Chiral Metamaterial Structure Research Articles

Low frequency bandgap characteristics of a 3D chiral acoustic metamaterial structure

It has been proved that bandgaps in acoustic metamaterials can block elastic waves in certain frequencies, which offer an unprecedented solution to the low-frequency vibration control. However, there are still challenges of low bandgap frequencies and wide bandwidth. Meanwhile, chirality breaks the symmetry of the structures, which could also contribute to the formation of bandgaps in acoustic metamaterials. Based on research background mentioned above, this work proposes a type of novel 3D chiral acoustic metamaterial structures. The dispersion relations are investigated by using the Bloch's theorem. Then, the specimens are fabricated by 3D printing combined with glue processing. A series of tests is carried out to study their vibration characteristics. It is shown that the simulated results of the structures are in good agreement with experimental results, which verified the validity of the numerical models. The dispersion relations and corresponding frequency-response curves are plotted. After embedding fillers, the bandgaps of structures decrease about 62.4 % and total bandwidth decrease about 46.5 %. In addition, the regularity of bandgaps in structures with different parameters are analyzed and the structure optimized accordingly shows good vibration reduction performance in low-frequencies. This work can provide a new possibility for engineering application in the field of vibration and noise control.

Observation of Chirality‐Induced Roton‐Like Dispersion in a 3D Micropolar Elastic Metamaterial

AbstractA theoretical paper based on chiral micropolar effective‐medium theory suggested the possibility of unusual roton‐like acoustical‐phonon dispersion relations in 3D elastic materials. Here, as a first novelty, the corresponding inverse problem is solved, that is, a specific 3D chiral elastic metamaterial structure is designed, the behavior of which follows this effective‐medium description. The metamaterial structure is based on a simple‐cubic lattice of cubes, each of which not only has three translational but also three rotational degrees of freedom. The additional rotational degrees of freedom are crucial within micropolar elasticity. The cubes and their degrees of freedom are coupled by a chiral network of slender rods. As a second novelty, this complex metamaterial is manufactured in polymer form by 3D laser printing and its behavior is characterized experimentally by phonon‐band‐structure measurements. The results of these measurements, microstructure finite‐element calculations, and solutions of micropolar effective‐medium theory are in good agreement. The roton‐like dispersion behavior of the lowest phonon branch results from two aspects. First, chirality splits the transverse acoustical branches as well as the transverse optical branches. Second, chirality leads to an ultrastrong coupling and hybridization of chiral acoustical and optical phonons at finite wavevectors.

Spiral Chiral Metamaterial Structure Shape for Optical Activity Improvements.

We report on a spiral structure suitable for obtaining a large optical response. We constructed a structural mechanics model of the shape of the planar spiral structure when deformed and verified the effectiveness of the model. As a verification structure, we fabricated a large-scale spiral structure that operates in the GHz band by laser processing. Based on the GHz radio wave experiments, a more uniform deformation structure exhibited a higher cross-polarization component. This result suggests that uniform deformation structures can improve circular dichroism. Since large-scale devices enable speedy prototype verification, the obtained knowledge can be exported to miniaturized-scale devices, such as MEMS terahertz metamaterials.

Strong and Broadband Pure Optical Activity in 3D Printed THz Chiral Metamaterials

AbstractOptical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization‐related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, a chiral metamaterial (CMM) structure composed of pairs of vertical U‐shape resonators of “twisted” arms is studied, and it is revealed that it demonstrates large pure optical activity (i.e., optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25° for an unmatched bandwidth of 1 THz (relative bandwidth 80%), from a 130 µm‐thickness structure, while theoretical optimizations show that the rotation can reach 45°. The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which also provides simple optimization rules for the enhancement of its chiral response. The proposed chiral design allows for easy fabrication via direct laser writing (DLW) and electroless metal plating, making the associated structures suitable candidates for polarization control applications.

Numerical investigation of low band gap and vibration suppression for novel four-ligament chiral composite metamaterial structures

Numerical investigation of low band gap and vibration suppression for novel four-ligament chiral composite metamaterial structures

Asymmetric transmission modular design via direction-dependent absorption of spoof surface plasmon polaritons

