Propagation of surface plasmon polaritons on semiconductor gratings

We present time-domain measurements of terahertz surface plasmon polaritons (SPPs) propagating on gratings structured on silicon surfaces. Using single-cycle pulses of terahertz radiation to excite SPPs in a broad frequency range, we observe that the efficient SPPs scattering on the semiconductor periodic structure introduces significant dispersion and modifies the SPPs propagation. A stop gap, or a frequency range where SPPs are Bragg reflected, is formed by the structure. This gap depends strongly on the Si doping density and type. The resonant scattering at the edge of the gap reduces the group velocity by more than a factor of 2. The measurements show a good agreement with our numerical calculations based on the reduced Rayleigh equation.     

介质填充浅槽周期结构表面上的太赫兹表面等离子体激元

通过在金属表面刻成浅的垂直凹槽,并在槽内填充不同的介质,对金属表面浅槽周期结构上传播的表面等离子体激元的色散特性与填充介质的关系进行了研究.研究表明通过在周期凹槽内填充介质可以有效降低人工表面等离子体激元的渐近频率,并增强金属表面对电磁场的约束.分析了太赫兹波段金属的吸收损耗对人工表面等离子体激元特性的影响,结果显示基于填充介质的浅槽周期表面结构可以获得长距离传输以及场的亚波长约束.通过对波传输的数值仿真,验证了该表面结构在太赫兹波段良好的导波能力.这种表面结构对太赫兹波段新型集成导波器件的设计具有参考价值.     

介质填充浅槽周期结构表面上的太赫兹表面等离子体激元

通过在金属表面刻成浅的垂直凹槽,并在槽内填充不同的介质,对金属表面浅槽周期结构上传播的表面等离子体激元的色散特性与填充介质的关系进行了研究.研究表明通过在周期凹槽内填充介质可以有效降低人工表面等离子体激元的渐近频率,并增强金属表面对电磁场的约束.分析了太赫兹波段金属的吸收损耗对人工表面等离子体激元特性的影响,结果显示基于填充介质的浅槽周期表面结构可以获得长距离传输以及场的亚波长约束.通过对波传输的数值仿真,验证了该表面结构在太赫兹波段良好的导波能力.这种表面结构对太赫兹波段新型集成导波器件的设计具有参考价值.     

Terahertz surface plasmon polaritons in textured metal surfaces formed by square arrays of metallic pillars

In this paper, we investigate numerically the characteristics of surface plasmon polaritons (SPPs) sustained by two-dimensional arrays of metallic pillars protruding out of planar metal surfaces at terahertz (THz) frequencies. Various shapes of the pillars are analyzed, and it is shown that the pillar shape only has weak influence on the dispersion of spoof SPPs. However, the loss of spoof SPPs is closely dependent on the pillar shape. It is also shown that spoof SPPs on textured surfaces with pillars can exhibit much better confinement than those on pierced surfaces with holes.     

The excitation and emission of terahertz surface plasmon polaritons on metal wire waveguides

The development of effective techniques for guiding pulsed terahertz radiation is essential for the continued development of terahertz spectroscopy and imaging applications based on the technique of time-domain spectroscopy. Terahertz surface plasmon polaritons (SPPs) can be excited and guided on cylindrical metal wires with low loss and dispersion. This propagating surface wave, known as a Sommerfeld wave, possesses radial polarization, which is not well matched with conventional sources of pulsed terahertz radiation. A photoconductive terahertz antenna with radial symmetry produces radiation that more efficiently couples to the wire waveguide. At the end of the wire, terahertz SPPs emit radiation into free-space that exhibits frequency-dependent diffraction. To cite this article: J.A. Deibel et al., C. R. Physique 9 (2008).     

Focused terahertz radiation formed by coherently scattered surface plasmon polaritons from partially uncorrugated metal surfaces

We present that a focus of terahertz radiation can be tailored based on coherent scattering of surface plasmon polaritons (SPPs) from a partially defected metal corrugation based on numerical simulations. The introduction of teeth defects in the corrugation allows coupling of the guided SPPs with the radiation and the far-field behavior is tailored by the spatial arrangement of such defects. The proposed structures serve as a kind of planar lenses which are quite thin and inexpensive. Promising applications include interfacing lens antennas between terahertz plasmonic integrated circuits and the external free space, which make terahertz systems very compact and low-cost.     

