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。     

Resonant terahertz transmission in subwavelength metallic hole arrays of sub-skin-depth thickness

We study surface-plasmon-enhanced terahertz transmission through subwavelength metallic hole arrays of sub-skin-depth thickness. Dynamic evolution of surface-plasmon resonance in terms of array thickness is characterized by use of terahertz time-domain spectroscopy in the frequency range 0.1-4.5 THz. A critical thickness of lead array film is observed, above which surface-plasmon coupling of terahertz pulses begins and is enhanced rapidly as the array thickness is increased toward the skin depth. The experimental results indicate that high-efficiency extraordinary terahertz transmission can be achieved at an array thickness of only one third of skin depth.     

Thermal switching of the enhanced transmission of terahertz radiation through subwavelength apertures

We demonstrate that the extraordinary transmission of terahertz radiation through semiconductor gratings of subwavelength apertures can be switched completely by varying the temperature. The enhanced transmission, which is due to the resonant tunneling of surface plasmon polaritons that can be excited in semiconductors at terahertz frequencies, is controlled by thermally modifying the density of free carriers. The transmission through metal gratings cannot be switched in the same way since the carrier density is temperature independent. Thus semiconductors offer an interesting alternative to metals in enhancing the transmission of electromagnetic radiation.     

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.     

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.     

Time-domain differentiation of terahertz pulses

We report on the time-domain differentiation of light waves by metallic transmission gratings. Time-resolved terahertz experiments show that the first time derivative of an arbitrary waveform can be achieved by use of gratings of subwavelength period. The results are in accord with classical diffraction theory and may permit novel applications for tailoring few-cycle light pulses and ultrahigh-frequency optoelectronics.     

Terahertz transmission properties of thin, subwavelength metallic hole arrays

We present experimental results of the transmission magnitude and phase change of terahertz pulses through thin metallic films patterned with subwavelength hole arrays on silicon wafers. Terahertz time-domain spectroscopy measurements reveal a sharp phase peak centered on the surface plasmon resonance. Correspondingly, and consistent with the Kramers-Kronig relations, the measured transmission magnitude has the shape of the derivative of this peak. In addition, we determine that the aperture shape has a notable effect on the transmission properties of two-dimensional hole arrays.     

Terahertz artificial dielectric cuboid lens on substrate for super-resolution images

This paper presents an investigation of terajets formation by dielectric periodic structure at terahertz frequencies (0.1–0.2 THz) in effective medium regime (photonic metamaterial). The dispersions of effective permittivity for periodic structures formed by plastics (ABS, black) were analytically obtained for both regimes. The study of the interaction of a plane wave and artificial cubic dielectric on a substrate parallelepiped to form a subwavelength terahertz beams (terajet) was performed. The results of numerical simulation of S-parameters of structure are discussed. The subwavelength beam waist of terajets about 0.39λ–0.41λ were obtained. The quality factor of terajets are discussed. The results may be applied for creation of methods to obtain subwavelength beams in the terahertz frequency range, and devices based on them. These investigations are opening new research lines such as mesoscale focusing devices for different sensing applications.     

Time-resolved imaging of mode-conversion process of terahertz transients in subwavelength waveguides

We studied the mode-conversion process of terahertz pulses from a planar subwavelength waveguide to a tilted rectangular subwavelength waveguide. An unusual wavefront rotation, which led to an extra conversion time, was observed using a time-resolved imaging technique. We simulated the mode conversion process by a finite-difference time-domain method, and the results agreed well with the experiments. According to the simulations, the conversion time was demonstrated to become longer as the tilt angle or width of the rectangular waveguide increased. This work provides the possibility to optimize the future high-speed communications and terahertz integrated platforms.     

环状烯烃聚合物基底材料的太赫兹复介电常数和损耗特性研究

太赫兹时域光谱(THz-TDS)已被用于研究包括液体,半导体,爆炸物和气体等多种材料。然而自由空间太赫兹光谱系统存在着一些检测局限性,如微量物质难以被检测、系统尺寸难以实现微型化、空气中水的强烈吸收引起的信号衰减较大等问题。为了解决这些问题,研究人员设计了基于金属波导传输线结构的太赫兹芯片集成器件,通过飞秒红外激光激励传输线上的光电导材料实现太赫兹波的产生和检测。然而,在这些芯片上传输的太赫兹信号的频谱宽度很难达到自由空间太赫兹时域光谱系统的频带宽度,一个重要原因是由于传输信号受到随频率增加的传输线损耗所导致的衰减。通常这些损耗主要由三个部分组成：导体损耗,介电损耗和辐射损耗。研究表明：使用低介电常数材料作为共面传输线的衬底,将减少这种介电常数的失配,从而避免冲击波辐射损失;使用具有低损耗角正切的基底材料可以减少太赫兹传输线的损耗。环烯烃聚合物(COP)是一种具有环状烯烃结构的非晶性透明共聚物的材料,在太赫兹波段具有很高的透射率,为了探究这种材料是否能用于共面传输线的衬底,需要通过太赫兹时域光谱技术和介电函数理论分析它在太赫兹频段的光谱和介电特性,以及对这种材料作为基底时用在太赫兹传输线的传输特性进行仿真计算分析。使用透射式太赫兹时域光谱系统,对三种COP、熔融石英和PMMA进行了光谱测试,提取了它们的透射时域信号,采用Dorney和Duvillaret等提出的物理模型计算复介电函数。实验表明：与其他两种材料相比,COP材料在1 THz处的透过率更高,可以达到94.5%,同时介电损耗和介电常数更低,其中介电损耗在1 THz处达到4.31×10-4,因此将COP作为传输线基底材料时能有效减少基底的介电损耗。同时COP材料的介电常数在0.2～2.8 THz范围内维持在约2.3的水平,也有效减弱了辐射损耗。对实验材料基底组成的共面波导传输线进行了HFSS模拟,获得了它们的正向传输衰减系数(S₂₁ parameter),并对由基底引起的介电损耗和辐射损耗进行了计算分析。模拟和计算结果也表明在同一传输线结构下,与其他材料相比COP作为基底时的损耗更小。通过太赫兹时域光谱法与介电响应分析,表明了在太赫兹波段具有较低介电常数的COP材料更适合作为太赫兹传输线结构的基底材料,它可以有效的降低因基底引起的介电损耗和辐射损耗。这为太赫兹传输线的设计过程中衬底材料的选择与应用提供了实验和理论依据。     