A method of achieving asymmetric transmission is proposed by means of the propagation direction-dependent absorption of spoof surface plasmon polaritons (SSPPs). In this letter, trapezoidal wire arrays which act as absorbers have been combined with cutting metal wires which act as polarization rotators. By combining trapezoidal wire arrays with polarization rotators, the metamaterial can exhibit different properties according to the direction of the incident wave, thereby achieving asymmetric transmission. Incident waves can be efficiently converted to the SSPPs by trapezoidal wire arrays, and their propagation and/or absorption can be controlled by engineering the spatial dispersion of k-vector. Moreover, polarization rotators based on cutting metal wires can customize the polarization rotation performance by changing the length of the metal wire. The simulated and measured results verify the asymmetric transmission effect and design method. The proposed asymmetric transmission structure can be applied in electromagnetic devices for linearly polarized wave or polarization control. Compared with chiral metamaterial structures, this work improves the design customizability and efficiency as well as provides an alternative method of designing asymmetric transmission. In addition, the framework expands the application prospect of asymmetric transmission and builds a bridge from SSPPs to asymmetric transmission, facilitating the asymmetric transmission integrated with other SSPPs devices.

Enhanced Chiral Sensing at the Few-Molecule Level Using Negative Index Metamaterial Plasmonic Nanocuvettes.

Chirality is a fundamental property of biological molecules and some pharmaceutical molecules. Chiral molecules have a pair of chiral isomers (enantiomers) with opposite handedness. Although both enantiomers of the same molecule have identical chemical and physical properties, one enantiomer may be toxic to living organisms while the other one is harmless. The detection of these enantiomers is done using their small differential absorption between right and left circularly polarized light, known as circular dichroism (CD). Considering the macroscopic size of these molecules, combined with their small differential absorption, the obtained CD signal is very small, imposing a severe limitation on the minimal concentration that can be detected. Chiral plasmonic and metamaterial structures have been used to enhance the sensitivity of CD measurements by orders of magnitude through chiral density hot spots (super chiral fields). However, the large background signal due to these structures' intrinsic chirality limits the effectiveness of these methods. Contrary to absorption-based chiral sensing measurements (CD), fluorescence detection circular dichroism (FDCD) sensing can greatly improve chiral measurement sensitivity, down to the ultimate limit of a few and even a single chiral molecule. Like differential absorption, differential fluorescence also produces a weak signal at the few-chiral-molecule limit. However, here we demonstrate a negative-index metamaterial (NIM) cavity that acts as a "plasmonic nanocuvette" with globally enhanced volume super chiral fields. Moreover, the achiral structure of the plasmonic nanocuvette allows for completely background-free chiral sensing. We show that with NIM-cavity-enhanced FDCD, we can detect as low as a few tens of chiral molecules, well within the zeptomole range.

Broadband linear polarizer with high-efficient asymmetric transmission using a chiral metasurface

Asymmetric transmission, as a peculiar electromagnetic phenomenon in electromagnetic wave transmission, has been attracted extensive attention because of its important applications in polarization control and photodiode. This wok proposes a broadband linear polarizer based on a chiral metasurface with high-efficient asymmetric transmission performance. The designed polarizer consists of double arc-shaped structures interconnected via two circular cavities, which has the capacity to achieve broadband asymmetric transmission effect at 4–15 GHz for linear polarization conversion. The polarization conversion ratio exceeds 90% at 6.8–11.9 GHz and has a relative bandwidth of 54.5%. The asymmetric transmission parameter is above 0.59 in the frequency band 7.4–11.6 GHz. The microwave experiment results show a great consistency between simulation and experiment. The physical mechanism of polarization conversion and asymmetric transmission are further studied by introducing the surface current and electric field distributions. The designed polarizer has the superiority of uncomplicated structure and easy control, and can be extended to terahertz and optical regime. We believe that it has a broad application prospect in the field of chiral metamaterial structure design.

Deep-learning-assisted designing chiral terahertz metamaterials with asymmetric transmission properties

Chiral metamaterial induced asymmetric transmission (AT) possesses great potential for terahertz (THz) polarization applications, but its design has mainly relied on the conventional trial-and-error forward strategy. Here, based upon massive training samples, we propose a deep-learning-based approach to accelerate the design of chiral metamaterials. The deep learning framework includes two bidirectional networks that allow the model to self-autonomously decipher the nonintuitive relationship between chiral metamaterial structures and their corresponding electromagnetic responses. Our preliminary results show that our model can accurately predict THz responses for any kind of metamaterial structure and inversely retrieve structure parameters from given THz responses. It turns out that the deep-learning-assisted methodology exhibits more efficient design performance than the conventional physical-based metamaterial design approach. This work might provide another artificially intelligent design strategy for chiral metamaterials and shed light on the prosperous development of unprecedented THz applications.

Plasmon-Induced Transparency for Tunable Atom Trapping in a Chiral Metamaterial Structure.