Accurate calculation of terahertz attenuation of a dielectric-coated metal wire with an intervening air gap using the engineering approach

A dielectric-coated metal wire with an intervening air gap between the conductor and inner surface of the dielectric is presented and demonstrated by theoretical calculation at terahertz frequencies. The characteristic equation of such a modified Goubau surface-wave transmission line is derived for the general case of a lossy dielectric and imperfect conductor. The terahertz attenuation of the modified Goubau line is investigated by using the accurate classical relaxation-effect frequency dispersion model. The influences of the different dimensions, different metal and dielectric materials on terahertz attenuation are also analyzed. In addition, the errors introduced by adopting the traditional and much simpler classical skin-effect model are also quantified. By using various conductivity models, the variation of the conductor loss is changed from 2.8% to 5.5%, and the variation of total loss is changed from 2.4% to 4.7%. It is shown that for certain combinations of the electrical dimensions of the structure the improvement in the attenuation constant over the Goubau line can be higher than 5 dB and realize stronger field confinement at terahertz frequency. The numerical results are very useful for the development of the surface plasmon polaritons (SPPs) devices in the fields of terahertz spectroscopy, sensors and detectors.     

Effect of a thin dielectric layer on terahertz transmission characteristics for metal hole arrays

We studied the role of surface-plasmon polaritons (SPPs) in a bandpass transmission property of two-dimensional metal hole arrays (2D-MHAs) by investigating the effect of thin dielectric layers on the 2D-MHA surfaces. We measured zero-order transmission spectra of the 2D-MHAs by changing the thickness of the dielectric layer and found that the bandpass transmission peak shifted to the lower-frequency side with increasing layer thickness, owing to the change of the resonant frequency of the SPP. This result shows that SPPs play a crucial role in the transmission property of 2D-MHAs in the terahertz region.     

Nanofocusing of terahertz wave on conical metal wire waveguides

The nanofocusing of the terahertz (THz) radiation energy is studied. By using a conical metal nanowire waveguide, we focus the energy of the terahertz surface plasmon polaritons (THz SPPs) to several nanometers’ scale. Another interesting property of the THz SPPs propagation on the waveguide is that the peak electric field at the waveguide tip enhances many times. What is more, both the phase velocity and the attenuation coefficient versus the wire radius are obtained. The terahertz energy nanofocusing opens the way to observe terahertz propagating and imaging on the nanoscale.     

Anomalous transmission of terahertz wave through one-dimensional lamellar metallic grating

The anomalous transmission through one-dimensional lamellar metallic gratings was investigated in terahertz (THz) regime. The extraordinary optical transmission (EOT) is identified to originate from two possible ways: coupling of incident light with waveguide resonances and coupling of surface plasmon polaritons (SPPs) at the upper and lower interfaces of metal grating. The dual effects of SPPs have been clarified in this study: (i) the excitation of SPP modes at each individual interface results in the weakness of the THz wave transmission; and (ii) the coupling of SPP modes at two interfaces of metal grating is attributed to enhancement of THz wave transmission. The enhanced transmission is dominated by the coupling of incident light with transverse waveguide resonances. Numerical simulation based on finite-difference time-domain (FDTD) agrees well with experimental results.     

Surface plasmon wave propagation along single metal wire

Recently,the single metal wire(SW) has become attractive for its potential applications in the terahertz and higher frequency range.However,as the most simple and typical surface plasmon polariton(SPP) transmission line,its study seems far from enough.Many important transmission behaviours have not been explained satisfactorily from the viewpoint of physics.In this paper,making use of the modified Drude model(MDM) based on the Sommerfeld theory,the transmission behaviours of SPPs along SW are systemically investigated theoretically.Some important physical phenomena such as the mode transformation,the lifetime of the radiative mode and the resonance frequency are revealed,and their mechanisms are explored.The results obtained in the paper will facilitate a general understanding of the features and the physical essence of the SPP transmission,not only for SW itself but also for other SPP transmission lines.     

Spoof surface plasmon-based bandpass filter with extremely wide upper stopband

We investigate the guiding modes of spoof surface plasmon polaritons(SPPs) on a symmetric ultra-thin plasmonic structure. From the analysis, we deduce the operating frequency region of the single-mode propagation. Based on this property, a spoof SPPs lowpass filter is then constituted in the microwave frequency. By introducing a transmission zero at the lower frequency band using a pair of stepped-impedance stubs, a wide passband filter is further realized. The proposed filter is fed by a transducer composed of a microstrip line with a flaring ground. The simulated results show that the presented filter has an extremely wide upper stopband in addition to excellent passband filtering characteristics such as low loss, wide band, and high square ratio. A prototype passband filter is also fabricated to validate the predicted performances.The proposed spoof-SPPs filter is believed to be very promising for other surface waveguide components in microwave and terahertz bands.     

