Fund Project:Project supported by the Youth Talent Lifting Project of the China Association for Science and Technology (Grant No. 17-JCJQ-QT-003), the National Defense Program of China (Grant No. 2019-JCJQ-JJ-081), and the Key Program of Natural Science Foundation of Shaanxi Province, China (Grant No. 2020JZ-33)
Received Date:26 January 2021
Accepted Date:17 February 2021
Available Online:07 June 2021
Published Online:05 August 2021
Abstract:With the development of intelligent technology, it is essential to develop polarization-conversion devices with adaptable electromagnetic (EM) performance for practical applications. Up to now, most of attempts have relied on PIN diodes and varactor diodes for electrical tuning, typically featuring simplicity and timelineness. However, the shortcomings are also notable, such as less degrees of freedom (DoFs), more complex circuits and more expensive. In view of this, here we propose a kind of spatial-order metasurface for reconfigurable polarization conversion based on kirigami concept. By adjusting the folding angle β, the interaction between neighboring dipoles can be progressively changed and thus the operation frequency of polarization conversion can be shifted. Such a mechanical reconfigurable strategy brings about more DoFs for tuning and is cheaper and extraordinary convenient in practice. To verify the feasibility of our concept, a proof-of-concept spatial-order kirigami metasurface is proposed for the dual-band reconfigurable linear polarization conversion based on asymmetric chiral split ring resonators (SRRs). Experimental results show that the linear polarization operates at 5 and 5.8 GHz when folding angle is β = 10°, these frequencies are shifted to 5.8 and 7.2 GHz when β = 45°: a tuning range is expanded by 18.5%. In addition, the Poisson’s ratio and relative density of proposed kirigami metasurface as a function of β are also theoretically analyzed. The results show that the Poisson’s ratio increases with the value of β increasing. The relative density can be reduced to 1.5% of its unfolded planar counterpart. Our spatial-order kirigami metasurface strategy paves the way for implementing the reconfigurable linear polarization conversion and multifunctional devices. Keywords:reconfigurable kirigami metasurfaces/ polarization conversion/ split ring resonators/ spatial-order
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2.原理分析、设计与数值仿真极化是电磁波的电场矢量按照某一规律振动的现象. 这里基于电磁场理论, 来分析电磁波入射到空间序构超表面结构后产生线极化转换及调控谐振频率的基本原理, 并为最终设计提供依据和指导. 如图2所示, 当超表面底部是金属板时, 电磁波垂直入射到超表面结构后, 透射系数为零, 当材料损耗较小时入射电磁波能高效率完美反射. 如图2(a)所示, 假设y (TM)极化电磁波向z轴负方向传播, 其中电场(E )沿y轴极化, 磁场(H )沿x轴方向. 设x-y轴绕着z轴逆时针方向旋转45°得到u-v轴. y极化电磁波入射到超表面结构后, 其电场矢量可分解为沿u和v轴的两个分量, 这里设z = 0, 所以电场可表示为 图 2 本文可重构空间序构剪纸超表面的线极化转换与频率调控原理示意图. u-v坐标系下的(a) y极化入射电磁波与(b) x极化反射电磁波; (c)任意两个空间放置磁偶极子的相互作用; (d)两个磁偶极子同向纵向耦合 Figure2. Schematic principle for linear polarization conversion and operation frequency control of reconfigurable spatial-order kirigami metasurfaces. The (a) incident y-polarized and (b) reflected x-polarized electromagnetic (EM) waves under u-v coordinate. (c) The interaction between two magnetic dipoles placed in free space. (d) The longitudinally coupled magnetic dipoles in identical direction respectively.
