Phase modulation of acoustic vortex beam with metasurfaces

We present a new acoustic metasurface structure to generate the stable acoustic vortex beams with controllable topological charge in the broadening of operating frequency. The proposed acoustic metasurface can dynamically achieve a full span of 360° phase modulation by three simple solutions. Acoustic plane beam and acoustic vortex beam can convert each other based on multi-metasurfaces combination. Potential applications of such phase modulation schemes can be anticipated.     

基于相位调控的超高透射声学超表面及其应用*

声学超表面可以灵活有效地对声波进行调控,实现多种新颖的现象和功能,在诸多声波调控领域有潜在的应用前景。该文引入了一种镀膜型迷宫结构单元,可以提供全范围(0~2π)的突变相位,具有高效的透射率(~100%)和较好的鲁棒性。基于相位调控,可利用镀膜型迷宫结构单元构建出超高透射超表面,实现异常声折射、声聚焦、声束沿任意凸轨迹弯曲传播以及声成像等应用。     

Broadband cross-circular polarization carpet cloaking based on a phase change material metasurface in the mid-infrared region

In view of the fact that most invisibility devices focus on linear polarization cloaking and that the characteristics of mid-infrared cloaking are rarely studied, we propose a cross-circularly polarized invisibility carpet cloaking device in the mid-infrared band. Based on the Pancharatnam–Berry phase principle, the unit cells with the cross-circular polarization gradient phase were carefully designed and constructed into a metasurface. In order to achieve tunable cross-circular polarization carpet cloaks, a phase change material is introduced into the design of the unit structure. When the phase change material is in amorphous and crystalline states, the proposed metasurface unit cells can achieve high-efficiency cross-polarization conversion, and reflection intensity can be tuned. According to the phase compensation principle of carpet cloaking, we construct a metasurface cloaking device with a phase gradient using the designed unit structure. From the near- and far-field distributions, the cross-circular polarization cloaking property is confirmed in the broadband wavelength range of 9.3–11.4 µm. The proposed cloaking device can effectively resist detection of cross-circular polarization.     

Acoustic metasurface for refracted wave manipulation

Here we present a design of a transmitted acoustic metasurface based on a single row of Helmholtz resonators with varying geometric parameters. The proposed metasurface can not only steer an acoustic beam as expected from the generalized Snell's law of refraction, but also exhibits various interesting properties and potential applications such as insulation of two quasi-intersecting transmitted sound waves, ultrasonic Bessel beam generator, frequency broadening effect of anomalous refraction and focusing.     

声学超材料与超表面研究进展

声学超材料是一种人工设计结构的材料,具有超越自然界材料行为的特性,如负折射、反常多普勒效应、平面聚焦等.本文主要介绍了声学超材料近二十年来的相关研究进展,重点论述了超原子声学超材料、超分子声学超材料、超原子簇和超分子簇声学超材料.最后简要介绍了近五年来声学超表面的研究概况和发展趋势.     

Broadband high-efficiency controllable asymmetric propagation by pentamode acoustic metasurface

We utilise the pentamode metasurface to realise broadband high-efficiency and controllable asymmetric transmission. The designed metasurface can manipulate the acoustic waves, as expected from the generalised Snell's law, and exhibits unique characteristics such as extraordinary broadband acoustic control, apparent negative refraction, and conversion from the propagating wave to surface mode. The asymmetric transmission features of positive refraction for the forward incidence (FI) and negative refraction for the reverse incidence (RI) can be realised within the range of 2600 Hz to 5600 Hz by controlling the incident angle from 0° to 35° with the transmission efficiency higher than 85.4% for the FI and RI. In addition, by further adjusting the angle of incidence in the range of 25° to 90°, asymmetric transmission characteristics can be expressed as surface wave transmission for the FI and transmitted wave transmission for the RI within the same frequency ranges.     

High-efficiency reflection phase tunable metasurface at near-infrared frequencies

The realization of active modulation of reflection phase based on metasurfaces is of great significance for flexible control of electromagnetic wavefront, which makes metasurfaces have practical application values in polarization conversion, beam steering, metalens, etc. In this paper, a reflection phase tunable gap-surface plasmon(GSP) metasurface based on phase change materials Ge_2Sb_2Te_5(GST) is designed and experimentally demonstrated. By virtue of the characteristics of large permittivities difference before and after GST phase transition and the existence of stable intermediate states, the continuous modulation of near-infrared reflection phase larger than 200°has been realized. At the same time, through the reasonable design of the structure sizes, the reflection has been maintained at about 0.4 and basically does not change with the GST phase transition, which improved the working efficiency of the metasurface significantly. In addition, the coupled-mode theory(CMT) is introduced to make a full analysis of the modulation mechanism of the reflection phase,which proves that the phase transition of GST can induce the transition of metasurface working state from overcoupling mode to critical coupling mode. The improvement of the metasurface working efficiency has practical values for wavefront modulation.     

