignment of the two symmetric ridges,fabrication distortion of rectangular ridges,and variation of ridge tip curvature radius,indicates that the proposed structure has enough fabrication error tolerance.Tunable single-mode transmission with good performance can be realized either by chemical doping or electrostatic gating.3.Current metamaterial-based filters still suffers some problems,such as insufficient modulation depth,complex structure,difficulty in flexible design,and inability to tune in real time.Utilizing the properties of localized SPP characteristics,dynamically tunable band stop filter based on metal-graphene metamaterials is proposed and numerically investigated at mid-infrared frequencies.A stable modulation depth up to-23.26 dB can be achieved.Due to the cooperative effect of the “bright-bright” elements,the amount of the gold strips in each unit cell determines the number of the stop-bands,providing a simple and flexible approach to develop multispectral devices.Further investigations illustrate that the location of the stop bands not only can be adjusted by varying the length of gold strips,but also can be dynamically controlled by tuning the Fermi energy level of graphene,and deep modulation is acquired through designing the carrier mobility.With the sensitivity as high as 2393nm/RIU of the resonances to the varieties of surrounding medium,the structure is also enabled to be an index based sensor.4.Dynamically and independently tunable absorbers based on multilayer metal-graphene metamaterials are proposed to achieve multi-band and ultra-wide-band absorbing properties at mid-infrared frequencies.Multiple bands absorption can be arbitrarily customized by etching the appropriate number of tandem gold strips in each meta-molecule,as well as stacking multiple metal-graphene layers.Through tuning the Fermi energy level of the graphene in each metal-graphene layer separately,the multiple absorption resonances can be dynamically and independently adjusted.With side-by-side arrangement of the gold strips in each supercell,the proposed structure is rendered to be a promising candidate for ultra-wide-band absorber.The extreme bandwidth exceeding 80% absorption up to 7.5THz can be achieved with a dual-layered structure,and the average peak absorption is 88.5% in the wide-band range for lossless insulating interlayer.For a triplelayered structure,the average peak absorption is 84.7% from 27.5THz to 38.4THz with a minimum of 60%.All these results will benefit the integrated microstructure research with simple structure and flexible tunability.更多还原