Design of an acoustic bending waveguide with acoustic metamaterials via transformation acoustics

Transformation acoustics is employed to design an acoustic bending waveguide. A two-dimensional square area with anisotropic and homogeneous material properties is transformed into a fan-shaped area with anisotropic and inhomogeneous material properties to rotate the direction of beam propagation. An alternating layered structure is considered to approximate a medium with anisotropic material properties. From the calculation results, the transformation medium can be realized by an alternating layered structure consisting of water and fluid with negative mass density. We propose that an acoustic metamaterial composed of three layers in water background can be designed to replace negative mass density fluid. The effective mass density and bulk modulus of the system that is composed of the acoustic metamaterial and water are dependent on the incident frequency and the geometric size of the acoustic metamaterial. We tune the geometric size of the acoustic metamaterial to approach the corresponding mass density distribution of the negative mass density fluid at a specific frequency. Thereby, the acoustic bending waveguide designed by using transformation acoustics can be achieved by the acoustic metamaterials.     

Acoustic Absorption Characteristics of New Underwater Omnidirectional Absorber

We investigate a new underwater omnidirectional absorber with acoustic black hole effect to realize a broadband omnidirectional acoustic wave absorption. Based on multiple scattering theory, a two-dimensional axisymmetric model of underwater omnidirectional absorber comprised of an acoustic gradient refractive index structure and a hollow core is developed, and the mechanisms of omnidirectional absorption and dissipation of acoustic waves are studied. The numerical results indicate that the omnidirectional absorber developed here can achieve the omnidirectional absorption of incident acoustic waves in a broadband frequency and can effectively reduce the backscattering of acoustic waves. It potentially provides a new notion for underwater acoustic coating design.     

Three-dimensional surface current loops in broadband responsive negative refractive metamaterial with isotropy

We propose a bulk negative refractive index (NRI) metamaterial composed of periodic array of tightly coupled metallic cross-pairs printed on the six sides of a cube for applications of superlenses. The structural characteristics of the three-dimensional (3D) metamaterial consist in the high symmetry and the superposition of metallic cross-pairs, which can increase the magnetic inductive coupling between adjacent cross-pairs and realize a broadband and isotropic NRI. The proposed 3D structure is simulated using the CST Microwave Studio 2006 to verify the design validity. The simulation results show that the proposed structure can not only realize simultaneously an electric and magnetic response to an incident electromagnetic (EM) wave, but also exhibit a broadband NRI whose relative bandwidth can reach up to 56.7%. In addition, the NRI band is insensitive to the polarization and the incident angle of the incident EM wave. Therefore, the proposed metamaterial is a good candidate material as three-dimensional broadband isotropic NRI metamaterial.     

Light propagation in anisotropic metamaterials. II. Reflection from the half-space

We consider reflection of light from half-space of anisotropic metamaterial at arbitrary direction of optical axis in the plane of light incidence. We obtain conditions at which total reflection and refraction do not depend on the polarization of incident light and the reflection does not depend on also the angle of incidence. We also study the possibilities and conditions for using the considered system as beam splitter, omnidirectional reflector, phase retarder, and so on.     

Controlling flexural waves in thin plates by using transformation acoustic metamaterials

In this study, we design periodic grille structures on a single homogenous thin plate to achieve anisotropic acoustic metamaterials that can control flexural waves. The metamaterials can achieve the bending control of flexural waves in a thin plate at will by designing only one dimension in the thickness direction, which makes it easier to use this metamaterial to design transformation acoustic devices. The numerical simulation results show that the metamaterials can accurately control the bending waves over a wide frequency range. The experimental results verify the validity of the theoretical analysis. This research provides a more practical theoretical method of controlling flexural waves in thin-plate structures.     

声学宽带多焦点聚焦

针对声学宽带多焦点聚焦的问题,设计出结构简洁的声学人工结构。通过控制细槽深度,对反射声波的波前进行任意操控。数值仿真结果验证了所设计的声人工结构的聚焦效果。这种人工结构还允许对焦点的相对位置及数目进行精确而自由的调控,当尺寸大于波长的障碍物存在时仍可通过合理设计来产生聚焦效果。该方案具有设计简洁及焦点可控等优点,有望在生物医学及无损检测等场合产生广泛应用。      

Broadband low-frequency acoustic absorber based on metaporous composite

Broadband absorption of low-frequency sound waves via a deep subwavelength structure is of great and ongoing interest in research and engineering. Here, we numerically and experimentally present a design of a broadband low-frequency absorber based on an acoustic metaporous composite (AMC). The AMC absorber is constructed by embedding a single metamaterial resonator into a porous layer. The finite element simulations show that a high absorption (absorptance A>0.8) can be achieved within a broad frequency range (from 290 Hz to 1074 Hz), while the thickness of AMC is 1/13 of the corresponding wavelength at 290 Hz. The broadband and high-efficiency performances of the absorber are attributed to the coupling between the two resonant absorptions and the trapped mode. The numerical simulations and experimental results are obtained to be in good agreement with each other. Moreover, the high broadband absorption can be maintained under random incident acoustic waves. The proposed absorber provides potential applications in low-frequency noise reduction especially when limited space is demanded.     

