Multifunctional Device for Circular to Linear Polarization Conversion and Absorption

In this paper, a metastructure multifunctional device for circular-to-linear polarization conversion (PC) and perfect absorption is proposed in which the electrical conductivity of the silicon material is controlled by light, thus changing the function of the device. The paper also explores three methods of optimizing bandwidth and their mechanisms, which are analyzed by means of current and energy density diagrams. The unit structure of this device adopts a 2 × 2 array, which is used for differentiated reflection of circular polarization waves, and forms linear polarization waves after reflection. In the other state, ultrawideband absorption can be achieved by changing the conductivity of silicon by external optical pumping, and the bandwidth is widened by inserting air resonators. In general, the device can form a PC at 0.89–1.31 THz with a relative bandwidth of 38% when there is no illumination. The resulting linear polarization wave has an axial ratio greater than 19 dB. When the silicon is excited by light resulting in a stable conductivity of around 9000 S m⁻¹, the absorption band is 0.89–2.01 THz, the relative bandwidth is 77%, and the absorption rate is above 90%. This device can be used for communication, electromagnetic cloaking, and modulation.     

Metal–Graphene Hybrid Chiral Metamaterials for Tunable Circular Dichroism

In the field of optoelectronics, circular dichroism (CD) has caused great research interest because it is widely used in imaging and biosensing. A new method for dynamically controlling terahertz (THz) CD in metamaterials is proposed. By introducing chirality and graphene to metamaterials, a pair of chiral structures with completely opposite responses to left-handed circularly polarized (LCP) waves and right-handed circularly polarized (RCP) waves are designed. The influencing factors of CD are explored, including the gap of the structure, the linewidth of graphene, and the Fermi level of graphene. The largest CD (ΔR) is 77%. The CD can be actively modulated in a modulation range of 39–77% and the modulation depth is up to 38%. In addition, two-channel and four-channel chiral metasurfaces for near-field imaging are designed in this way. Good imaging effects and on (“1”) or off (“0”) effects of the multichannel metasurface are demonstrated. This work provides new ideas for the design of tunable metasurfaces and promotes the application of metasurfaces in THz dynamic imaging.     

Giant optical activity and circular dichroism in the terahertz region based on bi-layer Y-shaped chiral metamaterial

We present a bi-layer Y-shaped chiral metamaterial (CMM) that can realize a giant optical activity and circular dichroism (CD) effect to the incident linear polarization wave in the terahertz (THz) region. Numerical simulation results exhibit that the pronounced CD effect with a great difference between the transmission coefficients for the circularly polarized waves can be obtained at 5.06 THz, meanwhile the 90°-polarization rotation can be observed at 5.2 THz when a y-polarized wave is incident to this CMM propagating along the −z-axis. The mechanism of the optical activity and giant CD effect is illustrated by simulated surface current distributions. Further, the influences of the structural parameters of the proposed CMM to the optical activity and CD effect have been investigated numerically.     

All-silicon chiral metasurfaces and wavefront shaping assisted by interference

Chiral metasurfaces have different electromagnetic responses with circularly polarized lights, showing as circular dichroism and optical activity. Here, a novel kind of all-silicon chiral metasurface is proposed by introducing destructive interference between achiral meta-atoms. The maximum value of circular dichroism spectra can reach 0.49. By adding an antireflective layer at the side of the silicon substrate, the maximum circular dichroism reaches 0.54. What is more, the bandwidth of circular dichroism greater than 0.4 reaches 0.15 THz. Two samples are fabricated to verify the feasibility of this scheme, and the experimental results are in good agreement with the simulations. In addition, the proposed scheme can also be used to generate various interesting functions, such as beam control and vortex generator. This flexible and efficient implementation solution of chiral metasurface can bring new ideas to the development of chiral devices in the future.     