其中: ${E_{{\rm{r}}u}}$和${E_{{\rm{r}}v}}$代表的是沿着u和v轴的反射电场幅度, 且满足 $\left[ {\begin{array}{*{20}{c}} {{E_{{\rm{r}}u}}} \\ {{E_{{\rm{r}}v}}} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {{r_{xx}}}&{{r_{yx}}} \\ {{r_{xy}}}&{{r_{yy}}} \end{array}} \right]\left[ {\begin{array}{*{20}{c}} {{E_{{\rm{i}}u}}} \\ {{E_{{\rm{i}}v}}} \end{array}} \right]$, 下标x和y表示电磁波极化方向, i和r表示入射电磁波和反射电磁波, ${r_{yy}}$(${r_{xx}}$)表示y(x)极化波入射y(x)极化波的反射系数, ${r_{yx}}$(${r_{xy}}$)表示x(y)极化波入射y(x)极化波分量的反射系数; ${\varphi _u}$和${\varphi _v}$表示沿u和v轴方向的电场分量的相位; t表示电磁波在超表面结构上的作用时间. 假设${\varphi _u}$和${\varphi _v}$的相位差为$\Delta \varphi = {\varphi _u} - {\varphi _v}$, 当$\Delta \varphi = {\text{π}}$时, 也就是${E_{{\rm{r}}u}}$和${E_{{\rm{r}}v}}$反相, ${{\boldsymbol{E}}_r}$可表示为${{\boldsymbol{E}}_{\rm{r}}} = - {{\boldsymbol{e}}_u}{E_{{\rm{r}}u}}\cos (wt + {\varphi _v}) + $$ {{\boldsymbol{e}}_v}{E_{{\rm{r}}v}}\cos (wt + {\varphi _v})$, 当${E_{{\rm{r}}u}}/{E_{{\rm{r}}v}} = {E_{{\rm{i}}u}}/{E_{{\rm{i}}v}}$时合成的电磁波极化方向发生了90°旋转, 即反射后为x(TE)极化波, 产生了交叉极化转换, 如图2(b)所示. 这里定义y(x)极化入射电磁波线极化转化效率为${\rm{PCR}} = r_{yx}^2/(r_{yx}^2 + r_{xx}^2)$(${\rm{PCR}} = r_{xy}^2/(r_{xy}^2 + r_{yy}^2)$). 如图2(c)所示两个空间放置的磁偶极子会相互作用, 当两个磁偶极子为单纯的纵向耦合时它们之间的相互作用能为$V = \dfrac{{{M_1} \cdot {M_2}}}{{4{\text{π}}{\varepsilon _0}{r^3}}} = \dfrac{{{M_1} \cdot {M_2}}}{{4{\text{π}}{\varepsilon _0}{m^3}{{\sin }^3}\beta }}$(r对应后边提到的ly, 为两个磁偶极子之间的距离; M1和M2为两个磁偶极矩的幅值; m为介质板的长度; β为折叠角度). 如图2(d)所示当β减小, 两个同向纵向放置的磁偶极子之间距离会减少, 两个磁偶极子会相互吸引使得系统变得更加稳定, 从而谐振频率向低频移动. 如图3所示, 基于上述理论我们对单元进行了精心设计, 并采用CST Microwave Studio 2018软件对其建模和数值仿真. 在频域计算单元反射幅度时, x和y方向的边界条件设置为周期边界, 由于所设计单元工作于反射体系, Zmin方向边界条件设置为电边界, Zmax方向边界条件设为开放边界, 所有仿真均采用线极化平面波进行激励. 为进行全面分析, 选取了三种情形下的单元进行对比研究, 如图3(a)所示, 空间序构超表面单元仅在左侧斜面排列SRRs, 其在6.9 GHz处反射波电场矢量的两个分量幅度基本一致且相位差$\Delta \varphi $近似为180°, 根据上述理论分析可知超表面单元能实现线极化转换, 且极化转换效率可达99%以上. 如图3(b)所示, 在单元右侧设计了开口方环谐振器, 在5.5 GHz反射波电场矢量的两个分量幅度基本一致且相位差$\Delta \varphi $近似为180°, 同理该频率处可实现线极化转换, 且转换效率可达99%以上. 如图3(c)所示, 在空间序构超表面单元的左、右侧面上分别设计两种不同SRRs, 单元在两个频段处的反射波电场矢量幅度基本一致且相位差$\Delta \varphi $均近似为180°, 因此可实现双频线极化转换, 转换效率依然在99%以上. 为调控工作频带fm, 分析了y极化波入射下图3(c)的电流分布, 如图3(d)所示, 超表面单元左侧(右侧)开口圆环(方环)谐振器形成闭环电流, 为磁谐振, 可等效为磁偶极子, 与相邻磁偶极子相互作用构成同向纵向耦合, 由于两开口环谐振器放置具有一定的角度, 沿着u和v轴的磁场分量相反, 一个穿进圆环谐振器一个穿出方环谐振器, 因此两开口环电流方向相反. 根据上述提到的磁偶极子耦合理论, 当增加折叠角度β, 磁偶极子之间的距离会减小, 从而可降低线极化转换的工作频段fm, 反之则会使fm升高, 达到了调谐器件极化转换工作频段的效果. 图3(e)所示为最终设计空间序构超表面单元的结构参数. 图 3 三种不同情形下空间序构超表面单元的结构与数值仿真电磁特性(其中${\varphi _u}$(${\varphi _v}$)和ru(rv)分别表示沿u(v)轴方向电场分量反射相位和反射幅度, ${r_{yy}}$(${r_{xx}}$)表示y(x)极化波入射时同极化反射电磁波的幅度, ${r_{xy}}$(${r_{yx}}$)表示y(x)极化波入射时交叉极化反射电磁波的幅度) (a)单元只有开口圆环谐振器; (b)单元只有开口方环谐振器; (c)单元同时包含开口圆环和开口方环谐振器; (d)谐振频率f = 6.8和5.5 GHz处超表面单元SRRs上的表面电流分布; (e)最终空间序构超表面单元的结构, 结构参数依次为m = 18 mm, n = 6.7 mm, d = 0.1 mm, r2 = 3 mm, r1 = r2 – b1 = 2.4 mm, a1 = 6 mm, a2 = a1 – 2b2 = 4.8 mm和g = 0.44 mm, 黄色部分为金属铜, 蓝色部分为介质板, 介质板采为聚酰亚胺板, 介电常数为3.0, 电正切损耗为0.001 Figure3. Layout and numerical characterizations of the spatial-order meta-atoms in three different situations of (a) only circular SRRs along left slope, (b) square SRRs along right slope, and (c) both circular and square SRRs along both slopes. Here, ${\varphi _u}$(${\varphi _v}$) and ru (rv) represent the reflection phase and amplitude for components along u(v) axis, ${r_{yy}}$(${r_{xx}}$) represent the reflection amplitude of the incident y(x)-polarized and reflected y(x)-polarized EM waves, ${r_{xy}}$(${r_{yx}}$) represent the reflection amplitude of the incident y(x)-polarized and reflected x(y)-polarized EM waves. (d) The Surface current distribution on SRRs at resonant frequencies of f = 6.8 and 5.5 GHz. (e) Layout and geometrical parameters of the finally designed spatial-order meta-atom. They are m = 18 mm, n = 6.7 mm, d = 0.1 mm, r2 = 3 mm, r1 = r2 – b1 = 2.4 mm, a1 = 6 mm, a2 = a1 – 2b2 = 4.8 mm and g = 0.44 mm. The yellow color indicates metallic copper while blue represents dielectric slab, which is a FR4 board with dielectric constant of 3.0 and tangent loss of 0.001.
