Fabrication of terahertz metamaterials by laser printing

A laser printing technique was used to fabricate split-ring resonators (SRRs) on Si substrates for terahertz (THz) metamaterials and their resonance behavior evaluated by THz time-domain spectroscopy. The laser-printed Ag SRRs exhibited sharp edge definition and excellent thickness uniformity, which resulted in an electromagnetic response similar to that from identical Au SRR structures prepared by conventional photolithography. These results demonstrate that laser printing is a practical alternative to conventional photolithography for fabricating metamaterial structures at terahertz frequencies, since it allows their design to be easily modified and optimized.     

Identifying magnetic response of split-ring resonators at microwave frequencies

In this study we provide experimental methods to identify the magnetic resonance of split ring resonantors (SRR) at the microwave frequency regime. Transmission measurements were performed on both single SRR unit cell and periodic arrays of SRRs. The magnetic response of the SRR structure was demonstrated by comparing the transmission spectra of SRRs with closed ring resonators (CRR). Effects of the changes in the effective dielectric constant of the SRR medium on the band-gaps of SRR are investigated experimentally. SRRs not only exhibit a magnetic resonance band gap but also a band gap due to the electric resonance. Finally, we present the effect of electric coupling to the magnetic resonance of bianisotropic SRRs by utilizing SRRs with different orientations, and incident electromagnetic wave polarizations.     

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.     

太赫兹频段开口环谐振器的可调谐振模式转换

在硅衬底上设计了一种单开口环谐振器，对其太赫兹频段内的透射性质进行了研究。假定通过光注入方式改变衬底硅的电导率，实现了谐振环的双谐振透射率可调。将砷化镓材料生长于该谐振环的开口处，通过光注入方式改变砷化镓材料的电导率，可以实现谐振环的双频LC共振和偶极子共振模式与单频闭合环共振模式之间的转换。这种通过光注入改变半导体材料电导率的方法，可以在不破坏原来谐振器件物理结构的前提下，实现谐振环谐振模式的可逆转换。     

基于Rydberg原子天线的太赫兹测量

Rydberg原子在微波和太赫兹频段具有极大的电偶极矩,利用量子干涉效应可实现对该频段电磁波场强的高灵敏探测,理论上灵敏度可达到远高于现有探测技术的水平.基于Rydberg原子量子效应的电磁场探测及精密测量技术在太赫兹的场强和功率计量、太赫兹通信和太赫兹成像等方面有着巨大的应用前景.本文回顾了基于Rydberg原子量子干涉效应实现电磁波电场自校准和可溯源测量的基本理论和实验技术,详细介绍了基于Rydberg原子的高灵敏太赫兹场强测量、太赫兹近场高速成像和太赫兹数字通信的基本原理和技术方案.最后简单介绍了本研究团队正在开展的基于Rydberg原子的太赫兹探测工作.     

Terahertz absorption by electrons and confined acoustic phonons in free-standing quantum wells

The acoustic phonon confinement in a free-standing quantum well (FSQW) results in an acoustic phonon energy quantization. Typical quantization energies are in the terahertz frequency range. Free electrons may absorb electromagnetic waves in this frequency range if they emit or absorb acoustic phonons. Therefore, the terahertz absorption reveals the characteristic features of the acoustic phonon spectrum in free-standing structures. We have calculated the absorption coefficient of an electromagnetic wave by free electrons in a FSQW in the terahertz frequency range. We took into account a time dependent electric field, an exact form of the acoustic phonon spectrum and eigenmodes, and electron interactions with confined acoustic phonons through the deformation potential. We demonstrate numerical results for GaAs FSQW of width 100 Å at low lattice temperatures in the frequency range 0.1-1 THz. The absorption coefficient exhibits several structures at frequencies corresponding to the lowest acoustic phonon modes. These features occur due to absorption of photons by electrons, which is accompanied by the emission of corresponding acoustic phonons.     

