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.     

Scattering characteristics of structures of lossy metamaterial–semiconductor cylinders

Here, we present for the first time the rigorous boundary problem solution of the Maxwell’s equations for the determination of scattering characteristics of a structure. The structure consists of a finite set of infinite parallel circular cylinders that can be made of different lossy isotropic materials. We numerically analyzed two structures that differ only in the symmetrical arrangement of semiconductor cylinders in relation to a central metamaterial cylinder. The electrical radii of cylinders can be arbitrary. Both polarizations of the incident microwave are considered in this work. The Poynting vector of the plane microwave that reflected from and transmitted through the structures analyzed here. We investigated dependency on the radius of an arc where are placed the semiconductor cylinders, the semiconductor-specific resistivity, the operating frequency at two radii of the metamaterial cylinder. We discovered that the structure can have features of a band gap photonic crystal dependent on the topology and the polarization of the incident microwave. We have found that the structure can operate as a microwave reflector at the certain radius of the arc on which are located thirteen n-Si cylinders. The Poynting vector is very sensitive to the change of semiconductor-specific resistivity when the incident microwave has the parallel polarization.     

Transmission-type reconfigurable metasurface for linear-to-circular and linear-to-linear polarization conversions

We present a transmission-type polarization conversion metamaterial (PCM) whose functions can be dynamically switched among the linear-to-circular (LTC) and linear-to-linear (LTL) polarization conversions. The proposed PCM consists of a grating, a polarization conversion surface and a reconfigurable polarization selective surface incorporated with PIN diodes. By changing the states of diodes, the PCM can achieve the reconfigurable manipulations for incident waves. The Fabry-Pérot (F-P) resonances excited by the PCM contribute to the polarization conversions, as is illustrated. Moreover, through establishing the F-P-like cavity model and analyzing the electric field components of the transmitted waves, the conditions for realizing LTC polarization conversion are revealed, which can guide the construction of PCM. The prototype of PCM is fabricated and measured, which can achieve LTC and LTL polarization conversions within 3.31-3.56 GHz and 2.76-4.24 GHz, respectively, the polarization conversion ratios of two functions are higher than 0.95. The measurement results are in agreement with the simulation data.     

Broadband 3D isotropic negative-index metamaterial based on fishnet structure

In this paper, we numerically demonstrate a broadband 3D isotropic negative index metamaterial (NIM) at microwave frequency ranges, which is composed of double periodic array metallic fishnet structure (FS) etched on the six sides of a cubic dielectric substrate. The electric and magnetic L-C resonance circuit models are constructed to demonstrate the broadband resonance properties of the proposed 3D metamaterial. The finite integration technology (FIT) simulation and standard S parameters retrieval methods are used to calculate and analyze the negative characteristics, isotropy and polarization of the 3D model. The numerical results show that the negative index bandwidth is about 7 GHz and relative bandwidth can be up nearly to 63%, the negative-index pass band is independent of the polarization of incident waves and is almost the same for different oblique incident angles. Thus, the proposed metamaterial is good candidate as a broad-band 3D isotropic NIMs.     

A pure dielectric metamaterial absorber with broadband and thin thickness based on a cross-hole array structure

A pure dielectric metamaterial absorber with broadband and thin thickness is proposed, whose structure is designed as a periodic cross-hole array. The pure dielectric metamaterial absorber with high permittivity is prepared by ceramic reinforced polymer composites. Compared with those with low permittivity, the absorber with high permittivity is more sensitive to structural parameters, which means that it is easier to optimize the equivalent electromagnetic parameters and achieve wide impedance matching by altering the size or shape of the unit cell. The optimized metamaterial absorber exhibits reflection loss below -10 dB in 7.93 GHz-35.76 GHz with a thickness of 3.5 mm, which shows favorable absorption properties under the oblique incidence of TE polarization (±45°). Whether it is a measured or simulated value, the strongest absorbing peak reaches below -45 dB, which exceeds that of most metamaterial absorbers. The distributions of power loss density and electric and magnetic fields are investigated to study the origin of their strong absorbing properties. Multiple resonance mechanisms are proposed to explain the phenomenon, including polarization relaxation of the dielectric and edge effects of the cross-hole array. This work overcomes the shortcomings of the narrow absorbing bandwidth of dielectrics. It demonstrates that the pure dielectric metamaterial absorber with high permittivity has great potential in the field of microwave absorption.     

