[论文解读] Space-Time Modulated Loaded-Wire Metagratings for Magnetless Nonreciprocity and Near-Complete Frequency Conversion

Yakir Hadad, Dimitrios L. Sounas|arXiv (Cornell University)|Jun 1, 2019

Metamaterials and Metasurfaces Applications被引用 5

一句话总结

该论文提出了一种无磁非互易超光栅，利用时空调制的电容加载导线，实现强异常反射和近乎完全的频率转换。通过利用时变极化率和离散偶极近似，该设计实现了高效、非互易的散射，对某一入射角的镜面反射被抑制，同时在相反方向保持高反射率，从而以极低的调制复杂度实现了实际应用。

ABSTRACT

In recent years a significant progress has been made in the development of magnet-less nonreciprocity using space-time modulation, both in electromagnetics and acoustics. This approach has so far resulted in a plethora of non-reciprocal devices, such as isolators and circulators, over different parts of the spectrum, for guided waves. On the other hand, very little work has been performed on non-reciprocal devices for waves propagating in free space, which can also have many practical applications. For example, it was shown theoretically that non-reciprocal scattering by a metasurface can be obtained if the surface-impedance operator is continuously modulated in space and time. However, the main challenge in the realization of such a metasurface is due to the high complexity required to modulate in space and time many sub-wavelength unit-cells of which the metasurface consists. In this paper we show that spatiotemporally modulated metagratings can lead to strong nonreciprocal responses, despite the fact that they are based on electrically-large unit cells. We specifically focus on wire metagratings loaded with time-modulated capacitances. We use the discrete-dipole-approximation and an ad-hoc generalization of the theory of polarizability for time-modulated particles, and demonstrate an effective nonreciprocal anomalous reflection (diffraction) with an efficient frequency conversion. Thus, our work opens a venue towards a practical design and implementation of highly non-reciprocal magnet-less metasurfaces in electromagnetics and acoustics.

研究动机与目标

• 解决在无外部磁体条件下实现实用化自由空间波器件的挑战。
• 克服在时空调制超表面中对密集亚波长单元结构进行调制的高复杂度问题。
• 利用电尺寸较大的单元结构实现高效、非互易的异常反射与频率转换。
• 通过每单元仅使用三个不同的调制区域，简化调制需求。
• 展示在电磁学与声学中实现可制造的非互易超表面的可行路径。

提出的方法

• 为时变调制谐振粒子开发广义极化率理论，以模拟电容加载导线的响应。
• 应用离散偶极近似分析二维时空调制导线阵列的散射特性。
• 通过电容的时空调制产生合成运动，打破时间反演对称性。
• 分析空间-时间谐波的激发，特别是(m,n) = (1,-1)和(0,1)阶次，以实现异常反射。
• 设计具有电尺寸较大单元结构（O(λ/2)）的超光栅，以支持传播衍射级次并抑制镜面反射。
• 优化调制参数，实现在下变频频率处向(1,-1)谐波的近乎完全的功率传输。

实验结果

研究问题

• RQ1 具有电尺寸较大单元结构的时空调制超光栅是否能在自由空间中实现强非互易散射？
• RQ2 如何以最小的调制复杂度实现非互易异常反射与高效频率转换？
• RQ3 时变极化率在实现基于导线的超光栅非互易响应中起什么作用？
• RQ4 离散偶极近似能否准确建模时空调制谐振导线阵列的散射响应？
• RQ5 不同空间-时间谐波的激发如何依赖于入射角，从而实现非互易性？

主要发现

• 对于入射角θⁱ = -30°，在基频ω处的镜面反射被完全抑制，几乎所有入射功率被转换至(m,n) = (1,-1)谐波，频率为ω₋₁ = ωᵣ - ωₘ。
• 在θʳ ≈ 42°处的主导反射波对应于(1,-1)谐波，表明实现了强非互易异常反射。
• (0,1)谐波被弱激发，功率通量可忽略不计，表明频率转换效率极高。
• 对于互补入射角θⁱ = 30°，镜面反射占主导，反射角θʳ = 30°，证实了非互易行为。
• 系统实现了近乎完全的频率转换，对其他谐波的损耗极小，从而实现了高效的非互易工作。
• 每单元仅需三个不同的调制区域，显著简化了实际实现与布线复杂度。