矩形波导中理想模式转换的亚折射率：理论与实验
Metagratings for Perfect Mode Conversion in Rectangular Waveguides: Theory and Experiment
摘要：
我们提出了一个完整的设计方案，从理论公式到实验验证，利用多功能性的metagratings（MG）设计矩形波导（RWG）$\mbox{TE}_{10}$ - $\mbox{TE}_{20}$模式转换器（MC）。由稀疏周期性定位的可极化粒子（元原子）形成的MG装置迄今为止主要用于束操纵应用。在本文中，我们表明，吸引人的衍射工程功能的MG在这种典型的自由空间周期性的情况下，可以利用波导（WG）内的有效模具领域。特别是，我们推导出一个分析模型，允许利用MG的概念，实现完美的模式转换RWG。方便地，该形式考虑了端接RWG的印刷电路板（PCB）MG，其作为反射模式MC操作。遵循典型的MG合成方法，该模型直接将元原子位置和几何形状与模态反射系数联系在一起，通过实施功能性约束来实现详细的制造就绪设计的解决方案：消除基本的$\mbox{TE}_{10}$反射和功率守恒（无源无损MG）。这种可靠的半解析方案，通过全波模拟和实验室测量验证，建立了一个简单而有效的替代常见的RWG MC，通常需要通过耗时的全波优化设计的WG具有挑战性的变形。此外，它突出了巨大的潜力MG涵盖了各种各样的应用超越光束操纵。
英文摘要：
We present a complete design scheme, from theoretical formulation to experimental validation, exploiting the versatility of metagratings (MGs) for designing a rectangular waveguide (RWG) $\mbox{TE}_{10}$ - $\mbox{TE}_{20}$ mode converter (MC). MG devices, formed by sparse periodically positioned polarizable particles (meta-atoms), were mostly used to date for beam manipulation applications. In this paper, we show that the appealing diffraction engineering features of the MGs in such typical free-space periodic scenarios can be utilized to efficiently mould fields inside waveguides (WGs). In particular, we derive an analytical model allowing harnessing of the MG concept for realization of perfect mode conversion in RWGs. Conveniently, the formalism considers a printed-circuit-board (PCB) MG terminating the RWG, operating as a reflect-mode MC. Following the typical MG synthesis approach, the model directly ties the meta-atom position and geometry with the modal reflection coefficients, enabling resolution of the detailed fabrication-ready design by enforcement of the functionality constraints: elimination of the fundamental $\mbox{TE}_{10}$ reflection and power conservation (passive lossless MG). This reliable semianaltyical scheme, verified via full-wave simulations and laboratory measurements, establishes a simple and efficient alternative to common RWG MCs, typically requiring challenging deformation of the WG designed through time-consuming full-wave optimization. In addition, it highlights the immense potential MGs encompass for a wide variety of applications beyond beam manipulation.
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