Frequency Reconfigurable and Multifunctional Metastructure Regulated by Nematic Liquid Crystal: Broadband Circular to Linear Polarization Converter

With the tunability of the nematic liquid crystal (NLC), a broadband frequency reconfigurable and versatile metastructure (MS) is proposed and theoretically investigated in this paper, combining circular-to-linear (CTL) polarization conversion (PC) and circular-to-circular (CTC) PC simultaneously. The MS is composed of two via-coupled modules, which can respond differently to the incident waves. Each module is connected utilizing a metal via a column, thus exceedingly enhancing the energy transmission and reducing the loss when transmitting. When the applied bias voltage (V_bias) is 0 V, the NCL molecules follow the initial orientation. The MS converts the incident right circular polarized (RCP) waves into linear polarized (LP) waves within 8.11–9.95 gigahertz (GHz) with a relative bandwidth of 20.38% and achieves the PC of left circular polarized (LCP) into RCP waves. As the V_bias reaches 20 V, the original operating interval reconfigures and shifts overall toward a lower frequency. The bandwidth of CTL is 7.66–9.02 GHz, and the CTC PC is broadened to 20.20%. Meanwhile, the structure justification is verified, and the inducing mechanism of PC is expounded. Possessing the merits of versatile collaborative processing and wide operating bands, such an MS is promising to be a polarization-controlled application candidate and enrich multifunctional designs.     

Bispectral Circular Dichroic Coding Metasurfaces

The frequency-dependent circular dichroism is proposed to extend the wave manipulating capability of coding metasurfaces. As a proof of concept, the bispectral circular dichroic coding metasurfaces (CDCMs) are realized using circular dichroism resonators (CDRs) implemented via introducing loss resistors into the circular polarization conversion resonators. The CDRs are distinguished into left-handed CDRs and right-handed CDRs. Left-handed CDRs absorb left-handed circularly polarized (LCP) wave and convert right-handed circularly polarized (RCP) wave into LCP wave. Conversely, they are defined as right-handed CDRs. Two bispectral CDRs are designed with the left-handed (right-handed) and right-handed (left-handed) working bands in 7–8.5 and 22.2–22.5 GHz, respectively. And then, 1 bit bispectral CDCM with 0101…/1010… coding sequence is designed. Simulated results indicate that the designed CDCM strongly absorb the incident LCP (RCP) waves in the frequency region 7–8.5 GHz (22.2–22.5 GHz), but the incident RCP (LCP) waves are anomalously reflected into four beams of the LCP (RCP) waves with high efficiency. The measured results agree well with the simulations. The results in this work may provide an effective solution for the inverse and helicity-dependent manipulation of the electromagnetic waves in two distinct frequency regions.     

Geometric Phase Based Circular Array for Multimode Vortex Beam Generation

A geometric phase model for electromagnetic radiating elements is proposed. By rotation of the radiating element, a frequency‐independent geometric phase occurs for circularly polarized components of radiation field along every direction in far field. In addition, the geometric phase is equal to the rotation angle for a circularly polarized source, which enables phase modulation ranging from 0 to 2π. In contrast, the Pancharatnam–Berry phase for circular polarization conversion components brought by optical element rotation is twice the rotation angle and is applicable only for the scattering waves propagating along the rotation axis. As a proof of principle, an antenna array is designed and fabricated in microwave regime to verify the phase modulation approach. Both the calculated and measured results verify that three different orbital angular momentum modes are generated simultaneously at 8.5 GHz and 11.5 GHz.     

Gradient Chiral Metamirrors for Spin‐Selective Anomalous Reflection

Metasurfaces, the phase‐engineered quasi‐2D interfaces, have attracted intensive interest due to their great capabilities in manipulating the reflection, refraction and transmission of electromagnetic waves. Here, we demonstrate the design and realization of a gradient chiral metamirror tailored for spin‐selective anomalous reflection based on the theory of Pancharatnam‐Berry phase. Asymmetric split ring resonators are employed as the basic meta‐atoms for strong circular dichroism. Dispersionless phase discontinuities are achieved by adjusting the orientation of the meta‐atoms, and spin‐dependent absorption is realized by introducing a chiral resonance. Theoretical results predict both broadband beam deflection and spin‐selective absorption for circularly polarized waves in a designer metamirror. Experimental verification of this bifunctional performance is implemented at microwave frequencies and the measured results agree well with the simulation ones. Such chiral metamirrors could pave an avenue towards spin‐selective modulation of the wavefront and might find promising applications in planar electromagnetic devices.     