A broadband planar acoustic metamaterial lens

Based on quasi-conformal mapping approach, we propose a two-dimensional broadband and low-loss planar lens in the acoustic regime, which can be realized using gradient-index sub-wavelength artificial structures with hard boundary. In this design, QCM is employed in the acoustic regime to convert a parabolic form into a planar one, which makes the resultant material approximately isotropic through minimizing the anisotropic factor. It is found that the planar lens can show the same performance as its parabolic counterparts over a broadband range of frequency. Moreover, the lens is fabricated via 3D printing technology and measured experimentally, which gives identical results with the theoretical prediction. The proposed technique could be extended to realize different acoustic devices and applied for a series of practical applications.     

Optimization of locally resonant acoustic metamaterials on underwater sound absorption characteristics

The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500 Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings.     

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.     

Membrane-based acoustic metamaterial with near-zero refractive index

We investigate a one-dimensional acoustic metamaterial with a refractive index of near zero(RINZ) using an array of very thin elastic membranes located along a narrow waveguide pipe. The characteristics of the effective density, refractive index, and phase velocity of the metamaterial indicate that, at the resonant frequency fm, the metamaterial has zero mass density and a phase transmission that is nearly uniform. We present a mechanism for dramatic acoustic energy squeezing and anomalous acoustic transmission by connecting the metamaterial to a normal waveguide with a larger cross-section. It is shown that at a specific frequency f_1, transmission enhancement and energy squeezing are achieved despite the strong geometrical mismatch between the metamaterial and the normal waveguide. Moreover, to confirm the energy transfer properties, the acoustic pressure distribution, acoustic wave reflection coefficient, and energy transmission coefficient are also calculated. These results prove that the RINZ metamaterial provides a new design method for acoustic energy squeezing,super coupling, wave front transformation, and acoustic wave filtering.     

各向异性超常介质中的反常线性现象

从麦克斯韦方程组出发,结合超常介质的特殊性质,分析了电磁波在一种具有线性色散关系的各向异性超常介质中的传播特性。在线性色散关系的情况下,得到了超常介质的介电张量和磁导率张量各元素之间的条件表达式。根据能量守恒定律和电磁场的边界连续性条件,在横电波(TE)和横磁波(TM)入射时,分析了折射波矢和能流与入射波矢和能流方向之间的关系。发现无论是TE波还是TM波,在这种介质中传输时,都产生了光准直的现象,且TE波和TM波的传输方式是相反的。     

Wide bandwidth acoustic transmission via coiled-up metamaterial with impedance matching layers

Many acoustic metamaterials suffer from a narrow bandwidth transmission because of the impedance mismatch at the airmetamaterial interface. In this paper, a two-dimensional impedance-matched metamaterial with broadband transmission performance is investigated. The impedance matching layer is introduced for a gradient variation of effective impedance from the inlet of the unit to the outlet. The effective medium theory and corresponding effective model are used to explain the underlying mechanism. The improved energy transmission of our designs is demonstrated by experiment and numerical simulation within a broad frequency bandwidth over 6 kHz. Our impedance-matched design can be used to enhance sound absorption, which is expected to present improved acoustic performance in the applications of acoustic damper and muffler.     

基于蘑菇型结构的双入射超宽带复合媒质材料设计与分析

通过设计一定的单元结构, 可以实现超宽带人工电磁材料. 基于蘑菇型金属结构, 提出了一种同时具有左右手通带无缝结合的超宽带双入射型复合媒质材料结构单元. 该结构由嵌入到介质板的两个反向对称的蘑菇型金属结构组成, 能够同时引发电谐振和磁谐振而得到左手通带. 通过利用CST软件仿真、等效电磁参数提取、折射率计算以及建立等效磁谐振电路模型等方法, 分析验证了该结构的双入射特性和左手特性. 仿真结果表明, 在电磁波垂直于介质板和平行于介质板入射两种情况下, 在X波段均表现出左手通带特性, 并具有1 GHz以上的左手带宽. 当电磁波垂直于介质板入射时, 在7.2 GHz-9.3 GHz频段为右手通带, 在9.3 GHz-11 GHz频段为左手通带; 当电磁波平行于介质板入射时, 在7.0 GHz-9.0 GHz频段为右手通带, 在9.0 GHz-10 GHz频段为左手通带. 在两种情况下分别于9.3 GHz与9.0 GHz处得到了零折射率, 从而构造了一种正-零-负复合媒质材料, 实现了具有3 GHz带宽的双入射超宽带平衡结构.     