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.     

混合谐振模式宽带长波红外超表面吸收器研究

为了满足红外探测器件集成化和对红外宽光谱范围吸收的需求,设计了一种工作在长波红外波段(8~14μm)的超宽带、高吸收、极化不敏感的超材料吸收器。通过在金属-介质-金属三层异质的超材料吸收器结构的顶部金属周围镶嵌一层介质形成超表面,以增加谐振强度和吸收带宽。在8~13.6μm的带宽范围内,该结构有超过90%的平均吸收率,覆盖了大部分长波红外大气窗口波段,对红外探测领域有着重要意义。研究结果表明:镶嵌的金属-介质组成的介质波导模式和谐振腔模式的结合以及传播型表面等离激元模式的激发是形成宽带高吸收的主要原因,并且谐振模式的谐振波长可以通过相关参数来进行调控。本文的研究结果为可调谐宽带长波红外吸收材料的设计提供参考,该设计方法可推广到中波红外波段、甚至长波红外或其它波段。     

An electrically tunable metasurface integrated with graphene for mid-infrared light modulation

We propose a low-cost plasmonic metasurface integrated with single-layer graphene for dynamic modulation of midinfrared light. The plasmonic metasurface is composed of an array of split magnetic resonators(MRs) where a nano slit is included. Extraordinary optical transmission(EOT) through the deep subwavelength slit is observed by excitation of magnetic plasmons in the split MRs. Furthermore, the introduction of the slit provides strongly enhanced fields around the graphene layer, leading to a large tuning effect on the EOT by changing the Fermi energy of the graphene. The proposed metasurface can be utilized as an optical modulator with a broad modulation width(15 μm) or an optical switch with a high on/off ratio( 100). Meanwhile, the overall thickness of the metasurface is 430 nm, which is tens of times smaller than the operating wavelength. This work may have potential applications in mid-infrared optoelectrical devices and give insights into reconfigurable flat optics and optoelectronics.     

二维磁流变包层声学超材料*

该文构建了以钨为内核、环氧树脂为基体、磁流变弹性体作为包层的二维局域共振型声学超材料,采用有限元方法分析了声波的能带结构、透射率、振动模。研究结果表明:利用外磁场可以调控磁流变弹性体包层的弹性模量,从而调节声学超材料带隙的中心位置和宽度,还可以通过改变磁流变弹性体包层的厚度来调节带隙的中心位置和宽度。这些方法对可调型声学超材料的设计有重要的参考意义。     

Real-time programmable coding metasurface antenna for multibeam switching and scanning

Novel electromagnetic wave modulation by programmable dynamic metasurface promotes the device design freedom, while multibeam antennas have sparked tremendous interest in wireless communications. A programmable coding antenna based on active metasurface elements (AMSEs) is proposed in this study, allowing scanning and state switching of multiple beams in real time. To obtain the planar array phase distribution in quick response, the aperture field superposition and discretization procedures are investigated. Without the need for a massive algorithm or elaborate design, this electronically controlled antenna with integrated radiation and phase-shift functions can flexibly manipulate the scattering state of multiple beams under field-programmable gate array (FPGA) control. Simulation and experimental results show that the multiple directional beams dynamically generated in the metasurface upper half space have good radiation performance, with the main lobe directions closely matching the predesigned angles. This metasurface antenna has great potential for future applications in multitarget radar, satellite navigation, and reconfigurable intelligent metasurfaces.     

Breaking Malus’ law: Highly efficient,broadband, and angular robust asymmetric light transmitting metasurface

High efficiency, broad bandwidth, and robust angular tolerance are key considerations in photonic device design. Here, a few‐layer, asymmetric light transmitting metasurface that simultaneously satisfies all the above requirements is reported. The metasurface consists of coupled metallic sheets. It has a measured transmission efficiency of 80%, extinction ratio of 13.8 dB around 1.5 μm, and a full width half maximum bandwidth of 1.7 μm. It is as thin as 290 nm, has good performance tolerance against the angle of incidence and constituent nano‐structure geometry variations. This work demonstrates a practical asymmetric light transmission device with optimal performance for large scale manufacturing.

     

Multifunction switching by a flat structurally tunable acoustic metasurface for transmitted waves

In this paper a flat structurally tunable acoustic metasurface is constructed based on the helical unit cell. The length of the acoustic channel can be tuned by the screw-in depth of the helix. Accordingly, the wave phase for the transmitted acoustic wave can be tuned and the wavefront can be manipulated. Then multifunctions such as anomalous refraction, point focusing, beam focusing and self-bending can be realized and switched just by screwing in or out the helixes. At the same time, the broadband operating frequency is also realized. The experiments for anomalous refraction and point focusing are also performed, and the results show that the designed metasurface is effective. The present studies have important applications in dynamic manipulation of acoustic waves by metasurfaces.     