宽带超薄完美吸波体设计及在圆极化倾斜波束天线雷达散射截面缩减中的应用研究

为了缩减天线带内雷达散射截面(radar cross section, RCS), 在双频带完美吸波材料的基础上, 通过缩小两吸波率峰值之间的距离, 设计出了一种频带较宽的超薄完美吸波体.该吸波体由两层金属及其中间的有耗介质组成, 底面金属不刻蚀, 顶面由方形贴片和绕其四周的开口方环组成, 该结构具有低频点LC谐振和高频点偶极子谐振的特征.仿真和实验结果表明: 该吸波体具有极化不敏感和宽入射角的特征, 其在厚度小于0.01λ的条件下, 具有8.2%的半波功率相对带宽, 最大吸波率的峰值为91.6%和96.5%. 将吸波体用于圆极化的倾斜波束 (tilted beam, TB)天线, 仿真和测试结果表明: 该天线在保持增益不变的条件下, 不仅轴比得到改善, 有效带宽得到拓展, 且在5.5–6.5 GHz范围内TB天线的RCS缩减至少在3 dBsm以上, 在谐振频点处最大缩减幅度分别为11 dBsm和8 dBsm; 在两谐振点处鼻锥方向-36°–+36°范围内, TB天线的RCS缩减均有明显效果. 关键词： 超薄完美吸波体 TB天线 雷达散射截面 圆极化     

基于复合超构表面的宽带圆极化双功能器件设计

多功能器件的设计是推动新一代电磁系统发展的重要力量,而超构表面因其对电磁波的幅度、相位和极化等特性的灵活调控在多功能器件领域备受关注.传统的多功能超构表面是利用各向异性单元对相互正交的线极化波具有不同响应的特性,从而设计出适用于线极化的多功能器件.本文提出了一种缝隙加载的环I形复合超构表面单元,通过单元臂长和旋转角度的调整实现了对圆极化电磁波传输和几何相位的独立控制.利用上述两种相位的共同作用,打破了左旋和右旋圆极化电磁波操控中存在的固有关系,为圆极化双功能器件的设计提供了新的思路.在此基础上,利用复合超构表面分别设计了异面偏折器和定向/涡旋光束产生器,实验结果表明,本文设计的两种反射型圆极化双功能器件在9—13 GHz的宽频带范围内均能良好工作.     

An Ultra-Wideband Janus Metastructure with Graphene for Detector Based on Anapole Mode in the Terahertz Region

In this paper, an ultra-wideband Janus metastructure (MS) utilizing anapole mode for detector in the terahertz (THz) range by graphene is proposed. Specifically, when Fermi level (E_f) is set to 0.9 eV, the MS demonstrates ultra-broadband absorption exceeding 0.9 from 0.754 to 5 THz in the −z-direction with a relative bandwidth of 147.6 %, in which perfect absorption of over 98% develops from 3.24 to 5 THz. In the case of the +z-direction, the absorptivity maintains around 0.6 within the 0.745 ∼ 5 THz range. As E_f equals 0 eV, the difference in absorption between the −z-direction and +z-direction exceeds 0.9 from 4.49 to 4.76 THz. The study also explores the MS for refractive index sensing near 3.71 THz by a unique difference detection, measuring two refractive index ranges: 1.2 ∼ 2.6 and 4.5 ∼ 4.7, with corresponding sensitivities of 0.0450 and 0.0304, respectively. Owing to its highly symmetrical structure, the MS is insensitive to the polarization state of the electromagnetic (EM) waves, performing remarkable angular stability as the incident angle varies from 0 to 60 degrees in the −z-direction. These splendid properties make the design a good candidate for biomedical sensing, EM cloaking, and full-space EM wave control.     

