基于近零折射率材料的声非对称聚焦透镜

研究基于近零折射率声子晶体的声非对称聚焦透镜。声非对称聚焦透镜的两侧分别为竖直平面与圆弧面结构,由于近零折射率材料的声波方向选择性,当声波从竖直平面一侧入射时,可以通过透镜,实现高性能声聚焦效应;而当声波从圆弧面一侧任意方向入射时,声波无法通过透镜,从而实现同时具有聚焦与非对称传输特性的声学透镜。在此基础上,改变透镜圆弧面的曲率半径,可以调控非对称透镜的正向聚焦焦点位置,且声非对称传输性能保持不变.此外,透镜内部的刚性散射体对声非对称聚焦性能的影响较小。研究结果表明所设计的声非对称聚焦透镜具有多功能、单一结构及高鲁棒性等优点,为设计新型近零折射率声学器件提供相应的理论方案与实验参考.     

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。     

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。     

分布折射率平面微透镜阵列综述

就分布折射率平面微透镜阵列的发展现状,以及该器件的特性、制造工艺、测试手段和实际应用等作了简单回顾,并分折了该器件的发展趋势。一、概述梯度透镜是一种折射率按某种规律变化的透镜。     

具有单相单向换能器的水平剪切声表面波压力传感器插入损耗特性

为实现水平剪切声表面波压力传感器低损耗设计,系统研究了均匀叉指换能器(IDT)和单相单向换能器(SPUDT)器件的声波能量损耗性能。在确定最优反射系数的SPUDT结构的基础上,建立3D周期性有限元仿真模型,计算器件表面振动位移和双向输出电压瞬态响应。通过对电压信号进行傅里叶变换获得器件模型的插入损耗,并制备两种不同换能器的声学传感器,测试其频率特性和灵敏度。与均匀IDT相比,基于SPUDT器件插入损耗的仿真和实验结果分别降低了5.2 dB和5.6 dB,SPUDT器件灵敏度约为均匀IDT器件的两倍。结果表明,SPUDT能有效降低SH-SAW压力传感器声波能量的单向损耗,提高检测灵敏度,且构建的3D周期性有限元仿真模型有助于声学传感器的声波损耗分析,实现高精度和小型化的声学测量系统。     

水下环形凹槽圆柱体散射声场空间指向性调控

本文提出一种具有深度梯度的环形凹槽结构,可用于调控水中有限长刚性圆柱体散射声场空间指向性.基于声学相位阵列理论分析了环形凹槽圆柱声散射空间指向性改变的机理,研究表明:凹槽深度方向相位延迟和凹槽间Bragg散射的相互作用使得平面声波垂直于圆柱方向入射其正横方向散射声波发生偏转.采用有限元方法讨论了凹槽结构参数如占空比、梯度等对圆柱散射声场空间分布特征的影响规律.多个不同深度梯度环形凹槽单元组合圆柱体散射声场数值计算和实验结果显示:具有环形凹槽结构圆柱体正横方向散射声波均匀偏转到预定的空间范围内,使得圆柱体声散射场空间指向性均衡化,改变了圆柱整体的散射特征,这为水下目标声隐身设计和声波定向传播提供了新的方法.     

一种测量变折射率透镜阵列（SLA）的传递函数的新方法

本文提出了一种与计算机技术相结合、利用ＣＣＤ像感器件测定变折射率透镜阵列传递函数的方法，该方法比较接近于变折射率透镜阵列有实际使用环境，精确度高，测量速度快，而且简便易行，测量结果也较为全面。该方法已用于光电图文扫描仪中变折射率透镜阵列的设计及测量，并获得了满意的结果。     

基于遗传算法的五模材料分层优化*

由于超材料的微结构尺度不能忽略,利用超材料微结构所集成的器件等效参数不是连续分布的,而是呈离散分布的。因此在利用超材料设计声学器件的过程中,需要对器件进行分层离散化,而分层方案的选取往往会影响器件的工作效果。该文基于五模材料水下隐身衣的设计理论,提出了一种基于遗传算法的五模材料隐身衣分层优化策略,并分别对优化前和优化后的分层隐身衣做了声学仿真。仿真结果表明采用优化策略后,隐身衣背景声场的散射能量和反射能量均有大幅度降低,并且对窄带探测信号和宽带探测信号均有较好的隐身效果,因此该分层优化策略能够显著提升隐身衣的隐身效果。该文所提出的分层优化策略为提升五模材料隐身衣性能提供了理论支持,并可以进一步推广至其他超材料器件的微结构设计中。     

Morse lens [Invited]

In this Letter, we find that Morse potential(proposed about 90 years ago) could be connected to Coulomb potential(or Newton potential) and harmonic potential(or Hooke potential) by conformal mappings. We thereby design a new conformal lens from Morse potential, Eaton lens, and Luneburg lens and propose a series of generalized Eaton/Luneburg lenses. We find that this Morse lens is a perfect self-focusing asymmetric lens that differs from a Mikaelian lens. Our theory provides a new insight to Morse potential and other traditional potentials, and revisits their classical applications on designing lenses.     

