Polarization-Independent Directional Beaming of Light by a Subwavelength Metal Slit

We present the directional beaming effect of light at the terahertz frequency by using a subwavelength slit in the metal film. The metal is dressed with anisotropic dielectric so that both the transverse electric （TE） and transverse magnetic （TM） polarized waves can be well guided on the metal surface and reach the phase matching. By using a periodical array of dielectric ridges and grooves around the slit, the guided waves can be scattered out of the slit and interfere with the transmitted light directly through the slit. The results performed by finite-difference at time-domain computations indicate that the directional beaming of light can be obtained simultaneously for both the TE and TM polarized waves after optimizing the geometric parameters. The structure may find great applications in polarization-independent optical devices such as couplers, connectors, beam collimator, and etc.     

Directional terahertz radiation through groove-assisted hole array

The authors theoretically demonstrate the terahertz beam shaping with metallic subwavelength holes array surrounded by concentric periodic grooves. High transmission and directional radiation can be obtained simultaneously for the resonant excitation of the surface wave in the combined structure. Finite-difference time-domain simulation results shows that the transmission mainly depends on the lattice constant of hole array and the features of incident surface around it, while the far-field angle distribution is dominated by the details of the output surface. This compact beam shaping structure is hoped to serve as a basic device for future terahertz systems.     

Investigation of the propagation properties of terahertz waves through a semiconductor subwavelength slit

The propagation properties of terahertz (THz) waves through semiconductor conductor-dielectric-conductor (CDC) structure have been investigated. The influence of geometrical parameters, radiation frequencies and temperatures on the propagation properties have been shown and discussed. The contour results demonstrate that as the length of the dielectric filling materials increase, the effective indices of the propagation modes increase. Compared with the results of the metal structure, the effective indices of surface modes through semiconductor InSb slits increase, the propagation length decrease. The effective indices of the surface modes decrease, the propagation lengths increase with the increasing of the carrier concentration. It is expected that the numerical results may be very helpful to have better understand the propagation mechanism of THz waves through semiconductor subwavelength slit.     

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.     

Effect of dielectric properties of metals on terahertz transmission subwavelength hole arrays

We study the influence of dielectric function of metals on the transmission properties of terahertz pulses through periodically patterned subwavelength holes. Because of a drastic increase in the value of dielectric constants, most metals become highly conductive at terahertz frequencies. Extraordinary terahertz transmission is observed in subwavelength hole arrays made from both good and poor electrical conductors. The measured transmittance of terahertz pulses is found to be enhanced with increasing ratio of the real to the imaginary dielectric constant of the constituent metals, for which the dielectric function follows the Drude model.     

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.     

Simulation investigation on waveguide properties of terahertz wave through subwavelength semiconductor gap

The waveguide propagation properties of terahertz wave through subwavelength semiconductor trench have been simulationally investigated. The effects of gap width, temperature, doping concentration and dielectric filling materials on waveguide property have been given and discussed. The results show that as temperature and doping concentration increases, the skin depth and the propagation constant decreases. In addition, the effective index increases and the propagation length decreases as the dielectric constant of filling materials increases.     

Compound polarized wavelength filters with a single subwavelength structure

A novel dual functional device that combines functions of polarizing and bandpass filtering together using a single subwavelength structure is proposed. Characteristics of both wavelength filtering and polarizing in the visible wavelength range with two different multi-layer subwavelength structures are investigated. It is found that both filtering and polarizing functions can be realized simultaneously by appropriate design of the substrate, a dielectric layer and a metallic layer system. Dependence of tunable filtering central wavelength on the properties of layered materials and structural dimensions are discussed in detail. Typical optimized multilayer structural parameters are obtained, in which ~ 75% passband transmission with > 30 dB polarization extinction ratio have been achieved simultaneously for three primary color (red, green and blue) filters. The results open new possibilities in designing and fabricating novel multi-functional polarizing and filtering photonic devices using a single subwavelength structure.     

