Spatiotemporal Visualization of THz Near-Fields in Metamaterial Arrays

We present an experimental approach to record the spatiotemporal electric field distribution of coherent broadband THz pulses propagating through planar metamaterial arrays. The electric field can be measured with sub-wavelength precision within a volume that is several wavelengths in size, thus, having the potential to map the near-field to far-field transition of the resonant structures constituting the metamaterial. To demonstrate the potential we present measurements of THz pulses propagating through a planar array of double split-ring resonators and their inverse analogues.     

太赫兹多波段的电磁诱导透明设计与分析

设计了一种超材料三维模型,由闭合方环和4个开口谐振方环通过正、反向双开口方环与闭合方环相互耦合来组成,在太赫兹范围内具有多波段电磁诱导透明(EIT)效应。该结构分别实现了在1.21、1.46、1.61、1.98 THz这四波段的电磁诱导透明现象,并且谐振强度均达到0.9左右。通过将结构单元进行拆分并相互对比分析,研究了该超材料结构产生多波段EIT效应的物理机理,并重点分析了开口大小、闭合方环尺寸对EIT强度与带宽的影响。通过对三维立体结构仿真分析可知,所设计的超材料不仅在多个波段获得了较高的折射率灵敏度,还具有高强度、多频点的慢光效应。因此,其在折射率传感与光缓存器件等领域,具有良好的应用前景。     

Modulating the Near Field Coupling through Resonator Displacement in Planar Terahertz Metamaterials

We present the effect of vertical displacements between the resonators inside the unit cell of planar coupled metamaterials on their near field coupling and hence on the terahertz (THz) wave modulation. The metamolecule design consists of two planar split- ring resonators (SRRs) in a unit cell which are coupled through their near fields. The numerically simulated transmission spectrum is found to have split resonances due to the resonance mode hybridization effect. With the increase in displacement between the near field coupled SRRs, this metamaterial system shows a transition from coupled to uncoupled state through merging of the split resonances to the single intrinsic resonance. We have used a semi-analytical model describing the effect of displacements between the resonators and determine that it can predict the numerically simulated results. The outcome could be useful in modulating the terahertz waves employing near field coupled metamaterials, hence, can be useful in the development of terahertz modulators and frequency tunable devices in future.     

A thermally tunable terahertz bandpass filter with insulator-metal phase transition of VO2 thin film

A terahertz bandpass filter with the sandwich structure consisting of thermally tunable vanadium dioxide （VO2） thin film, silica substrate and subwavelength rectangular Cu hole arrays is designed and theoretically analyzed. The results show that the transmittance of the filter can be actively tuned by controlling the temperature of VO2, the narrow band terahertz （THz） waves with the transmittance from 85.2% to 10.5% can be well selected at the frequency of 1.25 THz when the temperature changes from 50 ℃ to 80 ℃, and the maximum modulation depth of this terahertz bandpass fil- ter can achieve 74.7%.     

Evaluation of Cavity Modes in Superconducting Intrinsic-Josephson-Junction Oscillators for Terahertz-wave Generation

To realize antenna-coupled terahertz (THz)-wave oscillators using intrinsic Josephson junctions, rectangular mesas, 98-158 μm long, 78 μm wide and 1.04 μm high were fabricated on a Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductor, and the cavity resonant modes in the mesas were evaluated based on their radiative frequencies. For appropriate bias voltages in the return curve of an I-V characteristic of a 138-μm-long mesa, two peaks in the radiative power were observed. At the voltages where the peaks first appeared at high and low biases, frequencies of 0.646 and 0.499 THz were measured using a lamellar interferometer. The cavity modes corresponding to these two frequencies are TE₁₂₁ and TE₁₁₁ modes, respectively. The temperature and size dependencies of the calculated resonant frequencies based on a 3D cavity model agree with measurements, validating the mode assignments.     

Modeling of HTS Josephson junctions

We discuss recent developments for HTS Josephson junctions: (i) isolated (S/I) and resonantly coupled (S/I/S) Andreev interface states, (ii) d-wave models with resonantly coupled midgap states for ab-plane junctions and (iii) resonant tunneling models for c-axis junctions.     

六边形开口谐振器的太赫兹特性

电磁超材料通常是由一些在介质基底表面上周期分布的亚波长谐振器组成的。它的特性在很大程度上取决于基底的性质和谐振器的结构细节。由于开口环谐振器(SRRs)可实现多种功能,因此在超材料中应用广泛。通过设计一种二维六边形SRRs阵列,利用时域有限差分方法(FDTD)研究了单元开口形式、单元周期和介质基底性质对其传输特性的影响。研究结果表明,开口形式可使超材料在1~2.5 THz处产生谐振响应,且介质基底的性质对该谐振特性的影响权重较大。     

Terahertz Generation and Optical Properties of Lithium Ternary Chalcogenide Crystals

We have investigated the generation of THz radiation in lithium ternary compounds LiInSe₂, LiGaSe₂, LiInS₂, LiGaS₂ and characterized these materials by THz time-domain spectroscopy. Using 800 nm femtosecond excitation pulse, all crystals produce THz radiation due to an optical rectification corresponding to the nonlinear optical coefficient d ₃₃. We have measured refractive indices along the x-axis and the z-axis for all crystals in the range 150–700 μm and fitted them by using Sellmeier equation. With respect to the obtained results, velocity-matching between the incident laser pulse and the generated THz wave cannot be achieved at 800 nm, but for shorter wavelengths. Hence, an enhanced THz generation in Lithium ternary compounds may be observed by using a laser emitting below 800 nm.     

