Circular polarization converters based on bi-layered asymmetrical split ring metamaterials

In this paper, a kind of bi-layered asymmetrical split ring metamaterial was proposed as a circular polarization converter. Simulations and experiments at the microwave regime showed that the proposed structures can achieve the conversions from right-handed circularly polarized electromagnetic waves to left-handed ones and the reversed conversions in the opposite propagating direction. The linear to circular polarization transmission coefficients and the surface currents were investigated to understand the mechanism of the circular polarization conversions. Moreover, we optimized the proposed metamaterials by increasing the distance between the two metal layers. The proposed circular polarization converters have applications in microwave wave plates and metamaterial antennas.     

An ultra-broadband and highly-efficient tunable terahertz polarization converter based on composite metamaterial

A near perfect, ultra-broadband and highly-efficient terahertz reflective polarization converter based on multilayer metamaterial is proposed in this paper. The hybrid metamaterial unit structure consists of a split-ring metal pattern and a metal sheet separated by a dielectric resonator spacer. The polarization conversion ratio (PCR) is above 90% from 2.06 to 4.26 THz, with an optimal range between 2.98 and 4.16 THz where the efficiency is above 98% for normal incidence. It also shows excellent performance for oblique incidences. Moreover, the working band and the optimized frequency range for the polarization conversion can be manipulated by changing the open angle between the metallic arcs and the substrate thickness, respectively. Based on this design, two types of hybrid metamaterial converters are also investigated to swith off polarization conversion altogether or modify the polarization conversion bandwidth. A polarization converter with temperature controlled PCR is realized by adding a vanadium oxide (VO₂) mask on the designed structure. In addition, by integrating photoconductive silicon islands between the split-ring and metal bar a dual-band polarization converter can be realized. Combining with a polarizer, this window can act as an active THz filter.     

基于谐振环的太赫兹宽带偏振转换器件研究

提出了一个基于谐振环结构的宽带且高效的太赫兹线偏振转换器.该结构由金属-电介质-金属三层构成,位于顶层的是基于开口谐振环的超表面,中间为介质层,底部为金属板.实验结果表明,该结构可以在0.59-1.24 THz频率范围内将线偏振的太赫兹波偏振方向旋转90°,转换率超过80%.通过计算该结构在所研究的频率范围内反射光的偏振角和椭圆角,证实了该结构可以在较宽的频率范围内实现高效的线偏振转换.对该结构在偏振转换率高的频率下表面电流和电场进行仿真,分析了高偏振转换率和宽带的机理.同时,研究了该结构的偏振转换率对入射角以及偏振角的依赖性,结果表明该结构在0°-30°入射角范围内、-10°-10°偏振角范围内均有很好的偏振转换性能.     

Collisional Lifetimes of Elementary Excitations in Two-Dimensional Systems in the Field of a Strong Electromagnetic Wave

A two-dimensional system with two nonequivalent valleys in the field of a strong circularly polarized electromagnetic wave is considered. It is assumed that the optical selection rules for a given polarization of light allow band-to-band transitions only in valleys of one, optically active, type (two-dimensional layer based on transition metal dichalcogenides, gapped graphene, etc.). This leads to the formation of photon-coupled electron–hole pairs, or an “optical insulator” state. It is assumed that the valleys of the second type (optically inactive) are populated with an equilibrium electron gas. The relaxation of elementary excitations in this hybrid system consisting of an electron gas and a gas of electron–hole pairs caused by the Coulomb interaction between the particles is investigated.     

Dual-band and high-efficiency polarization converter based on metasurfaces at microwave frequencies

We present a dual-band and high-efficiency polarization converter in microwave regime. The proposed converter can convert a linearly polarized wave to its cross-polarized wave for two distinct bands: Ku (11.5–20.0 GHz) and Ka (28.8–34.0 GHz). It can also convert the linearly polarized wave to a circularly polarized wave at four other frequencies. The experimental results are in good agreement with simulation results for both frequency bands. The polarization conversion ratio is above 0.94 for the Ku-band and 0.90 for the Ka-band. Furthermore, the converter can achieve dual-band and high-efficiency polarization conversion over angles of incidence up to 45°. The converter is also polarization-selective in that only the x- and y-polarized waves can be converted. The physical mechanism of the dual-band polarization conversion effect is interpreted via decomposed electric field components that couple with different plasmon resonance modes of the structure.     