Inhomogenous dielectric metamaterials with space-variant polarizability

We experimentally demonstrate for the first time the focusing of optical beams within an inhomogeneous dielectric metamaterial with space-variant polarizability, implemented by etching subwavelength structures into a Silicon slab. Light focusing is obtained by creating an artificial slab material with graded refractive index profile. The local refractive index within the slab is modulated by controlling the duty cycle of the subwavelength structures. The demonstrated metamaterial based component can be integrated with various other building blocks towards the realization of devices and systems in free space optics on a chip configuration.     

Broadband Terahertz Polarization Converter and Asymmetric Transmission Based on Coupled Dielectric‐Metal Grating

Coupled dielectric‐metal gratings are investigated for broadband terahertz (THz) wave polarization conversion and asymmetric transmission by the experiments and numerical simulations, which are composed of the subwavelength Si grating and metallic wire grating layers. The dielectric grating layer with a large artificial birefringence and low dispersion is employed as a phase engineered waveplate, and the metal wire grating arranged with a 45° angle to the dielectric grating is utilized as a high‐efficiency polarizer. Due to the subwavelength integration, this coupled grating presents a local resonance coupling mechanism between dielectric and metal gratings, which greatly enhances the polarization rotation and expands the bandwidth, not a simple combination with dielectric and metallic gratings. The results demonstrate that a broadband asymmetric transmission with an extinction ratio of 30dB from 0.2 to 1.2 THz is achieved and the highest transmission of 90% can be obtained. It provides a simple way towards practical applications for THz artificial dispersion materials, polarization control and asymmetric transmission.     

A terahertz polarization insensitive dual band metamaterial absorber

Metamaterial absorbers have attracted considerable attention for applications in the terahertz range. In this Letter, we report the design, fabrication, and characterization of a terahertz dual band metamaterial absorber that shows two distinct absorption peaks with high absorption. By manipulating the periodic patterned structures as well as the dielectric layer thickness of the metal-dielectric-metal structure, significantly high absorption can be obtained at specific resonance frequencies. Finite-difference time-domain modeling is used to design the structure of the absorber. The fabricated devices have been characterized using a Fourier transform IR spectrometer. The experimental results show two distinct absorption peaks at 2.7 and 5.2?THz, which are in good agreement with the simulation. The absorption magnitudes at 2.7 and 5.2?THz are 0.68 and 0.74, respectively.     

All-optical switching of the transmission of electromagnetic radiation through subwavelength apertures

Unprecedented optical control of the surface plasmon polariton assisted transmission of terahertz radiation through subwavelength apertures is rendered possible by carrier-induced changes to the dielectric properties of a semiconductor grating. Although the study presented is static, the extension of our approach to dynamic switching and tuning is deemed straightforward, opening the way for the realization of ultrafast surface plasmon based devices.     

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.     

一维亚波长周期结构物成像的光子学研究

当用波长为λ的单色平行光垂直照射到光栅常数d<λ的亚波长周期结构衍射物时,会产生隐失波。由于快速衰减,这种亚波长周期物是不能成像的。但是,通过对亚波长周期结构物进行适当的编码后可得到均匀波,使这种携带了隐失波信息的均匀波通过经特别设计的光学系统并被放大到CCD所能识别的像素大小后,再进行必要的解码以滤掉编码波,即可得到原亚波长周期结构衍射物的像,最终达到超分辨的目的。基于这种新颖的成像技术,用常规仪器就实现了对亚波长周期结构物的成像。在运用光子学方法对实验和成像过程进行较为详尽的理论分析的同时,对编码器、解码器的位置以及它们相对物光栅的取向给出了设计性研究,对滤波器的选择给予了必要的说明。实验结果验证了该设计理论的正确性。     

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.     