Plasmon-induced transparency (PIT), usually observed in plasmonic metamaterial structure, remains an attractive topic for research due to its unique optical properties. However, there is almost no research on using the interaction of plasmonic metamaterial and high refractive index dielectric to realize PIT. Here, we report a novel nanophotonics system that makes it possible to realize PIT based on guided-mode resonance and numerically demonstrate its transmission and reflection characteristics by finite element method simulations. The system is composed of a high refractive-index dielectric material and a two-dimensional metallic photonic crystal with 4-fold asymmetric holes. The interaction mechanism of the proposed structure is analyzed by the coupled-mode theory, and the effects of the parameters on PIT are investigated in detail. In addition, we first consider this PIT phenomenon of such fields on atom trapping (87Rb), and the results show that a stable 3D atom trapping with a tunable range of position of about ~17 nm is achieved. Our work provides a novel, efficient way to realize PIT, and it further broadens the application of plasmonic metamaterial systems.

High-Efficiency Asymmetric Transmission of Red-Near-Infrared Light Based on Chiral Metamaterial

Designing and fabricating high-performance polarization converters that exhibit asymmetric transmission (AT), for light with different circularly/linearly polarized states with opposite propagating directions, are in high demand. The AT phenomenon leads to potential applications as isolators and circulators in information and communication systems. We propose a chiral metamaterial structure with high AT efficiency for two types of linearly orthogonal polarized red-near-IR light in two opposite incident directions. Theoretical results showed that the proposed chiral metamaterial structure achieves cross-polarization conversion where the polarization conversion ratio (PCR) is over 90%, in a broadband wavelength range from 715 to 810 nm, for both forward-propagating linearly polarized light and backward-propagating orthogonal linearly polarized light. The physical mechanisms of the polarization converter with the AT have been investigated. It was confirmed that the Fabry–Perot-like resonance and coupling between electric and magnetic dipoles lead to highly efficient asymmetric polarization conversion for two orthogonal linearly polarized light. Additionally, the conversion efficiency and bandwidth of the polarization converter are successfully optimized by adjusting the related structure parameters.

Kirigami Chiral Metamaterial Structure with Adjustable Shape after Deformation

Kirigami Chiral Metamaterial Structure with Adjustable Shape after Deformation

Analysis and design of new chiral metamaterials with asymmetric transmission characteristics

Asymmetric transmission (AT) metamaterials are extensively studied and applied in the fields of polarization converters and photodiodes. In order to further improve the properties of polarization conversion and unidirectional conduction in the high frequency band and to implement their tunability, the novel chiral electromagnetic metamaterials are studied. By the topology optimization technique, a new type of double-layer L-shaped variant metamaterial structure with excellent asymmetric transmission characteristics is designed. The objective function is to maximize the asymmetric transmission coefficient for the linear polarization wave. The rotationally symmetrical design domain is determined by considering polarization conversion and computation efficiency simultaneously. The design domain of upper layer is divided into two parts which are both the 180° rotationally symmetrical. The design domain of the upper layer and lower layer are the 90° rotationally symmetrical around the <i>x</i> and <i>z</i> axis respectively. Therefore, the number of design variables is only 18. Asymmetric transmission of linear polarization wave in the K band and Ka band are implemented. Numerical simulation results and experimental results show that the optimized chiral metamaterial has excellent asymmetric transmission characteristics, and its asymmetric transmission coefficient reaches 0.8562 at a frequency of 21.65 GHz and 0.8175 at a frequency of 28.575 GHz. Its asymmetric transmission mechanism is expounded by analyzing the electric field and surface current distribution at the resonance frequency. Based on the optimized chiral metamatertials, the reasonable geometric parameters are selected and the rotation angle of the metal layer is changed in order to further achieve the tunable AT characteristics. First, the influences of the dielectric substrate layer, the thickness of the metal layer and the side length of the grid on resonance frequency and asymmetric transmission coefficient are analyzed respectively, which provides the basis for the reasonable adjustment of the structural parameters to obtain better asymmetric transmission characteristics. After the reasonable geometric parameters are determined, the rotational angle of the upper metal layer and lower metal layer are changed. The linearly and circularly polarized wave are simultaneously achieved in the K band. In this article, the topology optimization technique is used to design the asymmetric transmission chiral metamaterial structure. The design process has a clear direction. The optimized asymmetric transmission chiral metamaterial has the simple structure type and the easy tunability of its asymmetric transmission characteristics. It can be used widely and easily in the fields of polarization converters and photodiodes. This design method has a broad application prospect in the chiral metamaterial field.

