Broadband Terahertz Transmission Modulation Based on Hybrid Graphene-Metal Metamaterial

A novel patterned metamaterial, composed of graphene layer and metal periodic array of split ring resonators (SRRs) and cross-shaped resonators (CSRs), with broadband terahertz (THz) wave modulation was proposed and theoretically studied. It demonstrated that a broad passband high transmission of over 96.1% in the frequency range from 1.02 THz to 1.66 THz and two narrow band resonance frequencies f₁ and f₂ could be generated. The modulation depth of transmission was 29.2% when the graphene layer was covered on the metal metamaterial surface, and the modulation depth could be further increased by increasing the Fermi energy of graphene layer and reached approximately 79.5% at 1.0 eV in a broadband THz frequency range. The resonance frequencies of f₁ and f₂ were blue-shifted, and their modulation depths reached about 63.2% and 18%, respectively. These results show that the ultrathin graphene-metal metamaterial exhibits potential to achieve high-performance active THz devices and may offer widespread applications.     

基于石墨烯超材料的三频段可调谐完美吸收器

为进一步降低太赫兹频率下高性能调控器件的结构复杂度,提出一种三频段可调谐超材料完美吸收器. 该吸收器由图案化的石墨烯层和经Si介质层隔开的Au接地平面组成,利用太赫兹下的石墨烯表面等离子体共振以及图案化石墨烯与电场耦合提供的电偶极子共振形成多个吸收峰. 数值仿真结果表明,在0.489 THz、1.492 THz和2.437 THz处实现了对入射波的共振吸收,各峰值处的幅值均大于99.9%. 由于吸收峰处的幅值可以通过外部施加的偏置电压改变石墨烯的费米能级进行控制,因而所提出的吸收器结构的工作状态可在反射器和吸收器之间灵活切换. 同时,通过对吸收器单元结构的对称设计实现了对极化角度的不敏感特性,且在宽入射角范围内仍能保持良好的吸收性能. 因此,所设计的基于石墨烯的太赫兹超材料功能器件在调制和传感方面具有巨大的潜力.     

基于石墨烯的可调谐高吸收率太赫兹超材料

提出一种基于石墨烯的双波段太赫兹超材料吸收体,它由金属-电介质-石墨烯3层超材料结构单元在水平方向上进行周期性拓展而成。仿真结果显示,其在太赫兹波段6.62 THz和 9.36 THz分别产生99.9%和98.9%的高吸收率;通过改变石墨烯的费米能级,可以灵活地控制吸收体的谐振频率和吸收强度,而吸收体的吸收强度也可以利用石墨烯的弛豫时间进行单独控制。另外,研究了吸收体中间介质层厚度和介质损耗对吸收率的影响,这为吸收体初始加工工艺参数的确定提供了依据。研究结果表明,提出的基于石墨烯的太赫兹超材料吸收体结构简单,易于加工,可通过偏置电压或者化学掺杂,简单地实现吸收体的可调谐性,为双波段高吸收率太赫兹超材料吸收体的设计提供了重要参考。     

Highly Sensitive Terahertz Sensing of Glycerol-Water Mixtures with Metamaterials

We demonstrate the highly sensitive terahertz (THz) sensing of a water-glycerol mixture poured on a metamaterial surface using reflection-based THz time domain spectroscopy. The reflectance at the resonant dip frequency changes significantly with glycerol concentration. The detection sensitivity is about 8 times higher than that compared to the case without a metamaterial structure. We also investigate how sensitivity depends on the Q factor of the metamaterial resonance for highly absorptive media, such as a water solution in the THz region.     

Active Graphene-Based Terahertz Dual-Band Modulator Implemented in the Presence of External Fields

In this work, we numerically demonstrate a dynamic graphene-based dual-band metamaterial modulator (gDMM) in the presence of an external magnetic field and gate electric field. With the objective of modulating terahertz waves at two separate channels, we utilize the proposed dual-field control method to dynamically modulate the optical conductivity of graphene, and thus the working frequencies of the gDMM. An interpretation for such dependence on the external fields is presented based on a quantum understanding of the energy structure of graphene, and a numerical method based on the finite element method (FEM) is employed to investigate the optical responses of our proposed gDMM. Our results show that, by varying the strength of external fields, one can switch the operation status of the two working channels located at 3.18 THz and 9.04 THz, with modulation depths exceeding 84.4%. Only 30 meV of energy is required for shifting the Fermi level to accomplish the switch, which is extremely low compared with methods in previous works using gate electric control alone. Simultaneous ON/OFF statuses are also realized. Such great tunability and controllability of our proposed gDMM over a wide frequency range may give rise to a new class of dynamic devices for terahertz and microwave applications.     