Tunable band gaps in acoustic metamaterials with periodic arrays of resonant shunted piezos

Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a two-dimensional (2D) acoustic metamaterial. The performance is characterized by the finite element method. More importantly, we propose an approach to solving the conventional issue of the nonlinear eigenvalue problem, and give a convenient solution to the dispersion properties of 2D metamaterials with periodic arrays of resonant shunts in this article. Based on this modeling method, the dispersion relations of a 2D metamaterial with periodic arrays of resonant shunted piezos are calculated. The results show that the internal resonances of the shunting system split the dispersion curves, thereby forming a locally resonant band gap. However, unlike the conventional locally resonant gap, the vibrations in this locally resonant gap are unable to be completely localized in oscillators consisting of shunting inductors and piezo-patches.     

宽入射角度偏振不敏感高效异常反射梯度超表面

偏振不敏感超表面在实际应用中具有重要意义, 本文提出了一种光通信波段的、对偏振不敏感的异常反射式梯度超表面, 这种超表面对于x-偏振和y-偏振入射光都能够实现高效率的异常反射, 表现出偏振不敏感特性, 为解决传统反射式超表面的偏振敏感性问题提供了一种新途径. 它采用金属(Au)-绝缘层(SiO₂)-金属(Au)结构, 超表面的超晶胞由五个各向同性的、尺寸不同的十字形基本结构单元组成. 仿真结果表明, 这种超表面结构对不同线偏振入射平面光波有几乎相同的相位和振幅响应; 合理的选取五个基本结构单元的尺寸, 在一个超晶胞内实现了2πup 相位的覆盖, 反射光波阵面畸变小, 而且反射光都集中到异常反射级次, 在工作波长1480 nm处具有较高的异常反射率(～ 70%). 此外, 这种结构的超表面在-30°–0°的宽入射角度范围内都具有偏振不敏感的异常反射特性. 在光通信、光信号处理、显示成像等领域具有潜在的应用前景.     

An acoustic switch

The benefits derived from the development of acoustic transistors which act as switches or amplifiers have been reported in the literature. Here we propose a model of acoustic switch. We theoretically demonstrate that the device works: the input signal is totally restored at the output when the switch is on whereas the output signal nulls when the switch is off. The switch, on or off, depends on a secondary acoustic field capable to manipulate the main acoustic field. The model relies on the attenuation effect of many oscillating bubbles on the main travelling wave in the liquid, as well as on the capacity of the secondary acoustic wave to move the bubbles. This model evidences the concept of acoustic switch (transistor) with 100% efficiency.     

Ultra-broadband and high-efficiency polarization conversion metasurface with multiple plasmon resonance modes

In this paper, we present a novel metasurface design that achieves a high-efficiency ultra-broadband cross polarization conversion. The metasurface is composed of an array of unit resonators, each of which combines an H-shaped structure and two rectangular metallic patches. Different plasmon resonance modes are excited in unit resonators and allow the polarization states to be manipulated. The bandwidth of the cross polarization converter is 82% of the central frequency,covering the range from 15.7 GHz to 37.5 GHz. The conversion efficiency of the innovative new design is higher than 90%.At 14.43 GHz and 40.95 GHz, the linearly polarized incident wave is converted into a circularly polarized wave.     

Broadband and broad-angle asymmetric acoustic transmission by unbalanced excitation of surface evanescent waves based on single-layer metasurface

Currently, complicated structure, incident-angle selectivity, and narrow frequency band are the key drawbacks of the asymmetric acoustic transmission (AAT) devices. Here we tackle these problems by proposing a class of single-layer lossy acoustic metasurfaces. The broadband AAT performance is realized in a broad range of incident angles. When the incident angle is in the range between two critical values, which are derived in this paper, an external sound wave can be converted into an evanescent mode, and the total internal reflection occurs for backward sound. The incident sound wave can be negatively refracted for forward sound if the evanescent mode conversion condition is broken, representing the realization of the AAT. However, the AAT phenomenon cannot be observed outside of the range defined above. The proposed design of highly efficient broad-angle AAT can find applications in sound sensing and noise control.     

薄膜超构表面声扩散体扩散性能研究*

针对传统的声扩散体结构复杂、加工不便的缺点,该文提出了一种薄膜型声学超材料扩散体,通过有限元软件建立了扩散体的有限元模型。分析了薄膜张力、质量块大小及背腔深度对声波相位调控的影响,并与传统施罗德扩散体的扩散性能进行比较,发现调控后的薄膜超构表面声扩散体对声能的吸收降低,并使入射声能更均匀地向全方向扩散。在相同的尺寸情况下,与传统施罗德扩散体相比,薄膜超构表面声扩散体在所需频段内扩散性能更优,结构更优,体积更小,质量更轻,加工更方便,更适用于工程应用。