An ultrathin transparent metamaterial polarization transformer based on a twist-split-ring resonator

In this paper, an ultrathin transparent metamaterial polarization transformer using a circular twist-split-ring resonator (TSRR) was proposed and investigated experimentally and numerically. The experimental and simulated results exhibit an asymmetric transmission only for forward and backward propagating linearly polarized waves. An incident linearly polarized wave can convert its polarization nearly completely to the cross direction after transmission under certain conditions. The simulated spatial evolution of the electric field further indicates that the twist structure functions as a perfect polarization transformer at certain frequencies.     

Circular-polarization-dependent study of the microwave photoconductivity in a two-dimensional electron system

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasioptical setup in which linear and any circular polarization can be produced in situ. The microwave induced resistance oscillations and the zero resistance regions are notably immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations between different polarizations occur only near the cyclotron resonance where an unprecedented large resistance response is observed.     

Broad band metamaterial absorber based on wheel resonators with lumped elements for microwave energy harvesting

A new metamaterial absorber structure is designed and characterized both numerically and experimentally for microwave energy harvesting applications. The proposed structure includes four wheel resonators with different dimensions, from which the overall response of the structure can then be obtained by summing all the overlapping frequency responses corresponding to each dimension. The essential operation frequency range of the wheels is selected in such a way that the energy used in wireless communications and found within the environment that we live is absorbed. The dimensions are obtained using parametric study and genetic algorithm to realize wideband absorption response. When the simulation and measurement results are taken into account, it is observed that the metamaterial absorber based harvester has potential to absorb and convert microwave energy with an absorption ratio lying within the range of 80 and 99% for the frequency band of 3–5.9 and 7.3–8 GHz. The conversion efficiency of the structure as a harvester is found to be greater than 0.8 in the interval of 2–5 GHz. Furthermore, the incident angle and polarization dependence of the wheel resonator based metamaterial absorber and harvester is also investigated and it is observed that the structure has both polarization and incident angle independent frequency response with good absorption characteristics in the entire working frequency band. Hence, the suggested design having good absorption, polarization and angle independent characteristics with wide bandwidth is a potential candidate for future energy harvester using wireless communication frequency band.     

人工异向介质调控电磁波极化特性的实验与仿真研究

电磁波的极化特性在通信、导航和雷达等方面已逐渐得到应用.为了有效控制电磁波的极化状态, 本文设计了一种基于开口环结构的人工异向介质.该人工异向介质由开口环结构,电介质基底和金属背板组成, 可以将入射的线性极化波完整地转换为圆极化波,椭圆极化波以及和入射波极化方向垂直的线极化波. 本文通过实验和仿真验证了本文的设计,实验结果和仿真结果符合较好.     

Optimal helix shape: Equality of dielectric,magnetic, and chiral susceptibilities

The possibility of linearly polarized electromagnetic microwave conversion into a circularly polarized wave using single-turn helices with preliminary calculated optimal parameters is demonstrated. The helices are characterized by equal dielectric, magnetic, and chiral susceptibilities. Such optimal helices can be further used, for example, to develop reflectionless coatings and metamaterials with negative electromagnetic wave refraction. It is demonstrated that the examined helices activated by both electric and magnetic fields, that is, for any arbitrary orientation of the incident wave polarization plane, have optimal characteristics. This is the advantage of the optimal helices over other possible metamaterial elements, for example, rectilinear vibrators or circular resonators.     

Asymmetric chiral metamaterial circular polarizer based on twisted split-ring resonator

An asymmetric chiral metamaterial (CMM) circular polarizer based on bilayer twisted split-ring resonator structure was proposed and investigated. Both numerical simulations and experiments reveal that when a y-polarized wave is incident on this CMM propagating along backward (?z) direction, the two linear components of the transmitted wave have nearly equal amplitudes and 90°(?90°) phase difference at the resonant frequencies. This means that the right-hand circular polarization and left-hand circular polarization are realized in transmission at 6.4 and 8.1 GHz, respectively. The surface current distributions are studied to illustrate the transformation behavior for both circular polarizations. Further, the influences of the structural parameters of the circular polarizer to the transformation transmissions spectra have been investigated numerically.     