Manipulation of terahertz radiation using metamaterials

During the past decade electromagnetic metamaterials have realized many exotic phenomena that are difficult or impossible using naturally occurring materials. It is their resonantly enhanced interaction with electromagnetic waves that underpins their attractive qualities, which are increasingly important in the terahertz frequency range. Passive and active terahertz metamaterials and devices have enabled novel functionality and unprecedented terahertz device performance. These demonstrations prove their potential to address the so‐called terahertz gap, a technology vacuum associated with the deficiency of natural materials with a desirable terahertz response.     

人工异向介质调控电磁波极化特性的实验与仿真研究

电磁波的极化特性在通信、导航和雷达等方面已逐渐得到应用.为了有效控制电磁波的极化状态, 本文设计了一种基于开口环结构的人工异向介质.该人工异向介质由开口环结构,电介质基底和金属背板组成, 可以将入射的线性极化波完整地转换为圆极化波,椭圆极化波以及和入射波极化方向垂直的线极化波. 本文通过实验和仿真验证了本文的设计,实验结果和仿真结果符合较好.     

射电天文用太赫兹三通带频率选择表面设计

本文设计一种基于单屏改进型开口谐振环(SRR)的太赫兹频率选择表面(FSS). 改进型 SRR 谐振单元由具有开口缝的金属贴片组成, 开口缝的物理尺寸会影响其阶跃特征阻抗特性. 本文通过对改进型 SRR 单元结构建立LC等效电路模型, 提取等效电路模型参数, 并结合传输线理论, 得到 FSS 的基频计算公式和谐波关系式. 相比于传统均匀 SRR, 本文所提出的改进型 SRR 多频带传输的控制更为灵活. 基于此特点, 设计了一款中心频率依次为 0.46 THz, 0.86 THz和 1.03 THz, 可应用于射电天文的三通带太赫兹 FSS. 采用电磁仿真软件对影响该 FSS 传输特性的关键参数、周期间隔、小型化程度以及入射角敏感性等重要指标进行分析研究. 结果表明, 改进型 SRR 三通带 FSS 在三个通带内的反射系数分别为 -37.6 dB, -13 dB和 -19.6 dB, 在0°–60° 范围内均具有稳定的频率响应特性, 且具有小型化程度高、损耗低等特点. 这种三通带 FSS 在太赫兹频段射电天文方面有潜在的应用价值.     

Generation of unipolar pulses in nonlinear media

Methods recently proposed for generating unipolar pulses in nonlinear media in terahertz and optical electromagnetic ranges are reviewed. Such pulses have nonzero “electric area” (time integral of the field strength over the entire duration of a pulse) and, correspondingly, a significant component of the field with zero frequency, thus exhibiting quasistatic properties. Effective generation of unipolar pulses would allow, e.g., transferring mechanical momentum to charged particles and, thereby, controlling the motion of wave packets of matter, which can be useful for compact accelerators of charged particles and for other applications.     

非对称开口六边形谐振单环的微波透射特性

受自然界树型结构的启发，设计了分叉树型微结构单元，用电路板刻蚀技术制作了非对称开口六边形谐振单环及其组合结构.采用计算机模拟和实验研究了单个和多个谐振环在微波段(7—12 GHz)的电磁响应行为.研究结果表明：该结构具有负磁导率特性，开口谐振环几何尺寸影响环的磁谐振频率；两环环间距较小时出现二次谐振；带高级分支的辐射状环列相对于不带分支环列透射峰向低频移动.实验和模拟结果相符. 关键词： 负磁导率 开口谐振环 透射率     

On the possibility of terahertz wave generation upon dense gas optical breakdown

The possibility of applying the physical principles underlying the operation of a solid-state terahertz oscillator to plasma gas-discharge media is analyzed. A new method of electromagnetic wave generation in the terahertz frequency range is proposed. It is based on the excitation of plasma-oscillation current by a static electric field in a laser spark produced by an axicon lens, followed by the radiation of an electromagnetic pulse through the Cherenkov mechanism.     

