基于蜷曲空间结构的近零折射率声聚焦透镜

研究基于蜷曲空间结构的近零折射率声聚焦透镜.根据近零折射率材料的声波方向选择机理,采用蜷曲空间结构为基本单元进行排列,设计具有特定入射与出射界面的几何结构,对透射声波的出射方向进行调控,实现了平面声波与柱面声波的聚焦效应,并深入讨论了透镜内部刚性散射体对声聚焦性能的影响.在此基础上,改变近零折射率透镜的出射界面,可以精确调控声波阵面的形状与方向.该类型透镜具有单一的单元结构、高聚焦性能及高鲁棒性等优点.研究结果为设计新型近零折射率声聚焦透镜提供了理论指导与实验参考,同时也为研究声波阵面的调控提供了新思路.     

矩形波导中宽带非对称声传输

目前已有的非对称性声传输模型大部分局限于二维模式,无法直接应用于三维实际系统,针对这一问题,文中研究了通过内置非对称几何结构来实现声单向传输的三维管道模型。该几何结构体内引入天然材料——氙气,利用氙气与空气两种天然气体折射率的差异以及结构的不对称,实现声波在管道中的非对称传输,即当声波正向入射时,可通过管道内结构到达另一侧;而当反向入射时,声波无法通过内置的非对称结构。理论计算结果和仿真结果表明声波在三维矩形波导管中的非对称声传输效果和结构的厚度相关:当结构厚度较薄时,可在较宽频带范围内实现声波的非对称传输。此外,该矩形波导设计具有结构简单、透明超轻等特点,为设计新型声波非对称传输器件提供了相应的理论参考。      

共形Mikaelian声透镜设计*

该文研究了利用共形变换设计声学器件的一般方法,在此基础上根据普通Mikaelian透镜的折射率分布规律,利用指数映射设计出了弧形的Mikaelian透镜,分析并讨论了弧形透镜的密度、模量和折射率分布规律。对160 k Hz的声波进行了仿真实验,仿真结果表明,在弧形透镜的理论预测焦点处出现能量汇聚的现象,即实现了弧形聚焦的效果。同时,声波在经过该透镜后传播方向产生了一定角度的偏转。该工作为实现弧形声学器件提供了理论方法,在水下声探测及水下声通讯等方面有着潜在的应用。     

超宽带声螺旋结构

超材料慢声器件具有非常灵活的相位控制能力,能够实现超薄的声透镜、声学整流器和声学自加速发生器等多种声学功能器件。但是,大多数慢声器件存在严重的色散、界面阻抗不匹配等问题,限制慢声器件只能在较窄的带宽工作。文章介绍了一种全新的螺旋型慢声器件,回顾了均匀螺距的螺旋结构超材料和螺距连续变化的螺旋结构超材料,深入分析了实现宽带零色散慢声器件和宽带高耦合效率的慢声器件的基本原理和设计规则。这些慢声器件可通过调整结构的螺距来灵活实现相位调节。最后,文章从数值和实验两个方面证明了按照一定规律排列的螺旋结构型超材料单元可以将入射的平面声波转换成按预设抛物线轨迹传输的艾里声束。     

负折射率平板透镜系统中的光束聚焦及相位补偿特性

利用平面角谱理论,建立厄米-高斯光束和拉盖尔-高斯光束在负折射率平板透镜系统中的傍轴传输模型,研究负折射率平板透镜中厄米-高斯光束和拉盖尔-高斯光束强度聚焦及相位补偿特性。结果发现:负折射率平板透镜产生的反向Gouy相移可以补偿空气中Gouy相移导致的相差,因而在满足相位匹配条件下,光束在物平面和像平面之间的相差为零;当强度聚焦和相位匹配条件同时满足的条件下,光束在物平面强度和相位分布可在像平面实现完全重构。     

声学超构表面

声学超构表面是当前声学领域的研究热点,其是一种由人工微单元构成的超薄平面结构,由于其具备平面、超薄等独特物理特性及对声波的灵活调控能力,使得其在降噪隔振、隐身技术、非接触操控物体等诸多声学领域具有重要的应用前景。文章以反射型、吸收型、透射型声学超构表面为框架,深入介绍超构表面高效调控声波的物理机制并举例展示了其构建复杂声场的能力,包括任意点反射聚焦、低频完美吸声、自弯曲声束、螺旋声波以及声能量非对称传输。     

调节声束宽度的超透镜

美国宾夕法尼亚州立大学的学者提出一种超常材料制成的声透镜设计方案,可以通过调整折射率控制声束。这是一个类似于照相机镜头的装置,能够实现声波对焦,采用不同的人工结构模块,可以放大或缩小通过透镜的平面波束宽度。科学家希望这种透镜能在医疗超声成像、无损检测、表面波传感器等应用领域发挥作用。     

基于变换声学的角反射器的设计*

基于超材料的新型声学器件是近年来的研究热点,变换声学为这些新型声学器件的实现带来了可能。该文提出了一种基于变换声学的声反射器模型的设计方法。通过简单的线性映射,该声反射器具有均匀而各向异性的参数。利用Biot流体等效介质理论,通过周期分层结构可以对该反射器进行实现。仿真结果显示,在一定角度范围内,该反射器可以将声波按照入射方向进行反射,同时也验证了该结构的宽频有效性。     

基于热声相控阵列的声聚焦效应

研究基于热声相控阵列的宽频带声聚焦效应.设计新型热声相位控制单元,通过改变单元的空气温度控制声波波速,实现声波透射与反射相位延迟覆盖2π区间.设计四种不同类型的热声相控阵列聚焦透镜,采用8种或2种热声相位控制单元分别实现了透射与反射声聚焦效应.与其他类型的声聚焦透镜相比,热声相控阵列聚焦透镜具有宽频带、高聚焦性能、设计方案简单等优点.     