一种基于旋转调谐的超材料

数值仿真研究了一种可调谐的双开口谐振环(DSRR)超材料.在平行入射的电磁波激励下,这种DSRR单元可以在不同的频段分别表现出磁谐振和电谐振.当外加电场E与DSRR的双开口平行时,DSRR受激励得到的磁谐振和电谐振强度最大.随着DSRR超材料沿外加磁场H方向顺时针旋转,其磁谐振和电谐振频率基本保持不变,但谐振强度均发生显著下降,同时对应透射相位的突变也逐渐降低.提出的超材料调谐方法只需要简单地旋转材料,而不需要改变原有超材料单元的结构或者增加额外的激励场,极大地简化了可调谐超材料的制备及应用,在电磁开关、相位调制等方面具有潜在的应用.同时,这种简单的方法有希望应用于更高频段的超材料调谐,可以有效地拓展太赫兹频段和光频段超材料的实际应用.     

Manipulate light polarizations with metamaterials: From microwave to visible

Polarization is an important characteristic of electromagnetic (EM) waves, and efficient manipulations over EM wave polarizations are always desirable in practical applications. Here, we review the recent efforts in controlling light polarizations with metamaterials, at frequencies ranged from microwave to visible. We first presented a 4 × 4 version transfer matrix method (TMM) to study the scatterings by an anisotropic metamaterial of EM waves with arbitrary propagating directions and polarizations. With the 4 × 4 TMM, we discovered several amazing polarization manipulation phenomena based on the reflection geometry and proposed corresponding model metamaterial systems to realize such effects. Metamaterial samples were fabricated with the help of finite-difference-time-domain (FDTD) simulations, and experiments were performed to successfully realize these ideas at both microwave and visible frequencies. Efforts in employing metamaterials to manipulate light polarizations based on the transmission geometry are also reviewed.     

Manipulate light polarizations with metamaterials: From microwave to visible

Polarization is an important characteristic of electromagnetic (EM) waves, and efficient manipulations over EM wave polarizations are always desirable in practical applications. Here, we review the recent efforts in controlling light polarizations with metamaterials, at frequencies ranged from microwave to visible. We first presented a 4 × 4 version transfer matrix method (TMM) to study the scatterings by an anisotropic metamaterial of EM waves with arbitrary propagating directions and polarizations. With the 4 × 4 TMM, we discovered several amazing polarization manipulation phenomena based on the reflection geometry and proposed corresponding model metamaterial systems to realize such effects. Metamaterial samples were fabricated with the help of finite-difference-time-domain (FDTD) simulations, and experiments were performed to successfully realize these ideas at both microwave and visible frequencies. Efforts in employing metamaterials to manipulate light polarizations based on the transmission geometry are also reviewed.     

Dual-band asymmetric electromagnetic wave transmission for dual polarizations in chiral metamaterial structure

In this paper, we propose a chiral metamaterial structure that enables dual-band asymmetric transmission effect for different linearly polarized electromagnetic waves. The metamaterial is composed of metallic spirals with two split-ring resonators sandwiching a dielectric slab and connecting with via hole. Strong one-way transmission of two orthogonally polarized waves at different frequency bands has been confirmed through both full-wave simulation and test on fabricated prototype at the microwave band. Analysis also shows such asymmetric transmission can be attributed to the induced asymmetric current distributions in the spiral that support strong polarization conversion and cross-polarization transmission. By scaling down the metamaterial structure, the concept could also be utilized at other frequency bands, such as submillimeter or even terahertz band and find applications in designing one-way electromagnetic wave devices or polarization spectral filters.     