Spatiotemporal Visualization of THz Near-Fields in Metamaterial Arrays

We present an experimental approach to record the spatiotemporal electric field distribution of coherent broadband THz pulses propagating through planar metamaterial arrays. The electric field can be measured with sub-wavelength precision within a volume that is several wavelengths in size, thus, having the potential to map the near-field to far-field transition of the resonant structures constituting the metamaterial. To demonstrate the potential we present measurements of THz pulses propagating through a planar array of double split-ring resonators and their inverse analogues.     

Realization of band-pass and low-pass filters on a single chip in terahertz regime

In this paper, we present the design, simulation, fabrication and characterization of a terahertz(THz) filter based on metamaterial consisting of the periodical double symmetric splits ring resonator(DS-SRR) array. We can observe that the metamaterial-based filter possesses a band-pass transmission when the electrical field is along y direction, and it possesses a low-pass transmission when the electrical field is along x direction. Our results demonstrate that the proposed filter can realize the switching between band-pass effect and low-pass effect by only changing the polarization direction of the incident electromagnetic wave. Moreover, the calculated surface current distributions are also used to analyze the switchable mechanism of the THz metamatrial filter. Therefore, the proposed THz wave filter has a potential application in THz wave communication systems.     

复合石墨烯/硅半球的宽带太赫兹超材料吸收器

提出了一种性能可调的宽带、极化与入射角不敏感的超材料太赫兹吸收器,该吸收器自上而下分为四层结构,分别是:硅半椭球/半球体复合结构、连续石墨烯层、PDMS介质层和金属背板。通过在TE波垂直入射条件下仿真,在已知结果基础上,对不同石墨烯化学势和不同结构条件下的电场结果分析表明,在硅半椭球/半球体亚波长复合结构所形成的连续、多模法布里-珀罗共振,以及由石墨烯所激发的多个离散的等离子体共振的协同作用下,其吸收光谱得到平滑和扩展,使该结构可实现吸收率宽范围可调,以及接近100%吸收率的宽频带吸收特性。特别的,当石墨烯化学势分别为0.2与0.9 eV时,其分别可获得约5.7 THz与7 THz的宽带太赫兹波吸收（吸收率超过90％）,且其最大吸收率接近完美吸收（约99.8％）。此外,该结构还具有360°极化不敏感和高于60°的入射角不敏感等优异特性,在以上角度范围内,吸收器吸收率仍可保持到90%以上。在太赫兹波探测、光谱成像以及隐身技术等方面具有潜在的应用前景。     

基于石墨烯超材料的宽频带可调太赫兹吸波体

基于二维材料石墨烯,设计了一款宽频带可调谐超材料太赫兹吸波体。该吸波体由三层结构组成,顶层为石墨烯超材料,中间层为二氧化硅,底层为金属薄膜。仿真结果表明,当石墨烯的费米能级为0.7 eV时,该吸波体在1.11~2.61 THz频率范围内吸收率超过90%,相对吸收带宽为80.6%。当石墨烯的费米能级从0 eV增大到0.7 eV时,该吸波体器件的峰值吸收率可以从20.32%增大到98.56%。此外,该吸波体器件还具有极化不敏感和广角吸收的特性。因此,它在太赫兹波段的热成像、热探测、隐身技术等领域具有潜在的应用价值。     

基于超材料结构的高Q太赫兹滤波器

在半导体微纳工艺基础上研究了基于镜像非对称结构超材料太赫兹(THz)滤波器的性能。通过时域有限差分理论,模拟分析了滤波器工作频率下太赫兹电场强度和电流密度在微结构的分布,并阐述了微结构太赫兹波电磁共振的机理。基于太赫兹滤波器电磁损耗机制和微纳加工工艺条件,设计并优化器件开口环结构,使相邻开口环中产生电磁场的干涉相消,减少太赫兹辐射损耗,提高品质因子Q值。采用太赫兹时域光谱(THz-TDS)系统测试了滤波器的太赫兹透射特性,实验结果表明,在工作频率0.923 THz下,镜像非对称结构的平面太赫兹滤波器品质因子达到12.5。同时,研究了太赫兹电场方向对滤波器性能的影响。高Q太赫兹滤波器的研制为制备高速太赫兹电调制器等太赫兹器件提供了重要的实验依据。     