基于极化旋转超表面的圆极化天线设计

本文通过设计出一种反射型极化旋转超表面,在8—12 GHz频域内实现高效的极化旋转,并将其加载于微带缝隙天线下方构成新型的极化旋转超表面天线,利用超表面的90°极化旋转效应,成功实现了天线的圆极化辐射调制.仿真与实验结果表明:圆极化天线的中心工作频率为GHz,阻抗带宽为8.3—10 GHz.当微带缝隙天线与极化旋转超表面的间距H=4.5mm时,天线在8.3—8.8 GHz频带内实现了圆极化辐射;当mm时,天线在8.8—9.3 GHz频带内实现了圆极化辐射;当=8mm时,天线在9.3—10 GHz频带内实现了圆极化辐射.实验结果与仿真结果相符,证明了此种设计方法的有效性,也为微带缝隙天线的圆极化设计提供了一种新的途径.     

基于圆台结构的超宽带极化不敏感太赫兹吸收器

本文提出一种基于圆台形吸收单元的超宽带、极化不敏感的超材料太赫兹吸收器. 该超材料吸收器采用金属薄膜金和介质层二氧化硅交替叠加的多层结构. 采用商业软件CST Studio Suite 2009时域求解器计算了其在0–10 THz波段内的吸收率A（ω）,在2–10 THz之间实现了对入射太赫兹波的超宽频带强吸收. 仿真结果表明,由于其圆台形单元结构,在器件垂直方向上形成一系列不同尺寸的微型吸收器,产生了吸收频点相连的多频吸收峰. 利用不同吸收峰的耦合叠加效应,获得超过8 THz的超宽带太赫兹波吸收,吸收强度达到92.3%以上. 这一结构具有超宽带强吸收,360°极化不敏感以及易于加工等优越特性,因而在太赫兹波探测器、光谱成像以及隐身技术方面具有潜在的应用. 关键词： 太赫兹波 超材料吸收器 圆台结构 超宽带     

Strong chirality in twisted bilayer α-MoO3

Chiral structures are promising in many applications, such as biological sensing and analytical chemistry, and have been extensively explored. In this paper, we theoretically investigate the chiral response of twisted bilayer α-MoO₃. Firstly, the analytical formula for the transmissivity is derived when the structure is illuminated with circularly polarized plane waves. Furthermore, the results demonstrate that the twisted bilayer α-MoO₃ can excite the strong chirality with the maximum circular dichroism (CD) of 0.89. In this case, the chirality is due to the simultaneous breaking the rotational symmetry and mirror symmetry, which originates from the relative rotation of two α-MoO₃ layers. To better understand the physical mechanism, the polarization conversion between the left-hand circular polarization (LCP) and right-hand circular polarization (RCP) waves is discussed as well. Moreover, it is found that the structure can maintain the strong chirality (CD> 0.8) when the twisted angle varies from 69° to 80°, which effectively reduces the strictness in the requirement for rotation angle. In addition, the CD can be larger than 0.85 when the incidence angle of circularly polarized plane wave is less than 40°, implying that the chirality is robust against the angle of incidence. Our work not only provides an insight into chirality induced by the twisted bilayer α-MoO₃, but also looks forward to applications in biological sensing.     

Design of a type of broadband metamaterial absorber based on metal and graphene

In this paper, we demonstrate a kind of broadband metamaterial perfect absorber using both graphene and metal resonator elements. Through step by step design and simulation, wider absorption band from about 4.22 THz to 7.48 THz with average absorption rate up to 98.21% is achieved in the absorption spectrum. In addition, the absorber has characteristics of polarization insensitivity and wide incident angle due to its inherent rotational symmetry. Moreover, the absorption band can be adjusted by changing the chemical potential of the graphene. The superiorities of broadband, high absorption rate, polarization independent and wide-angle characteristics make it have potential application prospects in electromagnetic wave absorbing, signal sensing and detection, and other optoelectronic devices.     