介质/金属结构太赫兹空芯光纤的传输特性

理论分析了金属、介质/金属结构空芯光纤在THz波段的模式结构和传输特性.金属空芯光纤支持TE11模式,介质/金属空芯光纤的介质膜厚在取最优值时支持HE11模式.对于波长为200μm的太赫兹波,内径为1 mm的两种空芯光纤,TE11和HE11模式的损耗分别为8.4 dB/m和2 dB/m.为优化介质/金属结构宅芯光纤的传输性能,分析了金属和介质材料的光学常数对衰减系数的影响.基于几种已发表的金属在太赫兹波段的光学常数,计算结果表明铝是最好的选择;初步测量结果显示,在各种树脂材料中聚乙烯在THz波段吸收较小,并且其折射率接近介质膜的最优值1.41,为太赫兹波空芯光纤中介质膜材料的理想选择.     

Multiband Terahertz Photonic Band Gaps of Subwavelength Planar Fractals

Optical transmission properties of subwavelength planar fractals in terahertz （THz） frequency regime are studied by means of time-domain spectroscopy. The transmission spectra with multiple pass bands and stop bands are observed. The tunable photonic band gaps are realized by changing the angle between the principle axis of planar fractal and the polarization of THz wave. The possible application of the subwavelength optical component is discussed. We attribute the detected transmittance from subwavelength fractals to localized resonances.     

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.     

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.     

亚波长金属块阵列中太赫兹波的传输特性研究

利用太赫兹时域光谱技术,研究了亚波长金属块阵列的太赫兹透射光谱特性及金属阵列结构的周期、金属块尺寸等因素对太赫兹透射特性的影响.结合时域有限差分方法,对实验结果进行了数值模拟,并分析了影响太赫兹透射的因素.结果表明:亚波长金属块阵列结构中,THz波的透射极小的位置由金属块的尺寸和周期决定,其透射谷的半高宽随其周期的增大而减小;透射峰值的位置由阵列的周期结构决定,其频率随周期的增加而减小;亚波长金属块结构的透射极小来源于金属块表面局域化电场等离子体的本征频率反射,该局域化电场与金属块结构密切相关,通过改变金属块结构,可以改变其表面电场分布与局域化.研究结果为研制太赫兹波段带阻滤波器提供了有益的参考.     

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.     

亚波长金属块阵列中太赫兹波的传输特性研究

利用太赫兹时域光谱技术,研究了亚波长金属块阵列的太赫兹透射光谱特性及金属阵列结构的周期、金属块尺寸等因素对太赫兹透射特性的影响.结合时域有限差分方法,对实验结果进行了数值模拟,并分析了影响太赫兹透射的因素.结果表明:亚波长金属块阵列结构中,THz波的透射极小的位置由金属块的尺寸和周期决定,其透射谷的半高宽随其周期的增大而减小;透射峰值的位置由阵列的周期结构决定,其频率随周期的增加而减小;亚波长金属块结构的透射极小来源于金属块表面局域化电场等离子体的本征频率反射,该局域化电场与金属块结构密切相关,通过改变金属块结构,可以改变其表面电场分布与局域化.研究结果为研制太赫兹波段带阻滤波器提供了有益的参考.     

Transmission properties of terahertz pulses through subwavelength double split-ring resonators

We present a terahertz time-domain spectroscopy study of the transmission properties of planar composite media made from subwavelength double split-ring resonators (SRRs). The measured amplitude transmission spectra reveal a resonance near 0.5 THz, the central frequency of most ultrafast terahertz systems, for one SRR orientation in normal-incidence geometry. This resonance is attributed to the effect of electric excitation of magnetic resonance of the SRR arrays. In addition, the influences of background substrate, lattice constant, and the shapes of the SRRs on the terahertz resonance are experimentally investigated and agree well with the results of recent numerical studies.     

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.     

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.