Terahertz electromagnetic waves emitted from semiconductor investigated using terahertz time domain spectroscopy

Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy (THz-TDS).The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material,thus,THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale....     

太赫兹波段谐振频率可调的开口谐振环结构

开口谐振环(SRRs) 结构可以激励磁谐振, 实现负的磁导率. 提出在SRRs两环间隙内引入相对交错的金属短线, 并研究了金属短线对谐振频率的影响. 结果表明: 随着金属短线数目的增多, 谐振频率显著降低, 同时金属短线的结构参数如长度、宽度及间距也会对谐振频率产生影响. 最后验证了金属短线的引入对减小器件尺寸有明显作用, 且不因介质基底的存在而受到影响. 新型磁谐振单元可为今后超材料的设计及实际应用提供新的参考. 关键词： 左手材料 开口谐振环 太赫兹 小型化     

基于微流控技术的磁流体载基液的太赫兹透射特性研究

太赫兹(THz)是指频率在0.1～10 THz的电磁波,其波长在30～3 000 μm范围内。由于自然界许多小分子的振动、转动等的频率均在太赫兹波段,并且太赫兹的低电子能特性使其在实验过程中不会对待测样品造成破坏,所以太赫兹技术被广泛地应用于无损检测、生物医学等领域。但是太赫兹在铁磁领域的相关报道还是较少的,因此本研究利用太赫兹时域光谱系统研究了一种新型磁性材料：磁流体的组成部分-载基液的太赫兹透射特性。磁流体是一种兼具液体流动性和固体磁性的新型功能材料,其打破了传统磁性材料的固体形态。磁流体由Fe₃O₄纳米级颗粒以及载基液构成。在前人的研究成果中发现磁性液体不仅具有良好的磁光效应,而且对于一定频率的太赫兹波具有高透射率;另外,在极低频电磁场作用下其可用于医学上的肿瘤治疗,可作为靶向治疗的载药系统。由于磁流体的组成部分-载基液成本较高,因此在实验中运用了微流控技术。微流控技术对检测样品的消耗少、检测速度快,并且可以根据实验需求自行设计沟道,因此是一种便捷的、灵活性好的检测方式。采用对太赫兹波具有高透过率的石英材料制成了夹心式的太赫兹微流控芯片。首先将两块3 cm×3 cm×2 mm的石英玻璃作为基片和盖片,再把强粘黏性双面胶剪刻成镂空样式,形成2 cm×2 cm的方形区域,然后把盖片和基片通过雕刻好的强粘黏性双面胶键合,其沟道厚度为50 μm,可以用于对少量液体的探测,并且可以使载基液呈薄膜状。之后将太赫兹技术和微流控技术相结合,利用太赫兹时域光谱(THz-TDS)系统研究了载基液的太赫兹透射特性,通过对太赫兹时域光谱以及频域光谱的研究发现,装有载基液的微流控芯片的信号强度高于空的微流控芯片,这一发现为载基液的应用和深入研究提供了技术支持。     

Energy filtration effect and the possibility of the generation of terahertz radiation in resonant tunneling structures with several quantum wells

The properties of a high-frequency response in resonant tunneling double-well nanostructures have been considered for various energies of electrons arriving to a structure of electrons, various frequencies of the external electromagnetic field, and various features associated with the interaction of electronic states in neighboring quantum wells in double-well nanostructures. The energy filtration effect that is caused by the breaking of the symmetry of the high-frequency response in double-well nanostructures in a static electric field has been revealed. This effect leads to a sharp increase in the gain under conditions of the quantum amplification regime and opens real prospects of a significant increase in the efficiency of solid amplifying and generating devices based on resonant tunneling double-well nanostructure in the subterahertz and terahertz frequency ranges.     

