1.School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China 2.School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos.11374066, 11374068)
Received Date:11 December 2020
Accepted Date:14 January 2021
Available Online:08 June 2021
Published Online:20 June 2021
Abstract:In this paper, the Schoch displacement at the interface between different two-dimensional triangular phononic crystal metamaterial and natural material is studied by using finite element software. As is well known, the Schoch displacement is highly dependent on the surface wave and leakage wave excited at the interface between different materials. So, the negative Schoch displacement can be more easily obtained by adding a suitable thickness of covering layer at the interface between metamaterial material and natural material. The numerical results show that when the negative Schoch displacement happens, the effective parameters of metamaterials are close to zero. It means that the effective refraction index is near to zero and the reduced frequency of the incident acoustic wave is correlated with the reduced frequency of the band gap. It is also found from the results that the reduced frequency of the incident acoustic wave is located at the edge of the band gap when the negative Schoch displacement occurs. The maximum of the metamaterial effective impedance and the maximum of the reflection coefficient are almost at the same frequency. The phase of the imaginary part of the reflection coefficient has a phase mutation in π rad at the corresponding frequency. The frequency of negative Schoch displacement is located in the first band gap of MK direction and near the upper boundary. The Schoch displacement at the interface between conventional materials is usually positive and negligible in previous reports. In this paper, the negative Schoch displacement is obtained by using the near-zero refraction index metamaterials. This not only enriches the physics contents of Schoch effect but also provides a theoretical reference for designing the acoustic devices based on acoustic wave displacement at the interface. Keywords:near-zero refractive index material/ effective parameter/ Schoch displacement
当入射声波的约化频率为F = fa/c0 = 0.456 = FS (伴有最大负向Schoch位移的约化频率记为FS), 反射声波具有明显的负向Schoch位移和较好的高斯波形, 如图2(a)所示, 其中黑色实线箭头代表声波真实的传播方向, 虚线箭头代表几何预测的方向, 在虚线箭头右侧还有一微弱的反射波束, 所以图中共有两反射波束. 反射过程中能流较多的一部分以表面波和漏波的形式分别在水银与水的界面和覆盖层中传播, 较少的一部分用于波束的正常反射. 左边的反射波束位置明显偏离了声波的入射点, 向左偏离了入射波束中心位置$8 a$, 此反射波束就是由表面波和漏波主导的负向反射[17]. 同时, 在两反射波束之间出现了空白区域, 这是由反射波束和出现在水层和超材料界面附近的漏波共同作用形成的[18,19]. 如图3(a)所示, 当有负向Schoch位移出现时, 在水层和超材料界面附近会形成大量的后向漏波. 改变F = 0.503作为对照, 没有Schoch位移出现其声压场图如图3(b), 此时在水层和超材料界面附近完全没有漏波出现. 研究还发现在这一结构模型下, 约化频率F仅在0.447—0.466范围内有负向Schoch位移出现, 如图2(b)所示, 并且在这一范围内负向Schoch位移随着F的增大先增大后减小为零, 在F = 0.456处位移取得最大负值$8 a$. 图 2 (a) 具有明显负向Schoch位移的声压场图($R = $$ 0.224 a$, F = fa/c0 = 0.456 = FS, a = 1 m), 其中纵轴和横轴分别表示本文结构的高度y和宽度x, 右侧的颜色条对应的物理量为总声压场强Pt; (b) 在FS = 0.456附近, 负向Schoch位移随约化频率F的变化 Figure2. (a) Acoustic pressure field with significant negative Schoch displacement ($R = 0.224 a$, F = fa/c0 = 0.456 = FS, a = 1 m); (b) relation of Schoch displacement to reduced frequency F near FS = 0.456.
图 3 (a) $F = 0.456$, 水层和超材料界面附近出现大量后向漏波; (b)$F = 0.{{503}}$, 水层和超材料界面附近完全没有漏波 Figure3. (a) $F = 0.456$, there are a large number of backward leaky Rayleigh waves appear near the interface between the water layer and the metamaterial; (b)$F = $$ 0.{{503}}$, there is no leaky Rayleigh wave near the water layer and the metamaterial interface.
