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实现异常折射和反射的关键是在超表面获得覆盖0~2π的相移以满足所谓的广义斯涅尔定律,表示如下[13]:
$$ {n_i}\sin {\theta _r} = {n_i}\sin {\theta _i} + \frac{{{\lambda _0}}}{{2\pi }}\frac{{{\rm{d}}\varPhi (x)}}{{{\rm{d}}x}} $$ (1) 式中:ni为入射介质的折射率;θi和θr分别为入射角与反射角;λ0是真空波长;dФ(x)/dx则是超表面上的相位梯度。公式(1)同样可以用于描述异常折射现象,只需将nisinθi替换为ntsinθt。假设在一个周期长度D内覆盖2π的相移,则正入射时的反常反射角可以通过下式计算:
$$ {\theta _r} = {\arcsin}\left( { \sin {\theta _i} + \frac{{{\lambda _0}}}{{2\pi }}\frac{{2\pi }}{D} } \right) = {\arcsin}\left( { \frac{{{\lambda _0}}}{D} } \right) $$ (2) 根据公式(2),很明显,异常反射角可以通过满足适当的相移来控制。当界面处没有相位变化时,公式(1)便又简化为斯涅尔定律的标准形式。因此,重点是找到一种方法来实现太赫兹波段的覆盖整个2π范围的相移。幸运的是,已经有通过亚波长介质光栅进行相位控制的研究。高折射率对比度光栅 (HCG) 阵列提供宽带高反射率,同时调整光栅的占空比可以实现相移[30-31]。这可以用有效介质理论来解释[32],即当入射波沿Z方向入射时,保持介质在Z方向的厚度不变,改变其X和Y方向的尺寸能够相应地改变在该方向上的有效折射率。HCG的一个显著优点是不会改变入射波的偏振,从而避免了因偏振转换效率带来的损耗。然而,仅使用HCG可能无法保证在所需频率下完全覆盖0~2π相移,并且随着光栅占空比的改变,反射率可能会下降,从而使得效率降低。解决这个问题的方法之一是通过宽带反射的平面来确保超表面的反射率,从而使得介质光栅的反射率不再重要,于是在选择光栅参数时可以优先考虑相移以确保覆盖了整个2π范围。
广义斯涅尔定律描述的现象可以用惠更斯原理来解释[33],它指出波前的每个点都是出射波的次级源,因此,超表面上的场分布决定了超表面的反射。基于这个想法,可以通过确认超表面上的场和能量流分布来探寻完美异常反射存在的条件。首先选取X方向上周期的长度D,使其满足λ<D<2λ,这样系统中便只有−1级,0级和+1级衍射,其中+1级是所需的异常反射,0级是正常镜面反射,−1级则是寄生反射,如图1所示。显然,最小化0级和−1级衍射能够使得器件工作效率最大化。假设φ−1, φ0, φ+1,r−1,r0,r+1分别是这三个衍射级的相位和振幅响应,便可以得出当TM波正入射时超表面上方的场的表达式为:
图 1 存在三个衍射级的超表面上的异常反射
Figure 1. Schematic of the anomalous reflection on metasurface with three diffraction orders
$$\begin{split} & {E_x} = {E_i}(1 - {r_0}{{\rm{e}}^{i{\varphi _0}}} - {r_1}\cos {\theta _r}{{\rm{e}}^{iGx + i{\varphi _1}}} - {r_{ - 1}}\cos {\theta _r}{{\rm{e}}^{ - iGx + i{\varphi _{ - 1}}}}) \\& {H_y} = \eta {E_i}(1 + {r_0}{{\rm{e}}^{i{\varphi _0}}} + {r_1}{{\rm{e}}^{iGx + i{\varphi _1}}} + {r_{ - 1}}{{\rm{e}}^{ - iGx + i{\varphi _{ - 1}}}}) \\[-10pt] \end{split}$$ (3) 式中:Ei为入射平面波的电场;
$ \eta = \sqrt {\varepsilon /\mu } $ ,$ G = 2\pi /D $ 是超表面的倒易矢量;θr是+1阶衍射对应的反射角度,这里省略了时间因子${\mathrm{e}}^{{{{i}}}\mathrm{\omega }\mathrm{t}}$ 。有了Ex和Hy,便可通过$ {S_{p {\textit{z}}}}(x) = \dfrac{1}{2}{{\rm{Re}}} ({E_x}H_y^ * ) $ 计算出超表面上的能流分布。当系统中没有吸收和透射时,根据能量守恒定律可得到$ r_0^2 + r_1^2\cos {\theta _r} + r_{ - 1}^2\cos {\theta _r} = 1 $ ,理想状态下r−1和r0应等于0来让效率最大化,此时可得$ {r_1} = 1/\sqrt {\cos {\theta _r}} $ 。于是,能流分布可以写作:$$ {S_{p {\textit{z}}}}(x) = \frac{1}{2}\eta E_i^2{r_1}(1 - \cos {\theta _r})\cos (Gx + {\phi _1}) $$ (4) 为了实现公式(4)中的条件,即
$ {S_{p {\textit{z}}}}(x) \ne 0 $ ,超表面需要周期性地在局部展示出得失响应,唯一的例外是$ {\theta _r} = 0 $ ,也就是没有异常反射现象的时候。一个很容易想到的解决方案就是使用有源和有损组件构建系统,但这并非良策,因为这些元件难以应用于实际。值得一提的是$ \int_x^{x + D} {{S_{p {\textit{z}}}}(x){\rm{d}}x} = 0 $ ,意味着能量在一个周期内是无损的,而局部响应则是增益或损耗。换句话说,文中需要的是一个非局域性的超表面,它可以使能量横向转移,并且还应该满足广义斯涅尔定律描述的相位不连续性。考虑到上面提到的介质光栅,当平面波入射时,不同阶的传播布洛赫波和更高阶的倏逝波被激发,它们会相互耦合[34-35]。可以将介电光栅视为一个耦合谐振器系统,其中每个凹槽都可以视为一个谐振腔。容易想到当改变这些凹槽的宽度时,可以调节这些布洛赫模的行为,以实现想要的能量转移。于是下一步是找到一组合适的参数来达到预期的高效率。基于这个想法,提出了如图2所示的由硅和二氧化硅组成的全电介质超表面,用于实现太赫兹波段的高效异常反射。布拉格反射镜保证了反射率并降低了损耗,而相位光栅可以提供所需的相位不连续性和非局部响应。FDTD仿真结果显示,当垂直入射时反射约为 40°,效率超过 99%。文中的仿真结果基于全波模拟软件CST Microwave Studio[21-22]。
High-efficiency terahertz wave anomalous reflector based on dielectric metasurface of phase gradient grating