基于石墨烯的太赫兹光电功能器件研究进展

作为一种新型光电材料,石墨烯独特的能带结构和电子输运特性,使其与太赫兹科学有着密切的内在关系：石墨烯内部的等离子体振荡频率在太赫兹频段;人为调谐石墨烯的禁带宽度在0~0.3 eV时,正好覆盖太赫兹频段;光电导率的外部可控性等,这些特点使得石墨烯有望成为太赫兹频段新一代高性能设备研制的基础。最近的研究显示,石墨烯在太赫兹波产生、调控、检测等光电功能器件的研制中取得了很好的成果。重点介绍了基于石墨烯的太赫兹光电功能器件,包括太赫兹源器件、可控调控器件及检测器研究的最新进展,并对这一快速发展的研究领域进行了展望。     

Diffraction of plane and cylindrical waves by a cylindrical metamaterial shell with a negative refractive index

The problems of scattering of plane and cylindrical waves by a cylindrical metamaterial shell are solved rigorously. The influence of the geometric dimension of the shell, the value of the negative refractive index of the metamaterial medium, and the location of the cylindrical wave source on the near (far) field structure is investigated. It is shown that, in the quasi-opticl region, a caustic with one cusp is formed inside an electrically thick shell and whispering-gallery waves or standing waves are formed in between the facets of an electrically thin shell. It is found that, in the resonance region, reactive (surface) fields with substantial amplitudes are observed near the boundaries of a thin shell.     

基于超材料的太赫兹幅度调制器设计

提出了一款基于超材料的太赫兹幅度调制器,其结构由开口"弓"形超材料结构、高电子迁移率晶体管、斜十字馈线和碳化硅衬底四部分构成."弓"形超材料结构开口处的连通和断开两种状态将对通过该结构的太赫兹波产生不同的响应.在开口处添加高电子迁移率晶体管可模拟开口连通和断开的效果.当对晶体管上的栅极不施加偏压时,超材料结构开口相当于导通,对太赫兹波透射系数高;当对晶体管上的栅极施加偏压时,超材料结构开口相当于断开,对太赫兹波透射系数低.仿真结果表明,在0.22 THz处,对晶体管栅极不施加偏压时,调制器的透射系数为0.579;对晶体管栅极施加偏压时,调制器的透射系数为0.040.通过公式计算得到其调制深度为93％,而且对x和y极化入射波具有不敏感的特性.同时,通过分析0.22 THz处的电场分布和表面电流分布研究了该太赫兹调制器的工作原理.所设计的太赫兹调制器具有调制深度高、结构简单和易于加工等特点,在太赫兹通信领域具有广阔的应用前景.     

High-Contrast Imaging of Graphene via Time-Domain Terahertz Spectroscopy

We demonstrate terahertz (THz) imaging and spectroscopy of single-layer graphene deposited on an intrinsic Si substrate using THz time-domain spectroscopy. A single-cycle THz pulse undergoes multiple internal reflections within the Si substrate, and the THz absorption by the graphene layer accumulates through the multiple interactions with the graphene/Si interface. We exploit the large absorption of the multiply reflected THz pulses to acquire high-contrast THz images of graphene. We obtain local sheet conductivity of the graphene layer analyzing the transmission data with thin-film Fresnel formula based on the Drude model.     

偏振选择可调谐双带太赫兹吸收器

提出并研究了一种偏振选择可调谐双带太赫兹吸收器。吸收器由顶层方形劈裂石墨烯环、中间SiO₂介质层以及底层金反射层组成。基于时域有限差分法的仿真结果显示,该吸收器在不同偏振光入射下均可以实现双带高效率吸收。x偏振光时在7.86和12.63THz处的吸收率分别为97.9%和91.2%;y偏振光时在6.30和10.52THz处的吸收率分别为94.1%和93.2%。通过改变石墨烯费米能级,可以对两个偏振的双带吸收峰波长进行调谐。此外,研究了介质层厚度和石墨烯劈裂环的物理参数对共振吸收峰的影响。因为在两个偏振状态下都能产生双带高吸收,所以此吸收器在太赫兹偏振成像、太赫兹传感、选择性光谱检测和偏振复用等领域有重要的潜在应用价值。     