金膜上亚波长小孔阵列表面等离激元颜色滤波器偏振性质

金属薄膜上制备的表面等离激元颜色滤波器具有很强的颜色可调性. 在200 nm厚的金膜上, 通过聚焦离子束刻蚀, 制备一系列周期逐渐变化的圆形、方形、矩形亚波长尺寸小孔方阵列表面等离激元颜色滤波器, 改变入射光的偏振方向, 观察其超透射滤波现象. 研究发现: 对于矩形小孔阵列, 其透射光颜色随入射光偏振方向的变化而改变; 而对于圆形、方形的小孔阵列, 其透射光颜色对入射光的偏振方向并不敏感. 分析表明, 对于金膜上刻蚀的小孔结构, 虽然结构的周期性导致的表面等离激元极化子会对透射光的颜色变化产生一定影响, 但是随小孔形状变化的局域表面等离激元共振才是影响透射光颜色的决定性因素. 如果入射光没有在小孔中激发出局域表面等离激元, 则表面等离激元极化子对透射光的影响也会消失. 根据不同形状小孔周期结构透射光颜色随入射光的偏振变化特点, 制备出了包含两种小孔形状的复合周期结构. 随着入射光偏振方向的改变, 该结构会显示出不同的颜色图案. 关键词： 表面等离激元极化子 局域表面等离激元 颜色滤波器 亚波长小孔阵列     

Switchable terahertz polarization converter based on VO2 metamaterial

A switchable terahertz (THz) polarization converter based on vanadium dioxide (VO₂) metamaterial is proposed. It is a 5-layer structure which containing metal split-ring-resonator (SRR), the first polyimide (PI) spacer, VO₂ film, the second PI spacer, and metal grating. It is an array structure and the period in x and y directions is 100 μm. The performance is simulated by using finite integration technology. The simulation results show that, when the VO₂ is in insulating state, the device is a transmission polarization converter. The cross-linear polarization conversion can be realized in a broadband of 0.70 THz, and the polarization conversion rate (PCR) is higher than 99%. Under thermal stimulus, the VO₂ changes from insulating state to metallic state, and the device is a reflective polarization converter. The linear-to-circular polarization conversion can be successfully realized in a broadband of 0.50 THz, and the PCR is higher than 88%.     

基于超材料的中波红外宽带偏振转换研究

基于硅纳米块阵列和亚波长金属光栅,硅纳米块长轴与金属光栅夹角为45°,本文设计了一种高效、宽带偏振转换结构.模拟计算表明该结构实现了线偏振光90°旋转,在3.4—4.5μm波段偏振转换率大于60%,在3—5μm光谱范围内的转换对比率大于10~4.由于该结构光学性能优异,制备难度低,可以应用于光传输控制.     

Linear dipole behavior of single quantum dots encased in metal oxide semiconductor nanoparticles films

Understanding of charge/energy exchange processes and interfacial interactions that occur between quantum dots (QDs) and the metal oxides is of critical importance to these QD-based optoelectronic devices. This work reports on linear dipole behavior of single near-infrared emitting CdSeTe/ZnS core/shell QDs which are encased in indium tin oxide (ITO) semiconductor nanoparticles films. A strong polarization anisotropy in photoluminescence emission is observed by defocused wide-field imaging and polarization measurement techniques, and the average polarization degree is up to 0.45. A possible mechanism for the observation is presented in which the electrons, locating at single QD surface from ITO by electron transfer due to the equilibration of the Fermi levels, result in a significant Stark distortion of the QD electron/hole wavefunctions. The Stark distortion results in the linear polarization property of the single QDs. The investigation of linear dipole behavior for single QDs encased in ITO films would be helpful for further improving QD-based device performance.     