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摘要: 太赫兹(THz)波由于其诸多独特的性质,有着广泛的应用前景。然而由于相关材料和器件的发展相对滞后,太赫兹技术在实际中的应用尚有诸多限制。超材料和超表面概念的提出,能够对太赫兹波的相位、振幅、偏振进行有效操控,为太赫兹技术的发展提供了许多新的思路。其重要的功能之一是依靠相位不连续将入射波反射到非镜面方向,即通称的广义斯涅尔定律。然而,此前报道的大多数异常反射装置的效率都相对较低,在实际应用中存在局限性。针对这一问题,文中提出了一种太赫兹超表面异常反射器,将法向入射光反射到 40° 方向且不改变其偏振,并从理论上阐述了提高效率的思路,且通过数值模拟展示其有效性。通过使用全介质材料构建超表面从而消除材料损耗,并利用不同布洛赫波的耦合以提供非局部响应,令器件的工作效率超过99%。此外,这一设计理念可以推广到偏振无关器件中,并且对其他类似的器件也有一定参考意义。这一工作有潜力被应用于太赫兹波激光器、太赫兹波腔谐振器等太赫兹波实际器件中。Abstract: Terahertz (THz) wave has broad application prospects because of its many unique properties. However, due to the relative lag in the development of related materials and devices, there are still many limitations in the practical application of terahertz technology. Metamaterials and metasurfaces can effectively manipulate the phase, amplitude, and polarization of terahertz waves, providing many new possibilities for the development of terahertz technology. One of its important functions is to reflect incident waves in non-specular directions through phase discontinuities, commonly known as generalized Snell's law. However, most of the previously reported anomalous reflection devices are relatively inefficient and have limitations in practical applications. To solve this problem, we propose a terahertz metasurface anomalous reflector that reflects normal incident light to a 40° direction without changing its polarization. We theoretically expound the idea of improving efficiency and demonstrate its effectiveness through numerical simulation. We construct a metasurface using all-dielectric materials to eliminate material losses and exploiting the coupling of different Bloch waves to provide a non-local response, the operating efficiency of the device can be increased to more than 99%. In addition, our design concept can be generalized to polarization-independent devices and could be useful for other similar devices. This work has the potential to be applied to practical terahertz devices such as terahertz lasers and terahertz cavity resonators.
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Key words:
- terahertz /
- metasurface /
- anomalous reflection /
- non-locality
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图 4 (a)仿真得到的异常反射效率;(b)超表面上方的异常反射场(X-Z平面);(c)超表面周围的能流分布;(d)第一个光栅的占空比对异常反射效率的影响
Figure 4. (a) Simulation results of anomalous reflection efficiency; (b) Electric field of anomalous reflection above the metasurface (X-Z plane); (c) Power flow around the metasurface; (d) Anomalous reflection efficiency when the duty ratio of the first grating is scanned
图 5 (a)偏振无关二维超表面示意图;(b)二维超表面上方的异常反射场(X-Z平面);(c) TE模与TM模的异常反射效率对比;(d)文中的结构(蓝色)与基于广义斯涅尔定律的线性相位梯度结构(红色)的异常反射效率对比
Figure 5. (a) Schematic of polarization-independent two dimensional metasurface; (b) Electric field of anomalous reflection above the two dimensional metasurface (X-Z plane); (c) Efficiency of TE mode and TM mode; (d) Anomalous reflection efficiency comparison between our design (blue) and the generalized Snell's law-based linear phase gradient profile design (red)
图 6 (a) 5°入射时超表面上的异常反射场;(b)文中的结构(蓝色)与基于广义斯涅尔定律的线性相位梯度结构(红色)在入射角 0°~20°下的异常反射效率对比
Figure 6. (a) Electric field anomalous reflection above the metasurface when incident angle is 5°; (b) Anomalous reflection efficiency comparison between our design (blue) and the generalized Snell's law-based linear phase gradient profile design (red) when incident angle changes from 0° to 20°
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