Multi-band giant circular dichroism based on conjugated bilayer twisted-semicircle nanostructure at optical frequency

Plasmonic circular dichroism (CD) effect has been drawn great attention increasingly for its wide application in the fields of bio-sensing, biological detection, pharmaceuticals, and analytical chemistry. In this paper, we propose a chiral metasurface (CMS) to achieve strong multi-band CD effect at optical frequency. The designed CMS is composed of a periodic array of conjugated bilayer twisted-semicircle nanostructures. The numerical simulation results show that the CMS can produce strong multi-band CD effect due to the different coupling resonance modes under the excitations of left-handed circular polarization (LCP) light and right-handed circular polarization (RCP) light. It is shown that the chiral-selective absorption peaks can reach 89.4% and 95% for LCP light, 79% and 78.2% for RCP light, and the maximum CD is about 0.69 and −0.61 at 198.75 THz and 352.25 THz, 0.69 and −0.54 at 291.75 THz and 402.25 THz, respectively. The mechanism of the giant CD effect of the CSM has been revealed by analyzing the coupling mode of electric dipoles on the top and bottom layer through surface current distributions. Furthermore, the geometric parameter dependences of CD effect in the proposed CMS have been also studied numerically. The present results will guide the design of plasmonic chiral nanostructures for enhancing the CD effect.     

Tunable Transmissive Metastructure for Precise or Broadband Polarization Conversion Modulation Based on Graphene

Based on the principle of Fabry–Perot (F–P) cavity resonance and the selective permeability of gratings to specific electromagnetic waves, a graphene-based metastructure (MS) is proposed for transmissive polarization conversion (PC). Using the full-wave numerical simulation, it is found that by varying the Fermi energy of graphene, the effective resonance range of the suggested MS can be dynamically adjusted from 0.47 to 0.348–0.714 THz, achieving the target of precise to ultra-broadband polarization modulation. In this paper, the plausibility of the structure is verified from multiple perspectives, and the correlation analyses of the electric and magnetic fields are the supporting illustrations. Additionally, the triggering mechanism of PC is visually illustrated in the study of the surface currents distributions. Simulation results reveal that the MS is superior in performance, functionality, and principle, and it is foreseen to hold excellent promise for integrated equipment in the terahertz (THz) band.     

Optical properties of a three-dimensional chiral metamaterial

A three-dimensional chiral metamaterial with four-fold rotational symmetry is designed, and its optical properties are investigated by numerical simulations. The results show that this chiral metamaterial has the following features: high polarization conversion, perfect circular dichroism, and asymmetric transmission of circularly polarized light. A comparison of the results of chiral metamaterials without and with weak coupling between the constituent nanostructures enables us to confirm that the optical properties of our proposed nanostructure are closely related to the coupling between the single nanoparticles. This means that the coupling between nanoparticles can enhance the polarization conversion, circular dichroism, and asymmetric transmission. Due to the excellent optical properties, our metamaterial might have potential applications in the development of future multi-functional optical devices.     

Diode-like asymmetric transmission of circularly polarized waves

Optical diode plays an important role in optical computing and information processing. In this paper, a kind of three-layered chiral metamaterial is proposed to achieve diode-like asymmetric transmission for forward and backward propagations of circularly polarized electromagnetic waves only. The metamaterial is composed of a ring–chain structure sandwiched between two identical S-shaped metallic layers with their corresponding substrates. A transparent transmission at normal incidence in one direction and a small intensity of transmission in the opposite direction are numerically demonstrated in this chiral structure. Besides, the results reveal that the bandwidth of the cross-polarization transmission of this designed structure can be tuned by varying the incident angle of circularly polarized waves.     

圆偏振光和线偏振光在正常色散材料中的时空自压缩

实验研究了圆偏振和线偏振高强度飞秒激光脉冲在正常色散材料中传输时的时空自压缩现象。实验中利用BK7玻璃作为正色散材料,比较研究了不同偏振入射情况下脉冲波形及频谱的变化规律。圆偏振光入射时,可以获得更短脉冲宽度的压缩脉冲和更窄的光谱宽度。在圆偏振光入射条件下,50 fs入射脉冲成功地自压缩到了19 fs,获得了大于2.5倍的压缩倍率。所以利用圆偏振光可以获得更短压缩脉冲,更大能量,更好光束质量的激光。