Dual-Band Circular Polarizers Based on a Planar Chiral Metamaterial Structure

A novel dual-band chiral metamaterial (CMM) circular polarizer is proposed based on a dual-armed wheel-like structure. The proposed CMM can convert a linearly polarized electromagnetic wave into a circularly polarized (CP) mode. The simulation and measured results show, with good consistency, that the CMM exhibits good CP characteristics, such as a polarization isolation higher than 18 dB and an axial ratio lower than 1 dB. In addition, our studies show that all the resonance frequencies of this CMM structure are independent. Finally, a simple design method is presented and verified by a simulation to change the resonance frequencies of the proposed dual-band CMM circular polarizers.

Low-Loss Left-Handed Gammadion-Fishnet Chiral Metamaterials

Negative refraction has many potential applications in the information and communications technology field. However, the high losses of most negative refraction materials prevent the use of this novel property in the implementation of practical devices in the HF range. In an attempt to overcome this inconvenience, we present two gammadion–fishnet chiral metamaterial structures, constituted by the combination of a lossy chiral metamaterial and different fishnet structures. The electromagnetic characterization of the proposed metamaterials provides a larger bandwidth with negative refraction as well as lower losses when compared with the initial structures. The results were obtained by using both numerical and experimental data, which support our designs.

A multi-functional polarization convertor based on chiral metamaterial for terahertz waves

Abstract In this paper, a multi-functional polarization convertor for terahertz (THz) light composed of a bilayer wire-split-ring (WSR) structure chiral metamaterial (CMM) was proposed and investigated numerically. Simulation results indicate that when normal incident light is propagating along the – z axis direction through the proposed CMM structure, the y-polarization is converted to the left- and right-circular polarization (LCP and RCP) at 1.14 THz and 1.34 THz, respectively. Meanwhile, the x-polarization is converted to y-polarization in the frequency range of 2.19 – 2.47 THz. The further calculation results show that the proposed CMM structure can transmit a nearly pure circular polarization light with a polarization extinction ratio (PER) of over 30 dB and an orthogonal linear polarization light with a polarization conversion ratio (PCR) of over 90%. The mechanism of the polarization conversion properties is illustrated by the simulated electrical field and surface current distribution. Due to its excellent multi-functional polarization conversion properties, the proposed CMM is useful for the development of the integrated THz polarization manipulation devices.

Microstrip Patch Antennas Covered with Chiral Metamaterial Structures

In this working we present gain characteristic of microstrip patch antennas covered with chiral metamaterial. In order to determine gain of antennas covered with chiral metamaterial structure, S11 parameters and radiation pattern of antennas with chiral metamaterial and without chiral metamaterial are plotted and compared each other. The simulation results show that antennas covered with chiral metamaterial structure increase either gain or radiation pattern or both at operation frequency.

2D to 3D convertible terahertz chiral metamaterial with integrated pneumatic actuator.

Aiming to achieve real 3D chiral metamaterial structure, a new strategy is proposed. During which a 2D to 3D conversion mechanism based on integrated pneumatic actuator is used to provide a relatively simple yet effective process solution. With this concept, only the lithography-based planar process is required, and a symmetric tuning capability over the shape of the 3D chiral metamaterial as well as its chirality both can be achieved through controlling the applied pressure. At the same time, the resultant optical activity and circular dichroism associated with this tunable 3D chiral metamaterial are also studied and exciting characteristics demonstrating enantiomeric symmetry have been successfully obtained as well.

Study on Circularly Polarized Patch Antenna With Asymmetric Chiral Metamaterial

A circularly polarized (CP) patch antenna with a new chiral metamaterial structure was proposed. The proposed CP antenna is composed of a conventional linearly polarized patch antenna and a chiral metamaterial (CMM) board. For the CMM unit cell, the geometric parameters of the two pieces of copper films that are patterned on opposite sides of an FR-4 board are quite different. The function that a linear-polarization wave is converted into a circular polarization wave can be realized by the designed structure through simulation verification. When the CMM board is placed in the front of a conventional linear-polarized patch antenna, the antenna polarization mode can be changed to the circular polarized mode. The antenna performances have been investigated numerically and experimentally. Meanwhile, the new CMM structure could be applied to the field of circular-polarized antenna.

New study of geometry effect on chiral metamateri-al nanostructures

In this paper, the Comsol Multiphysics version 5.0 is used to study the effect of geometric parameters on transmission of chiral metamaterial nanostructures. The angle of the chiral metamaterial element was varied to see his effect. Transmission coefficient (S21) and reflection coefficient (S11) are computed. In the case of nano chiral metamaterial structure, and depending on the application, the electromagnetic behavior can be adjusted by changing the angle of the chiral element.