Surface Plasmon Amplification in Planar Metal Films

The propagation and amplification of surface plasmon polaritons (SPPs) is studied at the interfaces between metals and active media. A permittivity renormalization technique is proposed and developed to obtain an explicit analytic expression for the critical gain required to achieve infinite SPP propagation length. A specific multiple quantum-well (MQW) system is identified as a prospective medium for demonstrating efficient SPP amplification at telecommunication frequencies. The proposed system may have a strong impact on a variety of photonic devices ranging from plasmonic nanocircuits, subwavelength transmission lines and plasmonic cavities to nanosized transducers.     

基于几何相位超表面的高效独立双频点圆偏振太赫兹波束调控

为了克服超表面普遍具有的波长依赖性,提出了一种基于几何相位的多功能超薄超表面,在双频点处对透射圆偏振太赫兹波实现独立波前调控。该超表面单元由表层金属层和中间介质层组成,其中表层金属图案相同,均是由双C型开口环谐振器、中间金属圆环和长方形金属片谐振器构成。通过分别旋转表层金属谐振器,可以控制交叉偏振透射光具有相同的振幅和不同的相位。将单元结构按照特定的规律排列,可对入射波的波前实现任意调控,例如,在低频f₁=0.701 THz,分别实现了携带拓扑电荷数+1、+2、+3、+4的涡旋波束,其纯度均在60%以上;在高频f₂=1.663 THz,实现了对入射圆偏振波的汇聚,且焦距误差仅为0.04。仿真结果表明,设计的超表面在双频点处对电磁波具有良好的调控。     

太赫兹表面等离激元及其应用

金属或半导体与介质分界面上的电子与光子互作用形成的光学表面等离激元(SPP)以及人工超构材料或二维原子晶体材料表面上的电子与太赫兹波或微波互作用形成的人工表面等离激元(SSP)是小型化与集成化太赫兹有源/无源器件和太赫兹超分辨率成像的重要物理基础。随着太赫兹科学技术的发展,太赫兹表面等离激元研究在国际上受到很大关注。本文介绍了传统的光学表面等离激元及其发展,详细阐述了太赫兹波段的人工表面等离激元(SSP)和石墨烯表面等离激元(GSP)的基本原理和发展历程,对表面等离激元在太赫兹波段的新型辐射源、无源器件、超分辨率成像及其他领域的应用进行了较为全面的总结和评述,并对该领域未来进一步发展的方向进行了展望。     

基于超材料的连续太赫兹波透射特性研究

以基于超材料的太赫兹波透射为目的,设计并制作了四种亚波长开环共振（SRR）超材料。采用连续太赫兹波作为入射激光源,实验测量了它们在1.04 ~4.25 THz波段的功率透射属性,并采用CST Studio进行仿真,结果显示这些超材料存在一个位于2.52 THz的全局透射峰和多个局部透射峰。全局透射峰与SRR阵列的微结构和图形配置等参数有关。为了寻找一个具有较高透射效率的太赫兹感应阵列,比较了四种不同超材料微结构的归一化功率透射性能和感应差别。从这些差别中找到特定图案配置的超材料器件用于太赫兹波感应具有借鉴意义。     

All-optical Control of surface plasmon polaritons based on metal slit structures

表面等离激元是束缚在金属—介质交界面上的一种电磁波模式,可突破衍射极限,被认为是下一代集成光子回路最有希望的信息载体。基于我们的研究工作,就几种表面等离激元金属狭缝结构的原理和应用做了简单概述。利用法布里-波罗谐振腔、法诺共振、多模干涉等光学效应,这些金属狭缝结构可对表面等离激元的传输行为进行有效地调控。在理论上和实验上,利用金属狭缝结构实现了亚波长表面等离激元单向激发器、亚微米宽带单向激发器、亚微米分束器、超紧凑纳米聚焦器件和亚微米全光开关等纳米光子器件。这些纳米光子器件在纳米集成光学中具有重要应用。     