生物基底材料的超宽频带太赫兹光谱研究

作为一种新兴的方式,太赫兹时域光谱和成像已经被广泛应用到研究不同生物组织的光学特性。在空气等离子体处施加偏置电场对太赫兹波脉冲进行外差式相干检测(air-biased-coherent-detection,ABCD)的太赫兹系统具有超宽频带和可以在较远距离进行成像的优点,十分适用于对生物组织进行超宽谱研究,而对生物组织进行光谱测量通常需要基底材料。利用太赫兹ABCD系统对四种典型的基底材料(石英,高密度聚乙烯,聚四氟乙烯和石蜡)的光学参数进行测定,并计算其在1~15THz频率范围内的吸收系数和折射率。结果表明,高密度聚乙烯和石蜡可以很好的被用作生物组织超宽频带太赫兹光谱测量的基底材料。同时,虽然石英和聚四氟乙烯都是窄带(0.1~3THz)太赫兹系统中常用的基底材料,但是由于它们在高于5THz的频率范围内对太赫兹波具有较强的吸收,所以不能用作超宽频带太赫兹光谱测量的基底材料。     

Plasma wave field effect transistor as a resonant detector for 1 terahertz imaging applications

The possibility of using plasma wave field effect transistor in a time domain terahertz (THz) spectroscopy setup is presented. We demonstrate that High Electron Mobility Transistors (HEMTs) is an efficient device for detection of pulsed terahertz electric fields generated with a femtosecond laser oscillator. The response was observed in the frequency range of about 1 THz, far above the cutoff frequency of the transistors at room temperature. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel and that significant improvement of the active device can be achieved by increasing the drain current. The two-dimensional terahertz imaging applications clearly demonstrate that plasma wave nanometer HEMT should be employed as efficient future detectors in a matrix configuration.     

Frequency-tunable terahertz electromagnetic wave emitters based on undoped GaAs/n-type GaAs epitaxial layer structures utilizing sub-picosecond-range carrier-transport processes

We have investigated sub-picosecond-range carrier-transport processes in undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial layer structures with various i-GaAs-layer thicknesses d ranging from 200 to 2000 nm, focusing on the relation between carrier-transport processes and terahertz electromagnetic wave frequency. Initially, using numerical simulation and photoreflectance measurement, we confirm that a decrease in d enhances the built-in electric field in the i-GaAs layer. In the time-domain terahertz waveform, it is observed that the intense monocycle oscillation induced by the surge current of photogenerated carriers, the so-called first burst, is followed by the oscillation patterns originating from the coherent GaAs longitudinal optical (LO) phonon. From the Fourier power spectra of the terahertz waveforms, it is clarified that the decrease in d causes a high frequency shift of the band of the first burst. Consequently, we conclude that, in the sub-picosecond time range, the photogenerated carriers are monotonously accelerated by the built-in electric field without being affected by intervalley scattering. The present conclusion signifies that the frequency-tunable terahertz emitters are realized by controlling i-GaAs-layer thickness. We also find the intensity of the coherent LO phonon band is enhanced by a decrease in d.     

不同厚度衬底上矩形孔金属结构的太赫兹透射特性数值分析

利用时域有限差分法对不同材料和厚度衬底上矩形孔金属结构太赫兹（THz）波的透射特性进行了数值分析。研究表明：矩形孔金属阵列对THz光谱具有频率选择特性,这为太赫兹滤波功能器件的开发提供了基础。在实际应用中,这种金属微结构常常需要制备在某种衬底上,而衬底材料及厚度无疑会对其太赫兹透射特性带来影响。研究了不同衬底材料和衬底厚度的矩形孔金属结构太赫兹透射性质,通过数值模拟研究发现,对于同一种衬底材料,随着衬底厚度的增加,矩形孔金属结构的透射峰峰位向低频移动。比较高阻硅和聚四氟乙烯两种衬底材料发现,高介电常数的硅衬底引起的透射峰移动更为明显。