Dynamically controlled asymmetric transmission of linearly polarized waves in VO2-integrated Dirac semimetal metamaterials

We numerically demonstrated a novel chiral metamaterial to achieve broadband asymmetric transmission (AT) of linearly polarized electromagnetic waves in terahertz (THz) band. The proposed metamaterial unit cell exhibits no rotational symmetry with vanadium dioxide (VO$_{2}$) inclusion embedded between Dirac semimetals (DSMs) pattern. The resonant frequency of AT can be dynamically tunable by varying the Fermi energy ($E_{\rm F}$) of the DSMs. The insulator-to-metal phase transition of VO$_{2}$ enables the amplitude of the AT to be dynamically tailored. The transmission coefficient $|T_{yx}|$ can be adjusted from 0.756 to nearly 0 by modifying the conductivity of VO$_{2}$. Meanwhile, the AT parameter intensity of linearly polarized incidence can be actively controlled from 0.55 to almost 0, leading to a switch for AT. When VO$_{2}$ is in its insulator state, the proposed device achieves broadband AT parameter greater than 0.5 from 1.21 THz to 1.80 THz with a bandwidth of 0.59 THz. When the incident wave propagates along the backward ($-z$) direction, the cross-polarized transmission $|T_{yx}|$ reaches a peak value 0.756 at 1.32 THz, whereas the value of $|T_{xy}|$ well below 0.157 in the concerned frequency. On the other hand, the co-polarized transmission $|T_{xx}|$ and $|T_{yy}|$ remained equal in the whole frequency range. This work provides a novel approach in developing broadband, tunable, as well as switchable AT electromagnetic devices.     

二维宽带相位梯度超表面设计及实验验证

针对圆极化波, 通过同极化反射超表面结构单元的空间排布, 设计实现了一种二维非色散高效相位梯度超表面. 同极化反射相位可以通过同极化反射超表面结构单元金属线的面内旋转来自由调控. 实现的相位梯度超表面可对左右旋入射波产生相反的相位梯度. 当线极化波入射到超表面上时, 反射波被分为两束向相反方向传播的圆极化波. 仿真了线极化波垂直入射时的反射功率密度谱, 仿真结果与理论上设计的异常反射方向一致. 制作了厚度为2 mm的超表面样品, 测试了其镜面反射率曲线. 实验结果表明, 线极化波垂直入射时, 超表面在9.5-19.0 GHz的镜面反射率降至-5 dB以下.     

An Ultra-Broadband Chirality Selective Metastructure Absorber using High Impedance Surface

In this paper, an ultra-wideband chirality selective metastructure absorber is proposed that enables differential absorption and reflection of circularly polarized waves in the terahertz (THz) range. The structure achieves circular dichroism (CD) by using asymmetrically split metal rings as fundamental meta-atoms. Most critically, the high impedance surface and air-resonant cavities are inserted separately in the meta-atoms and dielectric substrate to enhance CD and broaden the bandwidth of absorption. The metastructure absorber can achieve more than 90% absorption of right circularly polarized waves at 0.675–1.244 THz, and it can maintain more than 90% reflection of left circularly polarized waves at 0.607–1.229 THz without changing the direction of rotation. Besides, its CD can reach more than 80% at 0.687–1.213 THz with a relative bandwidth of 55.3%. Spin-selective absorption, which is closely related to breaking chiral symmetry, is investigated through power loss distribution, wide-angle incidence, and scan parameter optimization. The proposed strategy is further validated in the THz band, and the polarization selection and manipulation techniques can be applied to chiral sensing/radio-thermometry, circular polarization detectors/lasers, and molecular spectroscopy.     

两圆偏振光以及线偏振光和圆偏振光干涉图样的衬比度

考察了两束圆偏振光之间以及线偏振光与圆偏振光之间干涉所形成图样的衬比度,并与两束线偏振光之间干涉图样的衬比度进行了比较．指出在非共面多光束干涉中有可能利用圆偏振光以改善干涉图样的整体衬比度,并简要介绍了它在光子晶体制备中的应用．     

Ultra-wideband circular-polarization converter with micro-split Jerusalem-cross metasurfaces

An ultrathin micro-split Jerusalem-cross metasurface is proposed in this paper, which can efficiently convert the linear polarization of electromagnetic(EM) wave into the circular polarization in ultra-wideband. By symmetrically employing two micro-splits on the horizontal arm(in the x direction) of the Jerusalem-cross structure, the bandwidth of the proposed device is significantly extended. Both simulated and experimental results show that the proposed metasurface is able to convert linearly polarized waves into circularly polarized waves in a frequency range from 12.4 GHz to 21 GHz, with an axis ratio better than 1 d B. The simulated results also show that such a broadband and high-performance are maintained over a wide range of incident angle. The presented polarization converter can be used in a number of areas, such as spectroscopy and wireless communications.     