基于温度梯度分布的宽频带声聚焦效应

研究基于温度梯度分布的宽频带声聚焦效应. 利用两个恒温热源产生的温度梯度分布, 控制声波传播路径, 实现声波聚焦效应. 该机理源于温度场连续变化引起声折射率连续变化, 无反射能量损失, 具有宽频带及高聚焦性能等优点. 在此基础上, 分析入射声源频率、热源温度、分界面空间分布、热源位置、介质的衰减系数、热源温度不对称分布等因素对声聚焦性能的影响, 利用气凝胶材料验证单一介质中基于温度梯度分布的声聚焦系统的可行性.     

Flat acoustic lens by acoustic grating with curled slits

We design a flat sub-wavelength lens that can focus acoustic wave. We analytically study the transmission through an acoustic grating with curled slits, which can serve as a material with tunable impedance and refractive index for acoustic waves. The effective parameters rely on the geometry of the slits and are independent of frequency. A flat acoustic focusing lens by such acoustic grating with gradient effective refractive index is designed. The focusing effect is clearly observed in simulations and well predicted by the theory. We demonstrate that despite the large impedance mismatch between the acoustic lens and the matrix, the intensity at the focal point is still high due to Fabry–Perot resonance.     

Numerical study of a near-zero-index acoustic metamaterial

This Letter studies a two-dimensional, membrane-based acoustic metamaterial with a near-zero refractive index. It yields a frequency-dependent effective density that is near-zero at a narrow frequency band centered around its first resonant frequency. This effective density results in its near-zero refractive index. Numerical simulations are shown which demonstrate that the phase in this metamaterial undergoes small changes, and the metamaterial functions as an angular filter such that only a wave with a near-zero incident angle can transmit. Its ability to tailor acoustic phase pattern is also discussed in this Letter.     

A0模态Lamb波聚焦透镜的结构设计及实验研究

基于折射率的弯曲波透镜多在板结构上打孔或挖槽,这种设计破坏了原有的结构,降低了板的刚度和稳定性。鉴于此,通过在板表面镶嵌棱柱的方式设计了A0模态Lamb波的聚焦透镜。首先,详细讨论了棱柱的结构参数对Lamb波带隙的影响机理,获取了波速与结构尺寸的定量关系,并实现了透镜的结构设计;其次,有限元仿真了该聚焦透镜的工作性能,包括聚焦位置、焦点处能量分布、聚焦尺寸、工作带宽等;最后,通过实验验证了该透镜设计的正确性。研究结果表明,设计的透镜能够使弯曲波聚焦在预先设定位置,且在不改变结构参数的情况下具有一定的工作频率带宽。该透镜设计方法具有聚焦性能优越、刚度强、易于加工等优点,为声聚焦透镜在无损检测、能量收集等领域的实际应用提供了参考。     

An acoustic Maxwell's fish-eye lens based on gradient-index metamaterials

We have proposed a two-dimensional acoustic Maxwell's fish-eye lens by using the gradient-index metamaterials with space-coiling units. By adjusting the structural parameters of the units, the refractive index can be gradually varied, which is key role to design the acoustic fish-eye lens. As predicted by ray trajectories on a virtual sphere, the proposed lens has the capability to focus the acoustic wave irradiated from a point source at the surface of the lens on the diametrically opposite side of the lens. The broadband and low loss performance is further demonstrated for the lens. The proposed acoustic fish-eye lens is expected to have the potential applications in directional acoustic coupler or coherent ultrasonic imaging.     

Thermoacoustic-reflected focusing lens based on acoustic Bessel-like beam with phase manipulation

We report the realization of broadband reflected acoustic focusing lenses based on thermoacoustic phased arrays of Bessel-like beams, in which the units of phase manipulation are composed of three rigid insulated boundaries and a thermal insulation film in air with different temperatures. Based on these units, we realize a reflected focusing lens which can focus reflected acoustic energy on a line, and its fractional bandwidth can reach about 0.29. In addition, we discuss the influences of the base angle of Bessel-like beam, the number of basic unit, and the variation of unit temperature on focusing performances in details. Furthermore, the reflected focusing lens for the cylindrical acoustic wave based on the Bessel-like beam is also demonstrated. The proposed focusing lens has the advantages of a broad working bandwidth, large focus size,and high robustness, which may provide possibilities for the design and application of acoustic lenses.     

Subwavelength beam manipulation via multiple-metal slits coupled by disk-shaped nanocavity

A novel plasmonic structure consisting of three nano-scaled slits coupled by nano-disk-shaped nanocavities is pro- posed to produce subwavelength focusing and beam bending at optical frequencies. The incident light passes through the metal slits in the form of surface plasmon polaritons （SPPs） ,and then scatters into radiation fields. Numerical simulations using finite-difference time-domain （FDTD） method show that the transmitted fields through the design example can gener- ate light focusing and deflection by altering the refractive index of the coupled nanocavity. The simulation results indicate that the focal spot is beyond the diffraction limit. Light impinges on the surface at an angle to the optical axis will add an extra planar phase front that interferes with the asymmetric phase front of the plasmonic lens, leading to a larger bending angle off the axial direction. The advantages of the proposed plasmonic lens are smaller device size and ease of fabrication. Such geometries offer the potential to be controlled by using nano-positior!i0g systems for applications in dynamic beam shaping and scanning on the nanoscale.