Thermal radiation control in the terahertz region using the spoof surface plasmon mode

In this Letter, the enhancement of thermal radiation in the terahertz region is presented theoretically by using the spoof surface plasmon mode on the microcavity array. We found that the terahertz wave is radiated selectively in a vertical direction. As a result, a quasi-monochromatic terahertz source can be created.     

Understanding fractional-order surface plasmons

We show experimentally that diffraction-induced surface plasmon excitation can mimic enhanced transmission and cause a highly sensitive modulation by the coherent interference between zero-order and reflected first-order diffraction in select regions of the terahertz spectrum. Based on the study of a one-dimensional metallic grating, we obtain the physical mechanisms of the fractional-order surface plasmon resonances observed with the two-dimensional grating of the metallic hole array.     

Propagation of THz plasmon pulse on corrugated and flat metal surface

We report here experiments on surface plasmon excitation and propagation along corrugated and smooth aluminum surface in the terahertz frequency range. Narrowband plasmon excitation by a subpicosecond terahertz pulse is shown to be a transient process and plasmon propagation sufficiently changes its measured time profile. Plasmon propagation during its excitation and detection changes measured signal. We suggest to use parameters T (plasmon duration) and τ (plasmon lifetime) to describe the narrowband THz plasmon pulse. Plasmon duration and lifetime were defined and plasmon propagation lengths on smooth and corrugated surface were measured. Plasmon propagation length on flat surface turned out to be much smaller than it is predicted by the Drude model.     

Confinement of THz surface waves on the subwavelength size metal waveguide

We investigate surface plasmon waves propagating on planar Goubau lines, so-called Goubau modes, using a guided-wave terahertz spectroscopy system based on a freely positionable electrooptic probe. We show the radial nature of the Goubau mode and its confinement around the Goubau line over a few tenths of microns (??/10).     

A diagnostic complex for studying terahertz surface plasmon polaritons generated by the Novosibirsk free electron laser

A diagnostic complex for studying surface plasmon polaritons generated by the monochromatic radiation of the Novosibirsk terahertz Free Electron Laser (FEL) is described. The complex contains three detection systems: an uncooled microbolometer focal plane array conjugated with a collecting lens, a Golay cell, and a superconducting hot-electron bolometer. Examples of experiments demonstrating the operation of this complex are presented.     

Plasmonic coupling from silver nanoparticle dimer array mediating surface plasmon resonant enhancement on the thin silver film

We investigate the optical absorption spectrum of a periodic array of silver nanoparticle dimer on a thin silver film using multiple-scattering formalism. Surface plasmon polariton mediated from silver nanoparticle dimer array is excited and enhanced by about four times compared with that from monomer array. This enhancement results from the coupling between the two nanoparticles’ plasmons of symmetry mode and anti-symmetry mode. We also illustrate the distance-dependent nanoparticle plasmonic coupling modes based on the polarized charge distribution in dimer geometry. The proposed silver nanoparticle dimer array can be used to enhance surface spectroscopy.     

Tunability and linewidth sharpening of plasmon resonances on a periodic gold nanowire array coupled to a thin textured silver film

We theoretically study the plasmon mode spectrum of a multilayer structure consisting of a periodic gold nanowire array and a spatially separated thin silver film with periodic slits. Results show that the plasmon mode spectrum of the multilayer structure depends sensitively on the relative lateral displacement of the Au nanowire array with respect to the textured silver film. This is due to the interaction between the localized surface plasmon (LSP) of the nanowire array and the plasmon modes, including the horizontal LSP and the antisymmetric short-range surface plasmon polariton (SPP), of the textured thin silver film. The strong coupling between the LSP and the antisymmetric short-range SPP results in a redshifted plasmon resonance with a significantly narrow linewidth and a large electromagnetic field enhancement. Moreover, the lateral displacement also has a great influence on the spacer layer controlled dipole-surface interaction. Therefore, this relative lateral displacement provides an efficient way to tune the optical properties of the multilayer structure, and this kind of highly tunable nanostructure can be used as a tunable plasmonic filter or a substrate for LSPs sensor.     

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.     