不同表面结构特征圆柱导体的太赫兹散射特性

太赫兹频段下导体表面的细微结构、粗糙度等细节将对目标电磁散射行为产生影响。为衡量这一影响程度,以圆柱导体为例研究了太赫兹频段下目标表面不同结构特征的电磁散射现象及其在图像域的表现规律。利用高频电磁计算方法获得了表面分别为理想光滑、带刻痕和周期粗糙的三种圆柱多姿态角、多频点单站散射场;基于转台成像算法重建了小转角下目标的二维图像。从仿真结果可以看出:m量级的细节特征在太赫兹雷达图像上有着显著的表现,表明太赫兹雷达能够获取更加丰富和精细的目标信息,从而为目标探测识别提供新的特征和技术手段。     

Coupled THz Waveguide Utilizing Surface Plasmon Polaritons on Thin Dielectric Slab Sandwiched between Two Corrugated Metallic Claddings

We present a comprehensive experimental study of terahertz （THz） wave propagation utilizing surface plasmon polaritons （SPPs） on the interfaces of a thin dielectric core layer sandwiched between two corrugated metallic claddings. THz wave impinges on the structured surfaces at normal incidence. Long-lasting oscillation propagation features are observed in the temporal waveform after traveling through the periodic arrays. The enhanced THz transmission can be achieved due to the coupling between incident waves to SPPs at the bottom and top interfaces. The finite element method is used to simulate the field distribution and the transmission mode in the waveguide. The hybrid waveguide with low absorption has great potential applications in THz integrated devices.     

基于双尺度4分离层优化的石墨烯太赫兹宽带吸波结构

为实现对未来远程太赫兹雷达的高效对抗与隐身,针对典型太赫兹雷达工作频率设计了一种石墨烯太赫兹宽带吸波结构。宽带吸波结构以表层金属层/石墨烯层/介质层/底层金属层为基本吸波结构单元,利用遗传算法对双尺度基本吸波结构单元进行4分离层优化设计,确定宽带吸波结构的各层结构参数。仿真结果表明:宽带吸波结构在0.138 THz～2 THz频率范围内吸收效率优于80%,在0.157 THz～2 THz频率范围内吸收效率优于97.46%,典型太赫兹雷达工作频率处吸收效率均优于92.27%,满足太赫兹雷达对抗与隐身要求。     

Terahertz surface plasmon-polariton propagation and focusing on periodically corrugated metal wires

In this Letter, we show how the dispersion relation of surface plasmon polaritons (SPPs) propagating along a perfectly conducting wire can be tailored by corrugating its surface with a periodic array of radial grooves. In this way, highly localized SPPs can be sustained in the terahertz region of the electromagnetic spectrum. Importantly, the propagation characteristics of these spoof SPPs can be controlled by the surface geometry, opening the way to important applications such as energy concentration on cylindrical wires and superfocusing using conical structures.     

基于石墨烯加载的不对称纳米天线对的表面等离激元单向耦合器

本文设计并数值研究了一种石墨烯加载的不对称金属纳米天线对结构.利用石墨烯费米能级的动态调控特性,实现了电控表面等离激元的单向传输.类似于传统的三明治型纳米天线结构,设计的不对称金属纳米天线对结构可以等效为两个共振的磁偶极子,由于磁偶极子辐射电磁波的干涉,将导致单向传输效应.通过计算腔中的电场分布,发现石墨烯的调谐能力与石墨烯区域的电场强度成正比关系.以上现象都可以通过等效电路模型进行理论解释.此外,该结构具有小尺寸、高效率、宽带宽和易于光电集成等优点,在未来的光子集成与光电子学领域将具有重要的应用.     

A New Theoretical Modeling of Surface Resistance in Normal Metals at Terahertz Frequencies

A number of researchers have reported discrepancies between surface resistance measurements and classical theoretical predictions for simple mm-wave and sub-mm-wave structures. We have developed a rigorous phenomenological model for analyzing surface resistance of normal metals. The model is based on quantum mechanical analysis for spatial dispersion within the metal. We have used the model to predict the surface resistance in normal metals at terahertz frequencies. Our analysis shows that the conductivity is not only frequency but also wave vector dependent. Moreover, we have compared the results of this model with the results of the classical skin-effect model and classical relaxation- effect model. We demonstrate that our model has good quantitative agreement with the published experimental data for the room temperature surface resistance of normal metals at terahertz frequencies range from the literature.     

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.     

频率色散表面阻抗对真空电子太赫兹源的影响

为了研究欧姆损耗对高频真空电子器件工作特性的影响,首先推导频率色散表面阻抗边界在三维共形粒子模拟软件UNIPIC-3D中的实现原理,并通过对有耗边界矩形谐振腔和圆波导进行模拟验证了该阻抗边界算法的正确性.采用有耗共形UNIPIC-3D模拟相对论太赫兹表面波振荡器和低电压平板格栅返波振荡器.模拟结果表明,对于表面波振荡器和平板BWO这种电磁场集中在金属慢波结构附近的太赫兹真空电子器件,欧姆损耗会对器件的运行带来极大影响,对于采用铜材料的器件,输出功率会下降一半左右,器件起振时间出现延迟,但器件工作频率几乎不变.为了提高相对论太赫兹表面波振荡器的效率,在二极管和慢波结构之间增加了反射腔,模拟结果表明,在考虑器件表面损耗的条件下,器件的工作频率保持不变,输出功率由41 MW提高到60 MW.