基于超材料的连续太赫兹波透射特性研究

以基于超材料的太赫兹波透射为目的,设计并制作了四种亚波长开环共振（SRR）超材料。采用连续太赫兹波作为入射激光源,实验测量了它们在1.04 ~4.25 THz波段的功率透射属性,并采用CST Studio进行仿真,结果显示这些超材料存在一个位于2.52 THz的全局透射峰和多个局部透射峰。全局透射峰与SRR阵列的微结构和图形配置等参数有关。为了寻找一个具有较高透射效率的太赫兹感应阵列,比较了四种不同超材料微结构的归一化功率透射性能和感应差别。从这些差别中找到特定图案配置的超材料器件用于太赫兹波感应具有借鉴意义。     

Synthesis of <Emphasis Type="Italic">c</Emphasis>-Axis Inclined AlN Films in an Off-Center System for Shear Wave Devices

AlN thin films are of continuing interest for excitation of acoustic waves in surface and bulk acoustic wave devices. We report herein on preparation and characterization of c-axis inclined AlN films by a new method of rotating the substrate holder plate to different angles in an off-center system. The microstructure of the c-axis inclined AlN films was investigated using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The analyses showed that polycrystalline AlN films with c-axis inclination of up to 12° could be obtained using the off-center system. Solidly mounted resonators based on the deposited c-axis inclined and vertical AlN films were successfully realized. The frequency responses showed dual-mode resonance characteristics located at 1.12 GHz and 1.87 GHz, corresponding to shear and longitudinal resonant modes, respectively.     

Ultrafast Relaxation of Charge Carriers Induced Switching in Terahertz Metamaterials

We demonstrate ultrafast switching of resonant mode in terahertz metamaterials through optical excitation of radiation-damaged silicon placed in the gap of a split-ring resonator. Upon optical excitation, we observe the dynamic transition of the fundamental resonance from ON-to-OFF state on a timescale of 4 picoseconds (ps) and then fast recovery of the resonance to the ON-state within the next 20 ps. Electric field distributions in the metamaterial unit cell derived through numerical simulations clearly support our experimental observations, showing that the high electric field at the resonator gaps, responsible for inductive-capacitive (LC) resonance, completely disappears and switches OFF the resonance after being optically excited. The ultrafast switching of the metamaterial resonance is attributed to the relaxation of the photo-carriers through the defect states of radiation-damaged silicon layer. Such ultrafast material–based active control of metamaterials can lead to the ultrafast terahertz metaphotonic devices.     

Investigation of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals at 2-MM wavelengths

Experimental investigations of superconductivity effects in single-phase and multiphase Bi₂Sr₂CaCu₂O₈ single-crystals have been carried out at 142 GHz frequency by means of the standing wave profile method [1]. Josephson harmonic generation has been observed to be responsible for the appearence of additional peaks on the standing wave profile of the open dielectric resonator loaded with a properly orientated multiphase high-T_c superconductor specimen. This leads to the conclusion that most of the Josephson junctions in multiphase crystals are located in certain crystallographic planes. The investigations of temperature dependencies showed that sharp resonant peaks of conductivity observed earlier [2] at 60 GHz could also be observed at 142 GHz.     

基于锑化铟的可调超材料太赫兹带阻滤波器

提出了一种工作在太赫兹频段, 基于半导体材料锑化铟的超材料带阻滤波器.由于锑化铟材料介电常数的特性, 该滤波器的谐振频率能够进行温度调节.同时, 通过有限积分法和等效LMC电路模型分析了滤波器的几何参数对其谐振频率的影响, 这两种方法得到的结果具有良好的一致性.在温度的取值范围是220～350 K时, 滤波器的谐振频率能够从0.91 THz动态调节到1.28 THz, 并且其阻带谐振频率的透射系数能够有限地被抑制.该滤波器的传输特性在30°入射角范围内具有良好的稳定性.设计的可调超材料带阻滤波器将在太赫兹无线通信、传感等方面有潜在的应用前景.     

