等离激元增强的石墨烯光吸收

石墨烯中等离激元具有特殊的光电性质,其和入射光的强烈耦合可以引起光吸收的增强.本文基于时域有限差分法和多体自洽场理论研究了等离激元对处于光学谐振腔中的石墨烯光吸收的影响.由于石墨烯中等离激元与入射光动量和能量不匹配而不能直接相互作用,因此石墨烯上施加了金属光栅结构.研究发现光栅结构能够对入射光进行动量补偿并且能够引起其下石墨烯中的电场强度产生很大程度增强,从而导致在该石墨烯结构中太赫兹等离激元和入射光发生强烈耦合而产生太赫兹等离极化激元,同时引起石墨烯光吸收的增强.希望本文能够加深对石墨烯光电特性的理解以及可以为基于石墨烯的太赫兹光电装置提供一定的理论依据.     

Band gaps in the spectra of terahertz surface plasmons on metallic diffraction gratings

The effect of the appearance of band gaps in the energy spectra of terahertz surface plasmons has been experimentally observed and investigated. The band gaps are formed due to the interference interaction of the surface plasmons excited by pulsed terahertz radiation on metallic diffraction gratings. It has been shown that the experimental dispersion curves of terahertz plasmons are in good agreement with the dispersion curves obtained in the numerical simulation.     

Optical generation of terahertz plasmons on comb-shaped surface of metal

The possibilities of a recently proposed (Opt. Express 17, 9323 (2009)) method for generating terahertz surface plasmons on a microstructured (comb-shaped) metal surface using a nonlinear polarization pulse that moves with a superluminal velocity and is induced by an ultrashort laser pulse in a strip of electrooptic material deposited on the surface are theoretically studied. For an arbitrary direction of motion of the nonlinear source along the comb-shaped surface, fields of excited terahertz plasmons and the angular spectral distribution of the radiated energy are calculated. It is shown that the spectral and energy characteristics of plasmons can be efficiently controlled by varying the direction of motion of the source. Conditions (parameters of the comb-shaped structure, direction and velocity of motion of the source) that ensure the maximal efficiency of the optical-to-terahertz conversion are found. The developed method of terahertz generation is promising for surface terahertz spectroscopy.     

Broadly tunable one-way terahertz plasmonic waveguide based on nonreciprocal surface magneto plasmons

One-way-propagating broadly tunable terahertz plasmonic waveguide at a subwavelength scale is proposed based on a metal-dielectric-semiconductor structure. Unlike other one-way plasmonic devices that are based on interference effects of surface plasmons, the proposed one-way device is based on nonreciprocal surface magneto plasmons under an external magnetic field. Theoretical and simulation results demonstrate that the one-way-propagating frequency band can be broadly tuned by the external magnetic fields. The proposed concept can be used to realize various high performance tunable plasmonic devices such as isolators, switches and splitters for ultracompact integrated plasmonic circuits.     

Tailoring terahertz near-field enhancement via two-dimensional plasmons

We suggest a novel possibility for electrically tunable terahertz near-field enhancement in flatland electronic materials supporting two-dimensional plasmons, including recently discovered graphene. We employ electric-field effect modulation of electron density in such materials and induce a periodic plasmonic lattice with a defect cavity. We demonstrate that the plasmons resonantly excited in such a periodic plasmonic lattice by an incident terahertz radiation can strongly pump the cavity plasmon modes leading to a deep subwavelength concentration of terahertz energy, beyond λ/1000, with giant electric-field enhancement factors up to 10(4), which is 2 orders of magnitude higher than achieved previously in metal-based terahertz field concentrators.     

从光子晶体效应到表面等离子波的实时演变

文章报道了激光诱导太赫兹表面等离子谐振效应。采用激光抽运-太赫兹波探测技术，实时改变单晶硅中的载流子浓度，使其介电特性从类绝缘体演变为类金属导体，以支持表面等离子谐振效应，进而实现太赫兹波在周期性亚波长单晶硅孔阵列中的实时可控制谐振增强传输。同时还通过实验观测到太赫兹波从光子晶体效应到表面等离子波的实时演变。文章作者采用Fano模型对实验结果进行模拟分析，获得了与实验数据一致的理论拟合。     

Localized surface plasmons-based transmission enhancement of terahertz radiation through metal aperture arrays

The extraordinary transmission spectrum of a copper film pierced by a periodic array of subwavelength rectangular holes is measured by the terahertz time-domain spectroscopy. The transmission coefficient is strongly dependent on the angle between the polarization of terahertz electric field and the latitudinal direction of the periodic apertures. When the angle increases from 0^o to 90^o, a peak becomes stronger and another peak reduces. The transmission is proposed to be the contributions of localized surface plasmons inside the apertures. The finite-difference-time-domain (FDTD) simulation results are in good agreement with experimental observations.     