Dynamically adjustable asymmetric transmission and polarization conversion for linearly polarized terahertz wave

The asymmetric transmission(AT) and polarization conversion of terahertz(THz) wave play a vital role in future THz communication,spectrum,and information processing.Generally,it is very difficult and complicated to actively control the AT of electromagnetic(EM) wave by using traditional devices.Here,we theoretically demonstrate a stereo-metamaterial(stereo-MM) consisting of a layer of metal structure and a layer of phase transition structure with a polyimide spacer in between.The performance of the device is simulated by using the finite-integration-technology(FIT).The results show that the AT and polarization conversion of linearly polarized wave can be dynamically controlled in a range of 1.0 THz-1.6 THz when the conductivity c,F of vanadium dioxide(VO2) is changed under the external stimulation.This study provides an example of actively controlling of the AT and polarization conversion of the EM wave.     

Interference of circularly polarized light: contrast and application in fabrication of three-dimensional periodic microstructures

The contrast of interference pattern formed by two circularly polarized waves and by a linearly polarized wave and a circularly polarized one is discussed. The results are compared with that by two linear beams. It shows that the use of circular light in holographic fabrication of three-dimensional periodic microstructures may remove the necessity of beam ratio and polarization optimization needed in the interference of three linear noncoplanar beams and improve the uniform contrast of resultant pattern simultaneously.     

Design of terahertz wave polarizing beam splitter

In this paper, we design a THz wave polarizer based on a periodic symmetrical thin film structure, which operates over a broadband THz frequency region (1.0–2.0 THz) and over an effective wide range of incident THz wave (72–84°). The spectral performance of this structure is characterized by transfer matrix method calculations. Results of simulations show that the polarizer is highly transmittance for TE polarized THz wave as well as highly reflecting for TM polarized THz wave.     

All-optical ultrafast polarization switching of terahertz radiation by impulsive molecular alignment

We experimentally demonstrate ultrafast polarization switching of terahertz (THz) radiation generated by dual-color driving pulses composed of orthogonally polarized fundamental and second-harmonic waves, which can be controlled by field-free molecular alignment in air by modulating the relative phase between the two field components as a transient dynamic wave plate. By fine-tuning the time delay to properly match the molecular alignment revivals, a significant polarization modulation of the THz radiation is observed and both linearly and elliptically polarized THz radiations can be obtained.     

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.     

Multi-band asymmetric transmissions based on bi-layer windmill-shaped metamaterial

This study proposes a bi-layer windmill-shaped metamaterial that consists of resonators, with similar shapes, on both sides of a dielectric substrate. In this study, the second layer is rotated clockwise around the substrate normal at 90° and thereafter flipped in the first layer. Due to the introduction of a windmill-like shape, the resonant structures result in new resonant modes and thus can achieve multi-band high-efficiency cross-polarization conversions and asymmetric transmissions (ATs) for a linearly polarized incident plane wave with a maximum asymmetric parameter of 0.72. Depending on the geometric parameters of our windmill-shaped structures, the AT effects can be flexibly modulated in a broad multi-band from 160 THz to 400 THz, which has not been reported in previous studies. These outstanding AT effects provide potential applications in optical diodes, polarization control switches, and other nano-devices.     

Manipulating Nonlinear Photocurrent from Interlayer Coupling in Bilayer Metamaterials for Polarized Terahertz Generation

Polarized terahertz (THz) wave generation is of great significance for chiral and anisotropic sensing applications. However, how to manipulate amplitude, polarization, and ellipticity of the THz generation is still a fundamental challenge. Herein, polarized THz wave generation is achieved from a bilayer metamaterial consisting of T-shaped structure (TSS) and split resonator rings (SRRs) by combining Maxwell and hydrodynamic equations. The elliptically polarized THz wave can be synthetized directly from horizontally and vertically polarized THz components due to the orthogonal nonlinear photocurrents along the arm-directions of TSS and SRRs, respectively. Besides, the ellipticity and the orientation angle of the THz polarization ellipse can be modulated by the twist angle between the SRRs and TSS layers. The maximum ellipticity can reach 0.34 while the orientation angle is tunable from −0.45 to 0.48π by tuning the twist angle. This work proposes an interlayer coupling method for the polarized THz sources based on metamaterials in potential circular dichroism and chiral sensing applications.     

