Broadband asymmetric transmission for linearly and circularly polarization based on sand-clock structured metamaterial

We proposed a sandwich structure to realize broadband asymmetric transmission(AT) for both linearly and circularly polarized waves in the near infrared spectral region. The structure composes of a silica substrate and two sand-clock-like gold layers on the opposite sides of the substrate. Due to the surface plasmons of gold, the structure shows that the AT parameters of linearly and circularly polarized waves can reach 0.436 and 0.403, respectively. Meanwhile, a broadband property is presented for the AT parameter is over 0.3 between 320 THz and 340 THz. The structure realizes a diode-like AT for linearly wave in forward and circularly wave in backward, respectively. The magnetic dipoles excited by current in the two gold layers contribute to the broadband AT. The current density in top and bottom metallic layers illustrates the mechanism of the polarization conversion for broadband AT in detail.     

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.     

Arbitrary linear THz wave polarization converter with cracked cross meta-surface

This Letter presents a double-layer structure combining a cracked cross meta-surface and grating surface to realize arbitrary incident linear terahertz(THz) wave polarization conversion. The arbitrary incident linear polarization THz wave will be induced with the same resonant modes in the unit cell, which results in polarization conversion insensitive to the linear polarization angle. Moreover, the zigzag-shaped resonant surface current leads to a strong magnetic resonance between the meta-surface and gratings, which enhances the conversion efficiency. The experimental results show that a more than 70% conversion rate can be achieved under arbitrary linear polarization within a wide frequency band. Moreover, around 0.89 THz nearly perfect polarization conversion is realized.     

基于IrMn/Fe/Pt交换偏置结构的无场自旋太赫兹源

与目前商用的太赫兹源相比,自旋太赫兹源具有超宽频谱、固态稳定以及成本低廉等优点,这使其成为下一代太赫兹源的主要研究焦点.但使用自旋太赫兹源时,通常需要外加磁场使铁磁层的磁化强度饱和,才能产生太赫兹波,这制约了其应用前景.基于此,本文制备了一种基于Ir Mn/Fe/Pt交换偏置结构的自旋太赫兹波发生器,通过Ir Mn/Fe中的交换偏置场和Fe/Pt中的超快自旋流注入与逆自旋霍尔效应相结合,在无外加磁场下产生了强度可观的太赫兹波.在Ir Mn和Fe的界面中插入超薄的Cu,可以使Fe在厚度很薄时零场下实现饱和磁化,并且其正向饱和场最高可达–10 m T,从而进一步提升无场下的太赫兹发射效率.零场下出射的太赫兹波的动态范围超过60 d B,达到可实用化的水平.通过旋转样品,发现产生的太赫兹波的偏振方向也会随之旋转,并且始终沿着面内垂直于交换偏置场的方向.此外,在此交换偏置结构的基础上,引入了一层自由的铁磁金属层Fe,设计了一种以Ir Mn/Fe/Pt/Fe为核心结构的自旋阀太赫兹源,发现产生的太赫兹强度在两层铁磁层反平行排列时比平行排列以及不引入自由铁磁金属层时均大约提升了40%.结果表明,基...     

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%.     

太赫兹波段电磁超材料吸波器折射率传感特性

太赫兹超材料吸波器作为一类重要的超材料功能器件,除了可以实现对入射太赫兹波的完美吸收外,还可以作为折射率传感器实现对周围环境信息变化的捕捉与监测.通常从优化表面金属谐振单元结构和改变介质层材料和形态两个方面出发,改善太赫兹超材料吸波器的传感特性.为深入研究中间介质层对太赫兹超材料吸波器传感特性的影响,本文基于金属开口谐振环阵列设计实现了具有连续介质层、非连续介质层和微腔结构的3款太赫兹超材料吸波器,并对其传感特性与传感机理进行了深入研究.结果表明,为了提高太赫兹超材料吸波器的折射率灵敏度、最大探测范围等传感特性,除了可以选用相对介电常数较小的材料作为中间介质层外,还可以改变中间介质层的形态,进而减小中间介质层对谐振场的束缚,增强谐振场与被测分析物之间的耦合.与传统的具有连续介质层的太赫兹超材料吸波器相比,具有非连续介质层和微腔结构的超材料吸波器具有更优越的传感特性,可应用于对待测分析物的高灵敏度、快速检测,在未来的传感领域具有更加广阔的应用前景.     

Broadband Terahertz Polarization Converter and Asymmetric Transmission Based on Coupled Dielectric‐Metal Grating

Coupled dielectric‐metal gratings are investigated for broadband terahertz (THz) wave polarization conversion and asymmetric transmission by the experiments and numerical simulations, which are composed of the subwavelength Si grating and metallic wire grating layers. The dielectric grating layer with a large artificial birefringence and low dispersion is employed as a phase engineered waveplate, and the metal wire grating arranged with a 45° angle to the dielectric grating is utilized as a high‐efficiency polarizer. Due to the subwavelength integration, this coupled grating presents a local resonance coupling mechanism between dielectric and metal gratings, which greatly enhances the polarization rotation and expands the bandwidth, not a simple combination with dielectric and metallic gratings. The results demonstrate that a broadband asymmetric transmission with an extinction ratio of 30dB from 0.2 to 1.2 THz is achieved and the highest transmission of 90% can be obtained. It provides a simple way towards practical applications for THz artificial dispersion materials, polarization control and asymmetric transmission.     

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.     