复合鱼鳞型超构材料中光控Fano谐振的设计

为解决传统超构材料存在的结构一旦形成,其谐振特性便无法进行动态可调的问题,本文将鱼鳞型超构材料与光电导材料硅相结合,实现了太赫兹波段Fano电磁响应的动态调控。该复合超构材料由鱼鳞型金属线、硅层以及聚酰亚胺组成。在鱼鳞型结构的金属弧线无、有缝隙两种情况下,研究了电磁波的入射角度和硅的电导率对Fano谐振的影响。当硅的电导率达到1×10³ S/m时,多频点电磁响应的调制深度都接近1。结果表明,调节缝隙宽度可以成为Fano谐振工作频率调控的有效方式。本文为实现超构材料中Fano谐振的可调谐特性提供了一种可行途径,对实际应用中太赫兹波的主动调控、传感等方面具有重要意义。     

光控电磁超材料研究进展

电磁超材料是由亚波长尺寸单元周期或非周期排列组成的人工结构,能对电磁波的频率、幅度、相位和极化等基本物理特征进行调控,突破了传统材料的限制,可实现很多自然界不存在的有趣物理现象及应用。过去二十余年,超材料因其强大的电磁调控能力一直是物理领域的研究热点。但无源超材料在电磁波调控中存在局限性,如工作频率固定、实现功能单一等...     

基于太赫兹人工超材料的带阻滤波器

鉴于太赫兹辐射的特殊性,其难以与自然界中多数材料发生电磁相互作用,导致太赫兹功能器件匮乏.人工超材料通过人工设计结构单元的周期排列组合,可实现太赫兹波段电磁响应的调控.本文设计一种由二氧化硅衬底上的单层金属方形谐振环结构构成的太赫兹带阻人工超材料,具有窄带宽、深带阻特性、偏振不敏感特性,通过近场电场和表面电流分析,带阻...     

Josephson array oscillator with microstrip resonators for submillimeter wave region

Shunted tunnel junctions with a small parasitic inductance have been developed for improving the operating frequency of Josephson array oscillators. The inductance was minimized by reducing the inductive length to 1 μm and was estimated to be about 40 fH. The analysis of resonant properties for the shunted junctions gave a high resonant frequency up to 1.4 THz. Josephson array oscillators were designed and fabricated to operate at near Nb gap frequency (700 GHz) using 11 shunted Nb/AlOx/Nb tunnel junctions with Nb microstrip resonators. Shapiro steps induced by Josephson oscillation were clearly observed above the Nb gap frequency (up to 830 GHz). By fitting the step height to the simulation result using the RLCSJ model, the output power of the Josephson oscillator to the load resistor was estimated to be about 0.1 μW at 680 GHz.     

Terahertz Oscillating Devices Based Upon the Intrinsic Josephson Junctions in a High Temperature Superconductor

Recent developments of coherent terahertz (THz) oscillators based on the intrinsic Josephson junctions (IJJs) in mesas of the high temperature superconductor Bi₂Sr₂CaCu₂O_8+δ are reviewed. Experimental and theoretical studies of the emission from equilateral, right-angled isosceles, and acute isosceles triangular mesas are compared with those obtained from rectangular, square, and disk mesas, in order to determine the role of the mesa geometry. The superconducting properties and emission frequency f spectra are presented for a variety of triangular mesa geometries. Analytic and finite difference time domain numerical calculations of the emissions from the internal electromagnetic (EM) cavity modes of triangular mesas are compared with experiment. The experimental f always satisfies the ac Josephson relation, and its narrow linewidth arises from the synchronized emissions from many IJJs. For some mesa geometries, f also strongly locks onto an EM cavity mode frequency, enhancing the emission’s stability and output power. For other geometries, such cavity mode locking is weak, and f is highly tunable.     

基于石墨烯超材料的三频段可调谐完美吸收器

为进一步降低太赫兹频率下高性能调控器件的结构复杂度,提出一种三频段可调谐超材料完美吸收器.该吸收器由图案化的石墨烯层和经Si介质层隔开的Au接地平面组成,利用太赫兹下的石墨烯表面等离子体共振以及图案化石墨烯与电场耦合提供的电偶板子共振形成多个吸收峰.数值仿真结果表明,在0.489 THz、1.492 THz和2.437...     

8mm波段Fabry-Perot谐振腔及其在约瑟夫森结特性测量中的应用

 通过三维电磁仿真技术设计并制作了8mm波段Fabry-Perot谐振腔,并建立起一套Fabry-Perot谐振腔高温超导约瑟夫森结特性测试系统.用它测量了Tl₂Ba₂CaCu₂O₈(Tl-2212)高温超导薄膜双晶约瑟夫森结的毫米波辐照特性,得到高质量的夏皮罗台阶,并与在U形波导测试系统中测得的结果进行了比较,发现利用Fabry-Perot谐振腔测量技术可显著提高外加电磁场与约瑟夫森结的耦合效率,为进一步研究毫米/亚毫米波信号检测、THz波信号源等研究打下坚实的基础.