Electrically tunable resonant terahertz transmission in subwavelength hole arrays

An actively enhanced resonant transmission in a plasmonic array of subwavelength holes is demonstrated by use of terahertz time-domain spectroscopy. By connecting this two-dimensional element into an electrical circuit, tunable resonance enhancement is observed in arrays made from good and relatively poor metals. The tunable feature is attributed to the nonlinear electric response of the periodic hole array film, which is confirmed by its voltage–current behavior. This finding could lead to a unique route to active plasmonic devices, such as tunable filters, spatial modulators, and integrated terahertz optoelectronic components.     

石墨烯等离激元的光学性质及其应用前景

石墨烯等离激元由于其独特的电学可调性、本征低衰减以及局域光场高度增强等特性, 引起了广泛的关注并迅速成长为一门新的学科分支--石墨烯表面等离激元光子学. 本文介绍了石墨烯等离激元的一些基本性质, 包括色散关系、局域的等离激元和传导的等离激元以及石墨烯等离激元对其周边介电环境的敏感性等. 在此基础上, 进一步介绍了石墨烯等离激元在太赫兹到中红外频段的应用, 比如有源光调制器的一些功能器件和增强的红外光谱探测等.     

Waveguided spoof surface plasmons with deep-subwavelength lateral confinement

We present a new type of waveguide scheme for terahertz circuitry based on the concept of spoof surface plasmons. This structure is composed of a one-dimensional array of L-shaped metallic elements horizontally attached to a metal surface. The dispersion relation of the surface electromagnetic modes supported by this system presents a very weak dependence with the lateral dimension and the modes are very deep-subwavelength confined with a long-enough propagation length.     

Making structured metals transparent for ultrabroadband electromagnetic waves and acoustic waves

In this review, we present our recent work on making structured metals transparent for broadband electromagnetic waves and acoustic waves via excitation of surface waves. First, we theoretically show that one-dimensional metallic gratings can become transparent and completely antireflective for extremely broadband electromagnetic waves by relying on surface plasmons or spoof surface plasmons. Second, we experimentally demonstrate that metallic gratings with narrow slits are highly transparent for broadband terahertz waves at oblique incidence and high transmission efficiency is insensitive to the metal thickness. Further, we significantly develop oblique metal gratings transparent for broadband electromagnetic waves (including optical waves and terahertz ones) under normal incidence. In the third, we find the principles of broadband transparency for structured metals can be extended from one-dimensional metallic gratings to two-dimensional cases. Moreover, similar phenomena are found in sonic artificially metallic structures, which present the transparency for broadband acoustic waves. These investigations provide guidelines to develop many novel materials and devices, such as transparent conducting panels, antireflective solar cells, and other broadband metamaterials and stealth technologies.     

Surface plasmon THz waves on gratings

Because of their long propagation length at a metal surface in the far infrared, surface plasmons make potentially feasible the design and realization of 2D integrated terahertz systems over a metallic substrate. The coupling of a terahertz beam to the surface plasmon wave is very efficiently achieved by diffraction gratings engraved at the metal surface. In this article, we present a review of some recent works we performed in view of characterizing this coupling phenomenon. The analysis of the experimental data supplied by terahertz time-domain spectroscopy allows us to point out the main parameters that govern this diffraction process and the propagation of a surface plasmon over a flat or corrugated metal surface. To cite this article: M. Nazarov et al., C. R. Physique 9 (2008).     

基于光导微探针的近场/远场可扫描太赫兹光谱技术

太赫兹技术已经成为涉及公共安全、军事国防和国民经济等国家核心利益的前沿研究领域.以往太赫兹测量技术中通常以远场测量为主,如常用的太赫兹时域光谱仪.近年来太赫兹近场技术得到了迅猛的发展,特别是基于光导天线的探针技术的发展,为可扫描的太赫兹近场测量提供了可能.本文详细报道了我们近期在可扫描太赫兹近场光谱仪研究中的进展.采用光纤耦合的光导微探针实现了方便灵活的太赫兹近场/远场三维扫描,并同时获得振幅和相位信息.该系统将有可能广泛应用于人工微结构、石墨烯、表面等离子激元、波导传输、近场成像、生物样品检测、芯片检测等研究领域.     