Analysis of plasmon terahertz waveguide formed by a dielectric ridge on semiconductor surface

We study, by the method of the effective index of refraction (EIR), the characteristics of plasmon terahertz waveguide formed by a quartz ridge above the surface of indium antimonide. EIR, propagation length, field distribution, and the condition of single-mode propagation are calculated for the fundamental TM00 mode at different sizes of the quartz ridge. It is shown that, as distinct from usual plasmon waveguides, high degree of field localization and long propagation length can be ensured simultaneously by optimum choice of ridge sizes. Comparison of field distributions calculated by the EIR technique and with COMSOL Multiphysics software indicates a good agreement, especially for not very small sizes of the ridge.     

Ultra-compact terahertz switch with graphene ring resonators

We propose and numerically demonstrate a compact terahertz wave switch which is composed of two graphene waveguides and three graphene ring resonators. Changing the bias voltage of the Fermi level in the center graphene ring, the resonant mode can be tuned when the plasmon waves in the waveguides and rings are coupled. We theoretically explain their mechanisms as being due to bias voltage change induced carrier density of graphene modification and the coupling coefficients of graphene plasmon effect after carrier density change, respectively. The mechanism of such a terahertz wave switch is further theoretically analyzed and numerically investigated with the aid of the finite element method. With an appropriate design, the proposed device offers the opportunity to ‘tune' the terahertz wave ON–OFF with an ultra-fast, high extinction ratio and compact size. This structure has the potential applications in terahertz wave integrated circuits.     

Optimization design of optical waveguide control by nanoslit-enhanced THz field

We discuss design issues of devices which were proposed recently [Opt. Lett. 37 (2012) 3903] for terahertz (THz) control of the propagation of an optical waveguide mode. The mode propagates through a nonlinear dielectric material placed in a metallic nanoslit illuminated by THz radiation. The THz field in the slit is strongly localized and thus significantly enhanced, facilitating nonlinear interactions with the dielectric waveguide material. This enhancement can lead to notable changes in the refractive index of the waveguide. The closer the waveguide is to the slit walls, the higher the nonlinear effects are, but with the cost of increasing propagation losses due to parasitic coupling to surface plasmon polaritons at the metal interfaces. We analyze several optical waveguide configurations and define a figure of merit that allows us to design the optimal configuration. We find that designs with less overlap of the THz and optical fields but also with lower losses are better than designs where both these parameters are higher. The estimated terahertz field incident onto the metallic nanoslit required to manipulate the waveguide mode has reasonable values which can be achieved in practice.     

THz波段亚波长半导体球形阵列光学特性的数值模拟

利用T-matrix方法对太赫兹波段亚波长半导体球形阵列进行了数值模拟并在数值模拟结果的基础上讨论了其光学特性。在太赫兹波段可以通过掺杂等手段调节半导体的表面等离子体特性。以半导体InSb为例并采用Drude模型,对单个亚波长球及两个或多个亚波长球组成的阵列进行了数值模拟,主要以归一化消光截面为参数,讨论了不同阵元半径、不同球形单元间距、不同单元数目及入射波不同极化方向对阵列特性的影响。     

Optical rectification of highly focused near-IR pulses in the plasmon waveguide

Generation of terahertz pulses via mixing the Fourier harmonics of strongly focused near-IR pumping laser pulses in a plasmon waveguide filled with a nonlinear medium is considered. At a certain choice of its parameters, this waveguide provides efficient transverse confinement of the terahertz mode and a low dispersion of its refractive index. As a result, the system becomes capable of generating short fairly intense terahertz pulses. Such an approach significantly (by a factor of 50) decreases the pump power compared with that in the conventional (waveguide-free) schemes, which generate terahertz pulses from single pumping pulses (optical rectification), and still yields the same intensity of the output terahertz pulse and conversion efficiency. For the Ag-GaP-Ag structure taken as an example, optimal parameters of the plasmon waveguide and pumping pulse are found and characteristics of the output terahertz pulse are calculated.     

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).     

Transmission characteristics of a microscale dielectric slab waveguide device with a deep groove and an embedded metallodielectric grating at low terahertz frequency

We discuss the transmission characteristics of a microscale dielectric waveguide device with a deep groove and an embedded metallodielectric grating illuminated by a continuous wave of TM and TE modes at low terahertz frequency. To study its performance we solve numerically the corresponding Maxwell equations by means of finite difference time domain method with uniaxial perfectly match layer as its boundary condition. By varying the angle of incident, grating filling factor and refractive index of analyte in the deep groove, it is found that the device exhibits a significant transmission enhancement for the TM mode due to the existence of surface plasmon interaction. We also demonstrate its potential application as a biosensor device.     