太赫兹矩形波导与共面波导耦合结构设计

太赫兹（THz）波位于微波与红外光波之间,现有微波和光波段波导技术应用正在向THz波段拓展。但是,由于水汽对THz波的强吸收及制造工艺等原因,THz器件主要是平面结构,而THz源及其传输需要用矩形波导。因此,矩形波导与共面波导之间的转换结构成为决定元件和系统性能的关键部分。该设计利用脊波导进行阻抗匹配及电磁场模式转换,实现THz波矩形波导到共面波导的高效率耦合。结果表明,在0.2~0.4 THz频段内,该转换结构的传输系数(S₂₁)高于?3 dB,可以对THz电磁场进行高效率转换。该结果可用于太赫兹分子探测、太赫兹通信等领域,为0.2 THz以上太赫兹的模式转换提供了一种可行方案。     

基于太赫兹人工超材料的带阻滤波器

鉴于太赫兹辐射的特殊性,其难以与自然界中多数材料发生电磁相互作用,导致太赫兹功能器件匮乏。人工超材料通过人工设计结构单元的周期排列组合,可实现太赫兹波段电磁响应的调控。本文设计一种由二氧化硅衬底上的单层金属方形谐振环结构构成的太赫兹带阻人工超材料,具有窄带宽、深带阻特性、偏振不敏感特性,通过近场电场和表面电流分析,带阻共振特性源于谐振环结构的电偶极共振。该设计结构简单,易于制备,在太赫兹调制器件、太赫兹通信、光电探测等领域具有应用价值。     

温度对太赫兹亚波长金属结构共振特性的影响

利用太赫兹时域光谱系统研究温度对GaAs基底上生长的太赫兹亚波长金属结构的透过率及共振特性的影响。实验发现,温度从低温80 K升高至380 K,样品的透过率逐渐降低,且低频共振频率处有轻微的红移现象。通过研究共振带区域及远离共振带区域的透过率情况,分析了总体透过率降低的根本原因在于温度升高导致GaAs基底的本征载流子浓度升高;共振凹陷减弱是由于基底载流子的变化致使透过率升高形成的;红移现象的产生是由于温度升高导致样品折射率增大。此外,该研究可以为太赫兹范围的功能器件在实际应用和生产制造中提供有意义的参考。     

动态可调的太赫兹超构表面

近年来,采用人工设计金属阵列的超构表面以实现对太赫兹波的调制受到越来越广泛的关注。设计了2种互补的亚波长金结构阵列超构表面,正、反结构2个超构表面对太赫兹波均有共振响应。利用光泵浦太赫兹时域透射光谱系统,通过控制泵浦光实现对太赫兹波的谱调制。仅需28 mW的外加泵浦光,反结构超构表面在0.91 THz处的振幅调制深度可达到95%。利用该反结构超构表面对太赫兹波的开关作用,进一步设计了太赫兹振幅全息图,希望利用该结构实现太赫兹波前的动态调控。初步的理论模拟验证了这一方法的可行性,可较好地实现对太赫兹波的动态调控。     

Terahertz Semiconductor Quantum Well Devices

For eventually providing terahertz science with compact and convenient devices,terahertz (1～10THz) quantum-well photodetectors and quantum-cascade lasers are investigated. The design and projected detector performance are presented together with experimental results for several test devices,all working at photon energies below and around optical phonons. Background limited infrared performance (BLIP) operations are observed for all samples (three in total) ,designed for different wavelengths. BLIP temperatures of 17,13, and 12K are achieved for peak detection frequencies of 9.7THz(31μm) ,5.4THz(56μm) ,and 3.2THz(93μm) ,respectively. A set of THz quantum-cascade lasers with identical device parameters except for doping concentration is studied. The δ-doping density for each period varies from 3.2 × 1010 to 4. 8 × 1010cm-2. We observe that the lasing threshold current density increases monotonically with doping concentration. Moreover, the measurements for devices with different cavity lengths provide evidence that the free carrier absorption causes the waveguide loss also to increase monotonically. Interestingly the observed maximum lasing temperature is best at a doping density of 3.6 × 1010cm-2.     