对电磁波极化不敏感超材料吸波体的研究

设计了一个在微波频段对极化不敏感的超材料吸波体。该超材料微观单元由4个互相垂直的电谐振环和短导线构成,这种结构克服了Landy提出的结构对电磁波极化敏感的缺点,对垂直极化和水平极化电磁波都有很好的吸收效果。采用数值仿真方法,在12-180Hz波段提取了这种超材料的S参数,计算了其吸波率。单层超材料吸波体在14GHz处达到吸波峰,吸波率达57．4％;多层组合吸波体在150Hz处吸波率峰值达到87．6％。     

“Quad-band flexible magnesium zinc ferrite (MgZnFe2O4)-based double negative metamaterial for microwave applications”

This article presents vertically coupled, rectangular complementary split-ring resonator-shaped quad-band double-negative (DNG) metamaterial unit cells, that is, having both negative permittivity and permeability, which redirect negative refractive and also are not found in nature. The metamaterial is fabricated on magnesium zinc ferrite-based flexible microwave substrates, and the flexible substrates are chosen with two different concentrations of magnesium (Mg) denoted by Mg₃₀ and Mg₅₀ for 30% and 50% of Mg, which possess dielectric constants of 4.32 and 3.15 and loss tangents of 0.003 and 0.005, respectively. The proposed metamaterials are demonstrated by utilizing the CST microwave simulator, and their effective parameters are extracted according to the Nicolson-Ross-Wire method. With Mg_30, the prepared, flexible metamaterial shows measured resonances at 3.70 GHz, 7 GHz, 8.60 GHz, and 9.78 GHz, whereas with Mg₅₀ it shows the measured resonances at 4.10 GHz, 7.70 GHz, 9.33 GHz, and 10.62 GHz. Very good effective medium ratios (EMR) along with DNG properties are obtained, namely 6.5 and 5.85 for Mg₃₀ and Mg_50, respectively, with a physical dimension of 12.5 × 9.5 mm² for both of the unit cells_. Also, the electric field, magnetic field, and surface current distribution at different resonances and the polarization insensitivity at different polarization angles were observed. Thus, the designed new flexible substrate microwave materials based on DNG metamaterials are potential candidates for S-, C- and X-band applications, as well as for flexible microwave technologies.     

Metamaterial absorber based on circular ring structure with and without copper lines

This paper presents the analysis of circular ring metamaterial absorber with the existing of copper lines. The structure is designed using lossy FR4 substrate with thin copper layers. The circular ring shape with copper lines is printed on the top surface of FR4 substrate, while at the bottom surface is printed with full copper ground plane. Parametric study is done to investigate the effect of copper lines on the resonance frequency. From the simulation, the circular ring metamaterial absorber with vertical copper lines can resonates at lower frequency, but this structure is polarized sensitive. This drawback can be improved by adding horizontal copper lines together with the vertical copper lines. The circular ring structure with vertical and horizontal copper lines is not only polarization insensitive, but it can works at wide operating angle of incident waves.     

3D broadband isotropic NRI metamaterial based on metallic cross-pairs

In this paper, a new type of 3D metamaterial composed of double periodic array of metallic cross-pairs printed on the six sides of a cubic dielectric substrate was proposed to obtain the characteristics of broadband NRI and isotropy for the applications of super lenses. The behaviors of NRI, isotropy and polarization were analyzed using the CST Microwave Studio. The results show that the proposed metamaterial exhibits not only a broadband NRI whose relative bandwidth can be up to 56.7%, but also polarization-independence and isotropy. Thus, the proposed metamaterial is a good candidate material for 3D broadband isotropic NRI metamaterial.     

Asymmetric chiral metamaterial circular polarizer based on four U-shaped split ring resonators

An asymmetric chiral metamaterial structure is constructed by using four double-layered U-shaped split ring resonators, which are each rotated by 90° with respect to their neighbors. The peculiarity of the suggested design is that the sizes of the electrically and magnetically excited rings are different, which allows for equalizing the orthogonal components of the electric field at the output interface with a 90° phase difference when the periodic structure is illuminated by an x-polarized wave. As a result, left-hand circular polarization and right-hand circular polarization are obtained in transmission at 5.1 GHz and 6.4 GHz, respectively. The experiment results are in good agreement with the numerical results.