下面分析了两个SRRs与工作频段之间的关系, 如图4所示. 图4(a)和图4(b)表示的是左侧开口圆环谐振器宽度的影响, 可以看出改变开口圆环谐振器宽度b1时, 仅会改变高频工作频带, 低频工作频带基本不受影响. 为进一步验证两个工作模式之间的关系, 下面分析了开口方环谐振器宽度b2的影响, 如图4(c)和图4(d)所示. 改变开口方环谐振器的宽度, 低频工作频带会偏移, 高频工作频带几乎不受影响. 再一次验证了开口圆环谐振器工作在高频, 开口方环谐振器工作在低频, 两个模式可独立调控. 通过调节SRRs宽度改变工作频带是由于谐振的原因, 减小SRRs宽度, 增大了结构的等效电感, 根据公式$f = 1/\big({2{\text{π}}\sqrt {LC} } \big)$可知谐振频率向低频移动. 当减小圆环的宽度b1, 增大方环的宽度b2, 高频工作频带会向低频移动, 低频工作频带会向高频移动, 两个工作频带不断靠近可调整到一个状态, 即提高极化转换的工作带宽. 图 4 最终设计的空间序构超表面单元在不同宽度开口圆环和方环谐振器的电磁波反射幅度仿真结果 (a) y极化和(b) x极化平面电磁波入射时的反射幅度随开口圆环宽度b1变化的关系; (c) y极化和(d) x极化平面电磁波入射时的反射幅度随开口方环宽度b2变化的关系 Figure4. Finite-difference time-domain (FDTD) calculated reflection amplitude of the finally designed spatial-order meta-atom based on circular SRRs and square SRRs for different widths. Reflection amplitude as a function of (a), (b) b1 and (c), (d) b2 under (a), (c) y-polarized and (b), (d) x-polarized plane wave of normal incidence.
为验证上述所设计空间序构超表面的性能, 图5给出了最终设计空间序构超表面单元在不同折叠角度β下y极化和x极化电磁波入射下的线极化转换工作频谱. 由于当β为0°和90°时, 沿着y轴的周期${l_y} = m\sin \beta $或者沿着x轴的周期${l_x} = $$ 2 m\cos \beta$为0, 在实际中没有物理意义, 因此下面选取$0^\circ < \beta < 90^\circ $. 如图5(a)和图5(c)所示, 当β为10°时, 线极化转换工作于4.7和5.7 GHz; 当β为45°时, 线极化转换工作频带被调谐到5.5和6.9 GHz, 平均频率调控范围达17.5%. 调控机理是因为β减小, 沿着y方向的周期${l_y} = m\sin \beta $减小, 根据磁偶极子耦合理论可知相邻两个面上磁偶极子之间的吸引力会增大, 所以谐振频率会向低频移动, 即极化转换的工作频带会向低频移动. 当β为45°时ly和lx达极大值, 入射电磁波在u和v方向上的分量相同, 根据第2节理论分析可知极化转换效率达到最大. 如图5(b)和图5(d)所示, β在其他角度时交叉极化转换效率减小, 当β = 5°时可作为同极化镜面反射器, 且β增大时, 极化产生切换且极化转换工作频带会向高频移动, 与理论分析一致. 图 5 最终设计空间序构超表面的电磁波反射幅度在不同折叠角度β下的仿真结果 (a), (b) y极化与(c), (d) x极化平面电磁波入射时的反射幅度 Figure5. FDTD calculated reflection amplitude of EM waves of the finally designed spatial-order metasurfaces at different folding angles β: the reflection amplitude of the incident (a), (b) y-polarized and (c), (d) x-polarized EM waves.