Dual-band asymmetric transmission of linearly polarized wave using Π-shaped metamaterial

A bilayered chiral metamaterial with Π-shaped structure is proposed, which demonstrates to exhibit dual-band asymmetric transmission (AT) of linearly polarized electromagnetic (EM) waves in two opposite directions. Incident x-polarized wave is almost converted to y-polarized wave while incident y-polarized wave is locked through the Π-shaped slab at 10.82 GHz; incident y-polarized wave is almost converted to x-polarized wave while incident x-polarized wave cannot pass through the Π-shaped slab at 14.1 GHz. The property of the AT is similar to the diode-switching characteristics, and the EM wave can be switched on/off by changing the polarization state of the incident wave. The surface current distributions of the Π-shaped structure are discussed to look into the physical mechanism.     

THz wave polarization-controlled spectroscopic imaging for anisotropic materials

We present a polarization-controlled terahertz (THz) wave spectroscopic imaging modality to investigate the anisotropy of the detected materials. The polarization of the emitted THz wave is controlled by changing the relative phase between the fundamental and second-harmonic waves in the two-color laser-induced air plasma THz generation configuration. The THz wave polarization direction is extracted by measuring the two electric field amplitudes when the polarization of the incident wave is controlled to be horizontal and vertical. The anisotropy of the industrial Sprayed-On-Foam-Insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrates that THz wave polarization-controlled imaging technique can be used for highly sensitive industrial nondestructive inspection and biological related characterization.     

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.     

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.     

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.     

Study on the polarization properties of electromagnetic waves with arbitrary magnetic declination in magnetized plasma

The polarization characteristics of oblique incidence electromagnetic waves in magnetized cold plasma layer are studied using the finite-difference time-domain (FDTD) method based on the trapezoidal recursive convolution (TRC) technology. The TRC-FDTD formulations are derived in detail and are confirmed by computing the reflection and transmission coefficients for the elliptically polarized wave through a magnetized cold plasma slab with arbitrary magnetic direction. Particularly, when the propagation direction of the EM wave is parallel to the magnetic direction, the right-circular-polarized and left-circular-polarized wave should be considered. When the propagation direction of the EM wave is perpendicular to the magnetic direction, the ordinary polarized wave and extraordinary polarized wave should be considered.     

多功能太赫兹超表面偏振控制器

本文提出了一种“金属栅-开口环/硅环-金属栅”结构的透射式超表面偏振控制器, 研究了入射角度和抽运光对该器件传输及偏振态控制性能的影响. 研究结果表明, 当线偏振太赫兹波垂直入射时, 可对0.39-1.11 THz频段的太赫兹波实现偏振方向90°旋转, 偏振旋转效率为99%, 损耗为1 dB. 对于斜入射的情况, 偏振转换性能在0-60°范围内基本保持不变, 且透过率达到90%以上. 同时, 通过调控抽运光强度的方式, 该器件能够实现对透射与反射太赫兹光束的强度调制, 调制深度均达到90%, 且可以实现太赫兹波偏振分束功能. 该器件可以作为未来太赫兹空间光通信和信息处理的宽带、角度不敏感、可调谐的偏振转换器和分束器.     

Compact multiband left-handed metamaterial at terahertz frequencies

We design and analyze a novel multiband left-handed metamaterial based on a fishnet-like structure at terahertz (THz) frequencies.The metamaterial exhibits simultaneous negative refractions around the frequencies of 0.48,1.05,and 1.19 THz for the electromagnetic (EM) wave normal incidence,and around the frequencies of 0.20,0.79,and 1.13 THz for parallel incidence.The simulated results verify the left-handed properties.A particularly important observation is the capability of the proposed metamaterial with a single geometrical structure to display multifrequency operations in a unit cell.The compact metamaterial is a major step toward the miniaturization of THz materials and devices suitable for multifrequencies.     

Terahertz polarization imaging with birefringent materials

We present a terahertz (THz) imaging method with a birefringent material which is sensitive to THz wave polarization. The subtle THz polarization modulations introduced by the target are extracted by the amplitudes of the two separated THz peaks passing through the birefringent material. The anisotropy of the industrial sprayed-on-foam-insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrated that this polarization sensitive THz imaging technique can be used for industrial inspection and biological related characterization.     

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.     

Multi-band terahertz two-handed metamaterial based on the combined ring and cross pairs

We present the design of a Multiband two-handed metamaterial (MM) with the composite structure combined ring and cross pairs (RP + CP) in the terahertz regime. The dual-band left-handed and single-band right-handed transmission properties of this composite structure were explored by the FDTD method. The MM exhibits left-handed and right-handed transmission pass bands around the frequencies of 0.43 THz, 1.32 THz and 0.85 THz for the terahertz electromagnetic (EM) wave normal incidence, respectively. The surface currents distributions are demonstrated to discuss the physical mechanism of the left-handed properties of the proposed MM. The retrieved equivalent EM parameters and the refraction phenomenon based on a wedge-shaped model verify the left-handed properties. Furthermore, the dual-band left-handed transmission properties of RP + CP structure can be tunable individually by changing the structural parameters. The presented design of terahertz two-handed MM offers a flexibility for investigation of their novel EM properties, and important terahertz device applications.     

基于二氧化钒宽、窄带可切换的双功能超材料吸收器研究

本文提出了一种宽、窄带可切换的双功能超材料吸收器.在超材料吸收器的结构中,引入了相变材料二氧化钒(VO2),仅利用单个可切换超表面就能实现不同的功能,其不同功能之间的相互转换通过VO2绝缘态和金属态之间的可逆相变特性实现.当VO2处于金属态时,设计的结构可以看作一个超材料宽带吸收器.仿真结果表明,在1.55THz至2....