基于双开口谐振环超表面的宽带太赫兹涡旋光束产生

提出了一种基于双开口谐振环单元结构超表面的太赫兹宽带涡旋光束产生器.该结构由金属-电介质两层构成,位于顶层的是基于双开口谐振环单元结构的超表面,底层为介质层.对单元结构阵列进行数值仿真,圆偏振的入射光可以被转换成相应的交叉偏振透射光,通过旋转表层金属谐振环,可以控制交叉偏振透射光具有相同的振幅和不同的相位.这些单元结构按照特定的规律排列,可以形成用以产生不同拓扑荷数的涡旋光束的涡旋相位板.以拓扑荷数1和2为例,设计了两种涡旋相位板,数值分析了圆偏振波垂直入射到该涡旋相位板生成交叉圆偏振涡旋光束的特性.结果表明,在1.39—1.91 THz的频率范围内产生了比较理想的不同拓扑荷数的涡旋光束,且透过率高于20%,最高可达到24%,接近单层透射式超表面的理论极限值.     

Outcoupling surface plasmons into propagation modes using a subwavelength dielectric grating for device applications

Coupling of surface plasmon polaritons to radiation modes by use of a one-dimensional subwavelength dielectric grating on a thin metal slab is discussed. The surface plasmon waves obtained in Kretschmann configuration are resonant outcoupled to radiation modes by using a subwavelength dielectric grating. A peak outcoupling efficiency is predicted to be 74.57% with rigorous coupled-wave analysis. In addition, potential applications of these results in the design and improvement of various optoelectronic devices, such as polarizers, wavelength filters and biochemical sensors are discussed.     

基于叠栅技术的二维亚波长周期结构成像设计

用光子学方法研究了叠栅技术中,当试件光栅被拉压和旋转后,叠栅条纹的空间周期和相对于基准光栅的取向,试件光栅被拉压后的节距等相关问题。根据衍射光波的空间周期可能大于试件光栅空间周期的特点,对二维亚波长周期结构衍射成像进行了设计研究。首先,对二维亚波长周期结构衍射物进行编码,以获得包含编码光栅空间信息的均匀波;其次,使编码得到的均匀波通过光学系统,并被放大到CCD相机所能辨识的大小;再次,经解码光栅解码,滤掉编码波,最终获得二维亚波长周期结构物的空间结构信息,达到超分辨的目的。同时,对成像过程进行了较为详尽的分析,对编码器、解码器的位置以及它们相对衍射物的取向进行了设计研究,对滤波器的选择给予了必要的说明,指出了取得超分辨成像的关键。     

Engineering the electromagnetic properties of periodic nanostructures using electrostatic resonances

Electromagnetic properties of periodic two-dimensional subwavelength structures consisting of closely packed inclusions of materials with negative dielectric permittivity epsilon in a dielectric host with positive epsilon(h) can be engineered using the concept of multiple electrostatic resonances. Fully electromagnetic solutions of Maxwell's equations reveal multiple wave propagation bands, with the wavelengths much longer than the nanostructure period. Some of these bands are described using the quasistatic theory of the effective dielectric permittivity epsilon(qs). Those bands exhibit multiple cutoffs and resonances which are found to be related to each other through a duality condition. An additional propagation band characterized by a negative magnetic permeability is found. Imaging with subwavelength resolution in that band is demonstrated.     

Analysis of artificial dielectric lens with metallic rectangular chips for terahertz wave band and physical explanation by periodic model

Optical devices for the terahertz wave band are being developed now and require better designs. This paper analyzes an artificial dielectric lens with metallic rectangular chips for the terahertz wave band. This paper also provides an explanation of the phenomena by use of a periodic model. The periodic analysis model, extracted from the full one by assuming periodicity, confirms the phase delay as the mechanism that produces the focusing effect. Furthermore, the results of the full model confirm that the focusing length is longer with the larger periodicity of rectangular metal chips along the direction transverse to the propagation direction. It also indicates a nonuniform change for the periodicity along the propagation direction and the longer focusing length with narrower rectangular chips. The results of the full model analysis are qualitatively consistent with those of the periodic model one. This implies that the design for an exact size lens is possible through use of the periodic model.