全向入射条件下一维固流周期结构中低频声裂隙变化特性研究

利用传递矩阵法, 从理论上建立了全向入射条件下一维固-流周期结构中的声传播模型, 在此基础上计算、分析并比较了无限周期结构的声能带结构和有限周期结构中的声传输特性. 研究结果表明, 当声波以一定的入射角入射时, 固-流周期结构的低频通带区域存在一个声裂隙, 该声裂隙所对应的入射角大小与构成周期结构的固体层和流体层的密度或结构尺寸无关, 而仅取决于构成该周期性结构材料的波速. 关键词： 传递矩阵 全向入射 固-流周期结构 声裂隙     

基于声学超材料的低频可调吸收器

在当今社会,噪声污染已经成为人类健康的一大威胁,如何有效地控制和消除噪声污染一直是科研领域的一个重要话题.本文以开口环嵌套结构为模型,设计并制备了一种声学超材料.通过理论分析、数值模拟和实验测试,发现由于模型内部空腔的强烈耦合共振效应,该超材料可以在低频区域实现接近完美的吸声效应.此外,通过简单地绕轴旋转其内腔开口方向,即可改变该超材料的相对阻抗值,进而在较宽的频带范围内实现对吸收峰位置的可调控制.由于该超材料具有深亚波长的尺寸,因此非常有利于低频吸声器件的小型化和集成化,同时该模型也为宽带吸收器的设计奠定了基础.     

Propagation of polarized photons through a cavity with an anisotropic metamaterial

We present a theoretical study of the propagation properties of polarized photons passing through the cavity with an anisotropic metamaterial. We find that the resonant peaks of transmission appear for photons polarized in a certain direction corresponding to a negative element of the permittivity tensor. This indicates the potential for applying such cavity structures as filters for photons with certain polarizations. The resonant peak of transmission for photons having a given frequency can be achieved by adjusting the thicknesses of the air and metamaterial. If the frequency of the incident photons and the thickness of the metamaterial are fixed, the cavity structure can be used as a photon switch controlled by the thickness of the air. The effect of the absorption is considered, and the result shows that the transmission peak always appears, even for metamaterials with large absorption. Finally, the polarization manipulation of such structures is explored.     

Integrated optical devices based on broadband epsilon-near-zero meta-atoms

We verify the feasibility of the proposed theoretical strategy for designing the broadband near-zero permittivity (ENZ) metamaterial at optical frequency range with numerical simulations. In addition, the designed broadband ENZ stack is used as meta-atoms to build functional nanophotonic devices with extraordinary properties, including an ultranarrow electromagnetic energy tunneling channel and an ENZ concave focusing lens.     

An acoustic bending waveguide designed by anisotropic density-near-zero metamaterial

Anisotropic metamaterial with only one component of the mass density tensor near zero(ADNZ) is proposed to control the sound wave propagation. We find that such an anisotropic metamaterial can be used to realize perfect bending waveguides. According to a coordinate transformation, the surface waves on the input and output interfaces of the ADNZ metamaterial induces the sound energy flow to be redistributed and match smoothly with the propagating modes inside the metamaterial waveguide. According to the theory of bending waveguide, we realize the "T"-type sound shunting and convergence, as well as acoustic channel selection by embedding small-sized defects. Numerical calculations are performed to confirm the above effects.     

Ultra-thin broadband metamaterial absorber

The design, fabrication, and measurements of a broadband metamaterial absorber are reported. The proposed metamaterial absorber consists of circular metallic patches and a metallic ground plane separated by a dielectric layer. Increasing the number of metallic patches can broaden the frequency range when their resonances are closely packed together, thereby resulting in a broadband resonance. Experimental results show that the proposed absorber has high absorptivity, with a full width at half maximum absorption bandwidth of 2.8 GHz and the relative FWHM absorption bandwidth of 25.3?%. In addition, the absorber can operate at a wide range of incident angles under both transverse electric and transverse magnetic polarizations.