宽频太赫兹异向介质吸收器研究

利用异向介质吸收器的传输线模型及CST仿真软件对其太赫兹波段的宽频吸收特性进行了研究。结果表明：异向介质吸收器对太赫兹电磁波的吸收主要来源于开口环共振单元(SRR)的LC共振。提出了两种使异向介质吸收器获得宽频太赫兹吸收的方法：一是增加LC共振中的等效电阻R,能有效扩展吸收带宽到100GHz以上;二是通过优化具有双吸收...     

Spectrum manipulation of a dual-stop-band metamaterial terahertz filter based on unit cell dimension changes

We numerically and experimentally proposed a dual-stop-band terahertz filter based on standard microelectronic fabrication method. The stop bands locate at 0.32 THz and 1.02 THz with 3 dB bandwidths of 0.26 THz and 0.55 THz, respectively. The resonance characteristics of the proposed device were discussed with the help of surface current maps and field density maps extracted from computer simulation software to better understand the working principle of the proposed device. On top of that, a total of seven devices with different dimensions were fabricated to fully discuss the dimension effects on the resonant frequency shift and bandwidth changes. This fabrication process is applicable for related integrated metamaterial devices and provides essential experiment evidences for effective ways of manipulating the transmission spectrum of the proposed filter.     

THz 波段电磁感应透明效应超常媒质的构建

本论文主要对基于金属的一种超常媒质的机理以及相关结构进行研究。在与传统的量子EIT 系统的比较中,探索这一现象的激发机理以及验证依据。由左手介质设计的理论基础进行推广,构建并设计了具有电磁感应透明效应的THz 超常媒质,对其中的理论问题进行了探讨并对相关材料的定义做出了解释。通过分别对亮部分（内环）和暗部分（外环）进行激发仿真,给出了整体激发的结果。此外,还对探针的位置以及SRR 环的厚度分别进行了分析。本研究工作对于推动以金属为基础的光频段材料、结构甚至集成电路、系统的发展和应用具有重要的意义。     

Spectroscopic Study on Ultrafast Carrier Dynamics and Terahertz Amplified Stimulated Emission in Optically Pumped Graphene

This paper reviews recent advances in spectroscopic study on ultrafast carrier dynamics and terahertz (THz) stimulated emission in optically pumped graphene. The gapless and linear energy spectra of electrons and holes in graphene can lead to nontrivial features such as negative dynamic conductivity in the THz spectral range, which may lead to the development of new types of THz lasers. First, the non-equilibrium carrier relaxation/recombination dynamics is formulated to show how photoexcited carriers equilibrate their energy and temperature via carrier-carrier and carrier-phonon scatterings and in what photon energies and in what time duration the dynamic conductivity can take negative values as functions of temperature, pumping photon energy/intensity, and carrier relaxation rates. Second, we conduct time-domain spectroscopic studies using an optical pump and a terahertz probe with an optical probe technique at room temperature and show that graphene sheets amplify an incoming terahertz field. Two different types of samples are prepared for the measurement; one is an exfoliated monolayer graphene on SiO₂/Si substrate and the other is a heteroepitaxially grown non-Bernal stacked multilayer graphene on a 3C-SiC/Si epi-wafer.     

复合鱼鳞型超构材料中光控Fano谐振的设计

为解决传统超构材料存在的结构一旦形成,其谐振特性便无法进行动态可调的问题,本文将鱼鳞型超构材料与光电导材料硅相结合,实现了太赫兹波段Fano电磁响应的动态调控。该复合超构材料由鱼鳞型金属线、硅层以及聚酰亚胺组成。在鱼鳞型结构的金属弧线无、有缝隙两种情况下,研究了电磁波的入射角度和硅的电导率对Fano谐振的影响。当硅的电导率达到1×10³ S/m时,多频点电磁响应的调制深度都接近1。结果表明,调节缝隙宽度可以成为Fano谐振工作频率调控的有效方式。本文为实现超构材料中Fano谐振的可调谐特性提供了一种可行途径,对实际应用中太赫兹波的主动调控、传感等方面具有重要意义。