Progress of Terahertz Devices Based on Graphene

Graphene is a one-atom-thick planar sheet of sp2-hybridized orbital bonded honeycomb carbon crystal. Its gapless and linear energy spectra of electrons and holes lead to the unique carrier transport and optical properties, such as giant carrier mobility and broadband flat optical response. As a novel material, graphene has been regarded to be extremely suitable and competent for the development of terahertz （THz） optical devices. In this paper, the fundamental electronic and optic properties of graphene are described. Based on the energy band structure and light transmittance properties of graphene, many novel graphene based THz devices have been proposed, including modulator, generator, detector, and imaging device. This progress has been reviewed. Future research directions of the graphene devices for THz applications are also proposed.     

High Mobility 3D Dirac Semimetal (Cd3As2) for Ultrafast Photoactive Terahertz Photonics

The Dirac semimetal cadmium arsenide (Cd₃As₂), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realize high-performance electronic and photonic devices at terahertz (1 THz = 4.1 meV) frequencies, where the performance of most dynamic materials are limited by the tradeoff between power-efficiency and switching speed. Here, all-optical, low-power, ultrafast broadband modulation of terahertz waves using an ultrathin film (100 nm, λ/3000) of Cd₃As₂ are experimentally demonstrated through active tailoring of the photoconductivity. The measurements reveal the photosensitive metallic behavior of Cd₃As₂ with high terahertz electron mobility of 7200 cm² (Vs)⁻¹. In addition, optical fluence dependent ultrafast charge carrier relaxation (15.5 ps), terahertz mobility, and long momentum scattering time (157 fs) comparable to superconductors that invoke kinetic inductance at terahertz frequencies are demonstrated. These remarkable properties of 3D Dirac topological semimetal envision a new class of power-efficient, high speed, compact, tunable electronic, and photonic devices.     

Ultrafast Plasmon Thermalization in Epitaxial Graphene Probed by Time-Resolved THz Spectroscopy

The control of carrier transport by electrical, chemical, or optical Fermi level tuning is central to graphene electronics. Here, an optical pump—terahertz (THz) probe spectroscopy—is applied to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H₂ intercalated films. The graphene layers display a prominent plasmonic response connected with induced THz transparency spectra on ultrashort timescale. It is generally believed that the plasmonic excitations appear due to wrinkles, and substrate terraces that bring about natural confinement potentials. It is shown that these potentials act within micrometer-sized domains with essentially isotropic character. The measured ultrafast dynamics are entirely controlled by the quasi-Fermi level of laser-excited carriers through their temperature. The photocarriers undergo a disorder-enabled super-collision cooling process with an initial picosecond transfer of the optically deposited heat to the lattice followed by a sub-nanosecond relaxation governed by the lattice cooling. The transient spectra is described by a two-temperature Drude-Lorentz model revealing the ultrafast evolution of the carrier temperature and chemical potential and providing crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path.     

Characterization of ErAs:GaAs and LuAs:GaAs Superlattice Structures for Continuous-Wave Terahertz Wave Generation through Plasmonic Photomixing

We investigate the impact of ErAs:GaAs and LuAs:GaAs superlattice structures with different LuAs/ErAs nanoparticle depositions and superlattice geometries on terahertz radiation properties of plasmonic photomixers operating at a 780-nm optical wavelength. Our analysis indicates the crucial impact of carrier drift velocity and carrier lifetime on the performance of plasmonic photomixers. While higher carrier drift velocities enable higher optical-to-terahertz conversion efficiencies by offering higher quantum efficiencies, shorter carrier lifetimes allow achieving higher optical-to-terahertz conversion efficiencies by mitigating the negative impact of destructive terahertz radiation from slow photocarriers and preventing the carrier screening effect.     