The exact solution of the relativistic equation of motion of acharged particle driven by an elliptically polarized electromagneticwave

An exact solution of the relativistic equation of motion of a charged particle driven by an elliptically polarized electromagnetic wave is obtained. The method used for this purpose is simple, and the results of earlier authors for the linearly and circularly polarized waves can be recovered as special cases of the solution     

基于双开口谐振环超表面的宽带太赫兹涡旋光束产生

提出了一种基于双开口谐振环单元结构超表面的太赫兹宽带涡旋光束产生器.该结构由金属-电介质两层构成,位于顶层的是基于双开口谐振环单元结构的超表面,底层为介质层.对单元结构阵列进行数值仿真,圆偏振的入射光可以被转换成相应的交叉偏振透射光,通过旋转表层金属谐振环,可以控制交叉偏振透射光具有相同的振幅和不同的相位.这些单元结构按照特定的规律排列,可以形成用以产生不同拓扑荷数的涡旋光束的涡旋相位板.以拓扑荷数1和2为例,设计了两种涡旋相位板,数值分析了圆偏振波垂直入射到该涡旋相位板生成交叉圆偏振涡旋光束的特性.结果表明,在1.39—1.91 THz的频率范围内产生了比较理想的不同拓扑荷数的涡旋光束,且透过率高于20%,最高可达到24%,接近单层透射式超表面的理论极限值.     

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.     

线偏光闪耀反射光栅衍射特性研究

通过数值方法对线偏光在Littrow装置中的理想导体闪耀光栅的偏振特性进行研究.用时域有限差分程序计算出任意偏振方向线偏光经过闪耀光栅衍射后的电磁场分布,提取衍射波的S偏振和P偏振分量,讨论了与光栅闪耀角和入射光波偏振角的关系.分析了在1530～1570nm光谱范围内,入射光波为线偏振波和园偏振波时,衍射光波的偏振色散特性与光栅闪耀角的关系,并给出了存在偏振色散的光栅闪耀角范围.这些分析和结果对工程设计具有一定的指导意义.     

Polarization conversions of diffractive wave plates based on orthogonal circular-polarization holography

A polarization holographic grating, which integrates the functions of a grating and a wave plate and is called a diffractive wave plate, is recorded by two beams(left and right circularly polarized) of a 532 nm laser in an azo polymer with a liquid-crystal structure. The polarization conversion characteristics of the diffractive wave plates are investigated with a detecting light of 650 nm by metering the polarization state of first-order diffracted light.It is confirmed that the diffractive wave plates convert the incident linear polarization into circular polarization for a linearly polarized probe laser and reverse the sense of rotation of the circular polarization when the detecting light is circularly polarized light.     

基于聚酰亚胺基底的太赫兹滤波器

超材料是通过人工方式做成的具有特殊电磁特性的亚波长周期性金属结构,通过合理的设计样品结构,可以实现自然界中传统材料无法实现的电磁现象。超材料可以广泛用于电磁隐身、完美吸收、负折射率等研究领域,近些年,随着太赫兹技术的发展,太赫兹超材料器件被广泛研究。由于硅（Si）对于太赫兹波的透过率较高,通常选取Si作为基底材料。但Si硬度较高、不易弯曲且易碎等缺点限制了THz超材料的应用。聚合物材料聚酰亚胺具有柔性,作为基底,克服了传统硅基底的缺点,透过率可以与Si匹敌,而且其表面光滑,适合传统光刻技术加工。对聚酰亚胺在太赫兹波段光学性质的测试结果表明,此种材料的折射率在1.9左右,透过率达到80%以上。设计了一种双开口谐振环结构,研究了其太赫兹波透射性质以及随太赫兹波的入射角度和样品曲率的变化规律,发现透射峰强度和峰位均不发生改变。此结果展示了将平面滤波延伸到曲面滤波领域的可能性,若将聚酰亚胺基底做薄,为今后太赫兹频段隐身衣的研究提供基础。将不同结构的两种样品叠加在一起制成宽谱滤波器, 50%的带宽达到181 GHz。此种宽带滤波器制作简单,滤波效果显著,为太赫兹波段宽带滤波器的制作提供一种新思路。     

Scattering of Circularly Polarized Terahertz Waves on a Graphene Nanoantenna

We present a surface current method to model the graphene rectangular nanoantenna scattering in the terahertz band with Comsol.Compared with the equivalent thin slab method,the results obtained by the surface current method are more accurate and efficient.Then the electromagnetic scattering of circularly polarized terahertz waves on graphene nanoantennas is numerically analyzed by utilizing the surface current method.The dependences of the antenna resonant frequency with the circularly polarized wave on width and length are consistent with those for the linear polarized waves.These results are proved to be useful to design efficient nanoantennas in terahertz wireless communications.