Near-field multiple super-resolution imaging from Mikaelian lens to generalized Maxwell's fish-eye lens

Super-resolution imaging is vital for optical applications, such as high capacity information transmission, real-time bio-molecular imaging, and nanolithography. In recent years, technologies and methods of super-resolution imaging have attracted much attention. Different kinds of novel lenses, from the superlens to the super-oscillatory lens, have been designed and fabricated to break through the diffraction limit. However, the effect of the super-resolution imaging in these lenses is not satisfactory due to intrinsic loss, aberration, large sidebands, and so on. Moreover, these lenses also cannot realize multiple super-resolution imaging. In this research, we introduce the solid immersion mechanism to Mikaelian lens (ML) for multiple super-resolution imaging. The effect is robust and valid for broadband frequencies. Based on conformal transformation optics as a bridge linking the solid immersion ML and generalized Maxwell's fish-eye lens (GMFEL), we also discovered the effect of multiple super-resolution imaging in the solid immersion GMFEL.     

Analysis of a conformal invisible device

We perform a semi-analytical calculation of the field distributions of a conformal invisible device via mode expansions. For a discrete set of frequencies in the regime of wave optics, the conformal invisible device is perfectly transparent, which stems from the special conformal mapping and the refractiveindex profile of the Mikaelian lens.     

Enhancement of pupil-masked wavelength-scale gas-filled flat acoustic lens based on anomaly apodization effect

In this letter, the improvement in focus by the use of a pupil mask produced in an acoustic mesoscale cuboid particle filled with CO₂ is reported. Thereby, the result shows that the pupil mask increases the sound intensity and also increases the resolution (or a reduction of the full width at half maximum, FWHM) in focus compared to the non-masked one. These results are important because they confirm the effect of abnormal amplitude apodization for a one-wavelength dimension acoustic lens and demonstrate that it is possible to improve sound focusing of a cuboid gas-filled lens with one wavelength dimension. This is the smallest size of an acoustic lens ever considered in this type of literature, with side dimensions of the cube equal to one wavelength and a diameter to focus ratio of 2.5, the sound amplification in focus is 5.4 dB at 4125 Hz, with the resolution near to the diffraction limit.     

Bending loss analyses of photonic crystal fibers based on the finite-difference time-domain method

This is a report on an effective simulation method for the bending loss analyses of photonic crystal fibers. This method is based on the two-dimensional finite-difference time-domain algorithm and a conformal transformation of the refractive index profile. We observed the temporal dynamics of light waves in a bent fiber in a simulation and obtained the bending loss as a function of bend radius and optical wavelength for the commercial photonic crystal fibers. The accuracy of this method was verified by good agreement between the simulation and experimental data.     

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.     

变换光学透镜天线研究进展

龙伯透镜天线是一种独特的渐变折射率透镜天线,但因为某些缺陷其应用受到限制。近年来,人们采用变换光学方法和超材料理论设计了许多以平板龙伯透镜为代表的新型透镜天线。本文对比总结了变换光学设计透镜的3种方法,即坐标变换法、保角变换法和准保角变换法;指出了准保角变换法由于设计灵活、可使用全介电材料制备而更具研究和应用前景;详细阐述了准保角变换法设计透镜的原理和步骤;介绍了国内外关于透镜变换和制备具有影响力的研究成果;指出了变换光学透镜天线今后的研究方向。     

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.     

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.     

Planar anisotropic Mikaelian lens on the basis of EBG-structure

The problem on the synthesis of a planar Mikaelian lens in the form of layered metal?dielectric waveguide with a nonuniform anisotropic dielectric layer is solved. The problem is reduced to solution of the dispersion equations for E and H modes of a homogeneous layered waveguide. As an example of implementation of the solution obtained, the synthesis and analysis of the Mikaelian lens is carried out on the basis of the EBG structure in the form of a planar corrugated open and closed layered metal?dielectric waveguide. For a special case of the closed waveguide, the solution of the problem of synthesis for both polarizations is obtained explicitly.