Design of terahertz beam splitter based on surface plasmon resonance transition

According to the resonance transition between propagating surface plasmon and localized surface plasmon, we demonstrate a design of beam splitter that can split terahertz wave beams in a relatively broad frequency range. The transmission properties of the beam splitter are analyzed utilizing the finite element method. The resonance transition between two kinds of plasmons can be explained by a model of coherent electron cloud displacement.     

基于导模共振效应提高石墨烯表面等离子体的局域特性

本文构建了一种包含石墨烯和亚波长光栅的复合结构, 借助衍射光栅的导模共振效应, 在石墨烯表面激发高局域性表面等离子体激元, 研究了石墨烯与光栅结构对表面等离子体激元局域特性的影响规律, 并借助基于有限元法的COMSOL软件, 分析了缓冲层厚度、光栅周期、载流子迁移率和费米能级对石墨烯的表面电场、品质因子Q和有效模式面积S_eff的影响. 结果表明, 石墨烯表面等离子体激元的局域性在特定的参数点获得显著提高: 当μ = 0.7 m²/(V·s)时, 品质因子达到最大值Q_max = 1793; 当p = 235 nm或E_F = 0.72 eV时, 表面电场达到了入射光的3000倍以上. 强烈的局域性导致强烈 的光-物质相互作用, 因而本文提出的复合结构可实现高灵敏度传感器和高效率的非线性光学设备, 极大地扩展了石墨烯在纳米光学领域中的应用.     

Absorption of surface plasmons in a metal-cladding layer-air structure in the terahertz frequency range

The effect of a thin dielectric cladding layer of a metal on the absorption of surface plasmons (SPs) in the terahertz frequency range is studied experimentally and numerically. It is found that, as the radiation wavelength increases, the attenuation of SPs caused by the cladding layer can increase by a factor of ～10⁴ as compared to the absorption of SPs propagating along the unperturbed metal-air interface. Data obtained in experiments with germanium-cladded aluminum specimens using radiation from a terahertz free-electron laser (v = 90 cm^?1) confirm that application of a dielectric cladding on the metal surface causes the SP absorption to increase.     

Percolation-enhanced generation of terahertz pulses by optical rectification on ultrathin gold films

Emission of pulses of electromagnetic radiation in the terahertz range is observed when ultrathin gold films on glass are illuminated with femtosecond near-IR laser pulses. A distinct maximum is observed in the emitted terahertz amplitude from films of average thickness just above the percolation threshold. Our measurements suggest that the emission is through a second-order nonlinear optical rectification process, enhanced by the excitation of localized surface plasmon hot spots on the percolated metal film.     

基于人工表面等离激元探针实现太赫兹波的紧聚焦和场增强

为提高太赫兹近场显微成像技术的分辨率,设计了一款在Teflon探针的尖锥形表面镀上厚度渐变、具有相同占空比的超薄金属银制条带的探针,用于实现探针尖端处人工表面等离激元的激发和太赫兹波的亚波长聚焦.研究表明,对于频率为0.1 THz的入射波,厚度渐变镀银条带探针产生的紧聚焦光场的尺寸可稳定在20μm左右(λ/150),探针尖端处最大电场强度为入射电场强度的849倍.研究还发现,周期性金属条带的数目和入射电场的偏振方向可对探针尖端处产生的紧聚焦光斑的尺寸和电场强度等进行灵活有效的调控.     

Extraordinary terahertz transmission through subwavelength spindle-like apertures in NbN film

We studied numerically the temperature dependent extraordinary terahertz transmission through niobium nitride(NbN) film perforated with subwavelength spindle-like apertures. Both the resonant frequency and intensity of extraordinary terahertz transmission peaks can be greatly modified by the transition of NbN film from the normal state to the superconducting state. An enhancement of the(±1, 0) NbN/magnesium oxide(MgO) peak intensity as high as 200% is demonstrated due to the combined contribution of both the superconducting transition and the excitation of localized surface plasmons(LSPs) around the apertures. The extraordinary terahertz transmission through spindle-like hole arrays patterned on the NbN film can pave the way for us to explore novel active tuning devices.     

THz emission from semiconductor surfaces

We provide a review of experimental and theoretical work on electromagnetic terahertz pulse emission from semiconductor surfaces excited by femtosecond laser radiation. The main terahertz emission mechanisms are analysed. The terahertz emission from InAs and Ge is explained by the photo-Dember effect and electric field induced optical rectification. Electronic band structure and carrier scattering mechanisms are investigated by means of terahertz emission and absorption spectroscopy in InAs, InSb and Ge. To cite this article: V.L. Malevich et al., C. R. Physique 9 (2008).