[论文解读] "Meta-atomless" architecture based on an irregular continuous fabric of coupling-tuned identical nanopillars enables highly efficient and achromatic metasurfaces

Hüseyin Bilge Yağcı, Hilmi Volkan Demir|arXiv (Cornell University)|Dec 11, 2020

Metamaterials and Metasurfaces Applications参考文献 36被引用 5

一句话总结

本文提出一种无需元原子的超表面架构，采用紧密排列、完全相同的介质纳米柱，并通过连续调节近场耦合，实现在可见光谱范围内的高效、非色差且偏振无关的聚焦。通过消除离散的元原子并利用相互耦合实现相位调控，该设计在400–700 nm波段内实现了超过85%的聚焦效率，并具备衍射极限性能。

ABSTRACT

Metasurfaces are subwavelength-thick constructs, consisting of discrete meta-atoms, providing discretized levels of phase accumulation that collectively approximate a designed optical functionality. The meta-atoms utilizing the Pancharatnam-Berry phase with polarization-converting structures produced encouraging implementations of optical components including metalenses. However, to date, a pending and fundamental problem of this approach has been the low device efficiency that such resulting metasurface components suffer, an unwanted side effect of large lattice constants that are used for preventing intercoupling of their meta-atoms. Although the use of near-field coupling for tuning electromagnetic resonances found its use in constructing efficient narrow-band designs, such structures fell short of providing high efficiency over a broad spectrum. Here, we propose and show that tightly packed fabrics of identical dielectric nanopillar waveguides with continuously tuned intercoupling distances make excellent and complete achromatic metasurface elements. This architecture enables the scatterers to interact with the incoming wave extremely efficiently. As a proof-of-concept demonstration, we showed an achromatic cylindrical metalens, constructed from strongly coupled dielectric nanopillars of a single geometry as continuously set phase elements in a meta-atomless fashion, working in the entirety of the 400-700 nm band. This metalens achieves over 85% focusing efficiency across this whole spectral range. To combat polarization sensitivity, we used hexagonally stacked nanopillars to build up a polarization-independent scatterer library. Finally, a circular metalens with polarization-independent operation and achromatic focusing was obtained. This is a paradigm shift in making an achromatic metasurface architecture by weaving identical nanopillars coupled into an irregular lattice laterally constructed via carefully tuned near-field coupling.

研究动机与目标

• 为克服基于非耦合元原子的传统超表面存在的低效率和光谱限制问题。
• 解决在非色差超表面中抑制相互耦合与实现高效率之间的根本性权衡。
• 利用单一纳米柱几何结构实现宽带、偏振无关且衍射极限的聚焦。
• 证明在二维晶格中连续调节相互耦合可替代共振或PB相位基的相位调控。

提出的方法

• 设计一种由相同介质纳米柱组成的二维非规则晶格，通过连续调节粒子间距离来调控近场耦合。
• 使用有限元时域（FDTD）仿真计算相位响应，并在可见光谱范围内优化耦合强度。
• 采用蜂窝堆叠结构，通过在纳米柱排布中破坏四重对称性，实现偏振无关性。
• 实施优化算法以最小化非规则晶格中的排布误差，确保波前整形的准确性。
• 通过仿真验证圆柱形和圆形金属透镜的性能，数值孔径NA = 0.26，焦距f = 11 µm。
• 利用Strehl比和光束轮廓分析量化聚焦性能及衍射极限工作状态。

实验结果

研究问题

• RQ1 能否将相同纳米柱之间的相互耦合作为连续相位调控机制，以替代离散的元原子？
• RQ2 这种耦合晶格能否在可见光谱范围内实现宽带非色差聚焦并保持高效率？
• RQ3 能否在不依赖复杂元原子几何形状或偏振复用技术的前提下实现偏振无关性？
• RQ4 连续耦合调节是否可消除超表面中对共振或PB相位基相位累积的需求？

主要发现

• 圆柱形金属透镜在400–700 nm整个光谱范围内均实现了超过85%的聚焦效率。
• 圆形金属透镜表现出衍射极限性能，Strehl比在所有波长下均达到理论极限。
• 焦斑尺寸和焦深呈线性波长依赖关系，光束轮廓在全光谱范围内变化极小。
• 通过六边形堆叠实现偏振无关工作，X-和Y-偏振输入情况下的对比度仅相差≤5%。
• 耦合晶格的相位响应与理想相位函数高度吻合，验证了连续耦合近似的有效性。
• 该设计在效率和非色差性能方面均优于近期最先进的超表面，尤其在宽带和偏振无关工作方面表现突出。