Optimal design of sub-wavelength metal rectangular gratings for polarizing beam splitter based on effective medium theory

A novel optimal design of sub-wavelength metal rectangular gratings for the polarizing beam splitter (PBS) is proposed. The method is based on effective medium theory and the method of designing single layer antireflection coating. The polarization performance of PBS is discussed by rigorous couple-wave analysis (RCWA) method at a wavelength of 1550~nm. The result shows that sub-wavelength metal rectangular grating is characterized by a high reflectivity, like metal films for TE polarization, and high transmissivity, like dielectric films for TM polarization. The optimal design accords well with the results simulated by RCWA method.     

A Compact Optical Switch via Plasmonics of Subwavelength Circular‐Sharp Hole Arrays in Metal Films

We studied numerically the enhanced optical transmission (EOT) through periodic subwavelength circular‐sharp hole arrays in metallic films with different edge sharp distribution features of unit structures. Detailed studies indicate that the unit structure edge sharp distribution features strongly influence the surface plasmons (SPs). These results demonstrate that the number of edge sharp activated the localized surface plamons (LSP) resonance on the unit structure is changed by rotating the polarization of the incident light, leading to change the infrared transmittance of the array. Moreover, a compact plasmonic switch via periodic circular‐sharp hole arrays based on the dependence of SPs on unit structural edge sharp distributions is proposed. The finding provides a new idea for designing plasmonics devices, and expands the application range of metal micro‐nano structure in the field of optical communications and information processing.     

Anomalously high low-frequency effective permittivity in a system of metal nanoislands

The low-frequency effective permittivity in a system of Co, W, and FeNi metal nanoislands in an electric field is investigated. The permittivity of films is determined from analyzing the dependences of the susceptance of the films on the temperature and the electric field. It is found that the effective permittivity of the structures under investigation has an anomalously high positive value (10⁷–10⁸) and that an increase in the electric field leads to a decrease in the effective permittivity and an increase in the electrical conductivity of the films. The nature of the high effective permittivity of the structures is governed by the island structure of the metal films and associated with the polarization of dipoles formed by pairs of negatively and positively charged metal nanoislands.     

微波电子回旋共振等离子体增强化学气相沉积法沉积氟化非晶碳薄膜的研究

使用三氟甲烷和苯的混合气体,利用微波电子回旋共振等离子体增强化学气相沉积法制备了F/C比在0.11—0.62之间的α-C∶F薄膜.研究了微波功率对薄膜沉积和结构的影响,发现微波功率的升高提高了薄膜的沉积速率,降低了薄膜的F/C比,也降低了薄膜中CF和CF₃基团的密度,而使CF₂基团的密度保持不变.在高微波功率下可以获得主要由CF₂基团和C=C结构组成的α-C∶F薄膜.薄膜的介电频率关系(1×10³—1×10⁶Hz)和损耗频率关系(1×10²—1×10⁵Hz)均呈指数规律减小,是缺陷中心间简单隧穿引起的跳跃导电所致.α-C∶F薄膜的介电极化主要来源于电子极化 关键词： 氟化非晶碳薄膜 ECR等离子体沉积 键结构 介电性质     

Experimental studies of extraordinary light transmission through periodic arrays of subwavelength square and rectangular holes in metal films

We fabricate a series of periodic arrays of subwavelength square and rectangular air holes on gold films, and measure the transmission spectra of these metallic nanostructures. By changing some geometrical and physical parameters, such as array period, air hole size and shape, and the incident light polarization, we verify that both global surface plasmon resonance and localized waveguide mode resonance are influential on enhancing the transmission of light through nanostructured metal films. These two resonances induce different behaviours of transmission peak shift. The transmission through the rectangular air-hole structures exhibits an obvious polarization effect dependent on the morphology. Numerical simulations are also made by a plane-wave transfer-matrix method and in good consistency with the experimental results.     