为了探究伴随着这种负向Schoch位移出现时超材料的物理特性, 参照周期性复合流固材料的有效参数[20,21], 计算并获取了超材料不同的物理参数随约化频率F的变化关系. 图4(a)显示当负向Schoch位移发生时, 超材料的reff和Zeff极大值出现在同一约化频率$F = $$ 0.456$(标记为A点), 这与图2(a)发生负向Schoch位移时FS的值相同; 图4(b)显示neff, ρeff, 1/κeff随F的变化关系, 图中添加两条平行坐标轴的虚线作为辅助线, 其交点记为B (0.456, 0), B点的横坐标取值与A点处的F相同, 图像右下角内嵌图形为B点附近的放大. 以B作为参考点, 在F = 0.456时, ρeff小于零, 1/κeff趋向于零, neff的值小于且接近零, 它们之间满足关系[22]$\mathop n\nolimits_{{\rm{eff}}}^2 =\! 1/{\kappa _{{\rm{eff}}}} \cdot\! {\rho _{{\rm{eff}}}}$, 此时超材料为近零折射率声学材料. 图 4 声学超材料不同的物理参数特性 ($R = 0.224 a$, FS = 0.456) (a) 相对阻抗(Zeff)和反射系数(reff)随F的变化; (b) 有效折射率(neff)、有效质量密度(ρeff)、有效体积模量的倒数(1/κeff)随F的变化, 图像右下角内嵌图形为B点附近的放大 Figure4. Different physical parameters of acoustic metamaterials ($R = 0.224 a$, FS = 0.456): (a) Relationship between relative impedance (Zeff) and reflection coefficient(reff) with F; (b) relationship of effective refractive index (neff)、effective mass density (ρeff) and inverse of effective volume modulus (1/κeff) to F, the embedded figure in the lower right corner of the image is an enlargement near point B.
古斯-汉欣位移主要是由反射波束的相位突变导致[23], Schoch位移是否也与此有关? 图5为$R = 0.224 a$, FS = 0.456条件下发生Schoch位移时超材料的另外两个物理特性. 图5(a)表示反射系数的虚部相位发生突变, 突变前后的相位差值为π rad. 图5(b)为是偶极子激发形成的本征模的声压场图的三分之一, 根据超材料形成的原理[24]知, 偶极子表征超材料有效质量密度为负, 与图4(b)中ρeff < 0相符. 图 5 (a) 反射系数虚部相位在$F = 0.456$处发生突变($R = $$ 0.224 a$, FS = 0.456); (b) 晶格的本征模声压场图 Figure5. (a) Phase of the imaginary part of the reflection coefficient mutates at $F = 0.456$ ($R = 0.224 a$, FS = 0.456); (b) eigen mode acoustic pressure field diagram of the lattice.
为了探究图4和图5中的物理参数特性与负向Schoch位移的联系, 改变散射体R, 寻找反射系数峰值对应的F, 当图4和图5中的物理参数特性同时得到满足时, 验证该约化频率下是否出现负向Schoch位移. 经过多次的仿真发现, 改变散射体R, 物理参数特性与$R = 0.2{{24}}a$时的结果类似. 发生负向Schoch位移时散射体半径R与FS(fa/c0)的变化关系以图6(a)表示, 整体来看约化频率FS与散射体橡胶半径R近似呈现线性关系, 随着R的增大FS逐渐减小. 图 6 (a) 发生Schoch位移时FS随R的变化; (b) 负向Schoch位移随覆盖层厚度${h_0}$的变化($R = 0.224 a$, FS = 0.456); (c) 负向Schoch位移随入射声波半腰宽wi的变化; (d) 反射声波半腰宽wr随覆盖层厚度的变化 Figure6. (a) Relationship of FS to R when the Schoch displacement happens; (b) relationship between negative Schoch displacement and overburden thickness ${h_0}$; (c) relationship between negative Schoch displacement and the half-waist width of the incident acoustic wave; (d) relationship between the half-width of the reflected acoustic wave and the thickness of the covering layer.