二维电子气等离激元太赫兹波器件

固态等离激元太赫兹波器件正成为微波毫米波电子器件技术和半导体激光器技术向太赫兹波段发展和融合的重要方向之一。本综述介绍AlGaN/GaN异质结高浓度和高迁移率二维电子气中的等离激元调控、激发及其在太赫兹波探测器、调制器和光源中应用的近期研究进展。通过光栅和太赫兹天线实现自由空间太赫兹波与二维电子气等离激元的耦合,通过太赫兹法布里-珀罗谐振腔进一步调制太赫兹波模式,增强太赫兹波与等离激元的耦合强度。在光栅-谐振腔耦合的二维电子气中验证了场效应栅控的等离激元色散关系,实现了等离激元模式与太赫兹波腔模强耦合产生的等离极化激元模式,演示了太赫兹波的调制和发射。在太赫兹天线耦合二维电子气中实现了等离激元共振与非共振的太赫兹波探测,建立了太赫兹场效应混频探测的物理模型,指导了室温高灵敏度自混频探测器的设计与优化。研究表明,基于非共振等离激元激发可发展形成室温高速高灵敏度的太赫兹探测器及其焦平面阵列技术。然而,固态等离激元的高损耗特性仍是制约基于等离激元共振的高效太赫兹光源和调制器的主要瓶颈。未来的研究重点将围绕高品质因子等离激元谐振腔的构筑,包括固态等离激元物理、等离激元谐振腔边界的调控、新型室温高迁移率二维电子材料的运用和高品质太赫兹谐振腔与等离激元器件的集成等。     

等离子体辅助平板波导的传输特性及应用研究

如何灵活地控制和操纵太赫波是目前研究的热点. 根据电磁波传输理论, 导出了等离子体辅助平板波导的场分布和色散关系表达式, 计算了其传输特性, 并通过全波仿真进行了证实. 结果表明, 等离子体辅助平板波导具有带阻特性, 上边带截止频率等于等离子体频率, 等离子体层越薄, 下边带截止频率越高, 带宽越窄; 阻带内存在两种不同的物理机理, 一种与等离子体和中间媒质的谐振耦合有关, 另一种与表面波的形成有关. 此外, 本文还研究了等离子体频率及碰撞频率对传输特性的影响, 提出了通过改变等离子体频率调谐平板波导滤波器特性的方法. 同时, 采用褶皱金属结构实现了等离子体层, 设计了平板波导传感模型, 通过改变凹槽内的材料的介电常数仿真了其传感特性, 结果表明当材料的介电常数变化0.1%时, 平均截止频率变化1.8 GHz; 通过检测截止频率的变化, 传感器能明显分辨氮、汽油、液态石蜡、甘油和水, 证实了其优良的太赫传感特性. 这项工作对研究太赫波的传输及太赫器件的设计和制备具有指导意义.     

Anomalous terahertz transmission in bow-tie plasmonic antenna apertures

Arrays of subwavelength dipole bow-tie apertures are designed and characterized at terahertz frequencies. For an incident terahertz field perpendicular to the longer axis of the bow tie, a strong resonance enhancement, line narrowing, and a nonmonotonic frequency shift were observed with increasing length of the tapered bow-tie arms. Such characteristic behaviors primarily originate from localized surface plasmon resonances. In addition, with a decreasing aperture size, the contribution of localized plasmons becomes prominent due to an increase in plasmonic lifetime as the terahertz pulses strongly couple with the metallic surface surrounding the bow-tie apertures.     

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.     

Dispersion of surface plasmon polaritons on metal wires in the terahertz frequency range

We report the experimental and theoretical study of the dispersive behavior of surface plasmon polaritons (SPPs) on cylindrical metal surfaces in the terahertz frequency range. Time-domain measurements of terahertz SPPs propagating on metal wires reveal a unique structure that is inconsistent with a simple extrapolation of the high frequency portion of the dispersion diagram for SPPs on a planar metal surface, and also distinct from that of SPPs on metal nanowires observed at visible and near-infrared frequencies. The results are consistent with a numerical solution of Maxwell's equations, showing that the dispersive behavior of SPPs on a cylindrical metal surface at terahertz frequencies is quite different from that of SPPs on a flat surface. These findings indicate the increasing importance of skin effects for SPPs in the terahertz range, as well as the enhancement of such effects on curved surfaces.