磁场下高温超导YBCO颗粒膜的非平衡微波响应

应用二维约瑟夫逊弱连接网络模型 ,讨论了磁场引起的二维超导体系中磁通“涡旋反涡旋”束缚对的激发问题 ,给出了自由涡旋分布 n( T,H )的分析表达式 ,对比 YBCO颗粒膜样品的非平衡微波响应测量数据发现 :两者具有相似的分布特征。该结果表明 ,高温超导颗粒膜的非平衡微波响应机制与磁通“涡旋反涡旋”束缚对的激发态可能存在某种内在联系。     

Ferromagnetic films with magnon bandgap periodic structures: Magnon crystals

A new type of photonic crystals is proposed. The new crystals have a forbidden gap in the microwave spectrum of magnetostatic spin waves, and, by analogy with photonic crystals, they are called magnon crystals. Specimens of such crystals were fabricated on the basis of yttrium iron garnet films. The surfaces of ferromagnetic films containing two-dimensional etched hole structures were studied by atomic force and magnetic force mag-netometry. The propagation of spin waves through the magnon crystals was investigated.     

p型半导体金刚石膜的磁阻效应

在p型硅(100)衬底上,采用衬底负偏压微波等离子体CVD方法进行了p型异质外延金刚石膜的生长.用O₂等离子体刻蚀技术将金刚石膜刻蚀成长条形,利用四探针法在0—5T的磁场范围内测量了样品的磁阻.实验结果表明,p型异质外延金刚石膜可以产生较大的磁阻.在Fuchs-Sondheimer(F-S)薄膜理论的基础上考虑晶格散射、杂质散射和表面散射,通过求解Boltzmann方程,利用并联电阻模型研究了p型异质外延金刚石膜的磁阻效应,给出了磁阻和金刚石膜厚度、迁移率、空穴密度及磁场的关系.讨论了表面散射和价带形变对p型异质外延金刚石膜磁阻的影响,初步解释了p型异质外延金刚石膜产生较大磁阻的原因 关键词： 金刚石膜 异质外延 磁阻效应 电导率     

Tunable wide-angle multi-band mid-infrared linear-to-linear polarization converter based on a graphene metasurface

We present a high-efficiency tunable wide-angle multi-band reflective linear-to-linear (LTL) polarization converter, which is composed of an array of two L-shaped graphene patches with different sizes. In the mid-infrared region, the proposed converter can transform x-polarized wave into y-polarized wave at four resonant frequencies. The polarization conversion ratios of the four bands reach 94.4%, 92.7%, 99.3%, and 93.1%, respectively. By carefully choosing the geometric parameter, triple-band LTL polarization conversion can also be realized. The three polarization conversion ratios reach 91.50%, 99.20%, and 97.22%, respectively. The influence of incident angle on the performances of the LTL polarization converter is investigated, and it is found that our polarization converter shows the angle insensitivity. Also, the dynamically tunable properties of the proposed polarization converter are numerically studied by changing Fermi energy. All the simulation results are conducted by finite element method.     

光子晶体高功率微波模式转换器设计

研究了传输线中非截面排列金属光子晶体电磁特性,并利用其作为移相器提出了一种紧凑型TEM—TE11模式转换器的设计方法.利用电磁软件cst microwave studio优化设计了一套L波段TEM—TE11模式转换器,在中心频率1.58 GHz上转换效率为98%.在1.56—1.625 GHz频率范围内,模式转换器转换效率大于90%,对应带宽4.1%.模式转换器功率容量为GW级,适用于高功率微波源系统.结合磁绝缘线振荡器开展了粒子模拟研究工作,发现模式转换器性能与设计结果相符,并且其引入不影响高功率微波器件的正常工作.     

Critical process of extraordinary optical transmission through periodic subwavelength hole array: Hole-assisted evanescent-field coupling

We present a new explanation of the extraordinary optical transmission through subwavelength hole array in metal films. By using the classical coupled-wave theory, we analyzed the coupling process between light and the surface plasmons wave on metal films with hole array. The analysis shows clearly the physical mechanism of extraordinary optical transmission. The calculation demonstrates that, instead of energy flux directly passing through the holes, the electromagnetic modes could exchange energy by overlapping the evanescent fields under the assistance of hole array. The periodicity of the array provides the momentum-matching condition to present the transmission peaks. The theory exhibits a good agreement with the experimental results reported in every detail.