亚波长金属光栅的凹槽深度对太赫兹伪表面等离子体影响

从理论上详细研究了一维亚波长金属光栅的凹槽深度对太赫兹伪表面等离子的影响。分别对一维标准亚波长金属光栅和缺陷亚波长金属光栅进行了研究。电场分布情况采用了COMSOL软件进行模拟。得到的结论是:对于一维标准亚波长金属光栅,沿金属光栅传播的表面等离子体取决于槽深度,较深的槽具有更强的束缚能力;对于具有缺陷的光栅结构,电场强度的分布特点取决于缺陷槽的深度,这归功于缺陷槽对光的反射和散射。基于这一理论研究,这两种不同的亚波长金属光栅结构能为太赫兹器件如波导、衰减器及滤波器发展提供新的途征。     

Tunable,Grating-Gated,Graphene-On-Polyimide Terahertz Modulators

An electrically switchable graphene terahertz (THz) modulator with a tunable-by-design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO₂/AlO_x gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation-optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies.     

Metamaterial Demonstrates Both a High Refractive Index and Extremely Low Reflection in the 0.3-THz Band

Communication and imaging in the terahertz waveband have advanced rapidly in offering industrial applications. However, optical elements such as collimated lenses in the terahertz waveband are bulky compared with the wavelength due to the lack of naturally occurring substances with a high refractive index and low loss. It is essential to miniaturize optical elements in the terahertz waveband for industrial application. Metamaterials consisting of subwavelength structures can arbitrarily control permittivity and permeability and provide a range of refractive indices. Here, we demonstrate a metamaterial with both a high refractive index and extremely low reflection consisting of symmetrically aligned paired cut metal wires with 18,800 units on the front and back surfaces of a dielectric substrate. Measurements by terahertz time-domain spectroscopy (THz-TDS) confirm a high effective refractive index of 6.66 + j0.123, extremely low reflection power of 1.16%, and the unprecedented high figure of merit (FOM = |n _real/n _imag|) of above 300 in the 0.3-THz band. Components with such specifications would enable miniature, high-performance optical elements in the terahertz waveband such as ultrathin flat antennas with high directivity. Further, the concept of the metamaterial with both a high refractive index and extremely low reflection potentially offers a wide range of attractive applications such as solid immersion lenses and cloaking devices.     

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.     

太赫兹无线和有线融合通信技术的研究与展望

随着5G的移动互联及物联网相交织等新型业务的蓬勃发展,对未来通信系统传输容量、传输速度以及误码率等要求愈来愈高。介于毫米波与远红外光之间的太赫兹频段兼有微波和光波的特性,具有低量子能量、大带宽、良好的穿透性。近年来太赫兹通信系统成为研究热点之一,但太赫兹无线通信存在视距传播以及较大路径损耗缺点,太赫兹无线和有线融合传输则兼具两者优点。本文分析了光子太赫兹信号产生、光子太赫兹无线链路传输和光子太赫兹光纤链路传输过程中涉及的器件和技术,重点介绍了太赫兹有线传输的研究现状,并通过基于强度调制直接检测实现1.485 GBaud 350 GHz的1 m太赫兹光纤有线实时传输视频实验,展现了太赫兹有线传输巨大的发展潜力。     

Cobalt-Based Ferrites Characterization Using Two Different Terahertz Time-Domain Spectrometers

We report an experimental study of the electrical properties of manganese cobalt (MnCo) and nickel cobalt (NiCo) ferrites in the terahertz (THz) frequency band. The study is motivated by the potential of MnCo, NiCo, and other magnetic ceramics for the fabrication of active and passive devices for THz wave communications. Using two different terahertz (THz) time-domain spectroscopy systems, we characterized the optical constants of cobalt ferrites doped with manganese and nickel in the technologically important 0.2–1 THz frequency band. The MnCo and NiCo ferrites investigated in our study exhibit a lower refractive index and absorption coefficient in the 0.2–1 THz frequency band than commercial strontium ferrite. We observed that using different valency ion oxide leads to a sudden change of the refractive index as a function of sample stoichiometries. Our experimental results provide evidence that microwave ferrite technology can be extended to the THz frequency band.