In a reciprocal medium, transmission of electromagnetic (EM) waves is symmetric along opposite directions which restrict design and implementation of various systems in optics and photonics. Asymmetric transmission (AT) is essential for designing isolators and circulators in optics and photonics, and it benefits other applications such as photovoltaic systems, lasers, cloaking, and EM shielding. While bulky nonreciprocal devices based on magnetic field biases have been well known, creating AT in subwavelength structures is more challenging, and structures with a subwavelength thickness that show AT have drawn a lot of attention over the last decade. Various approaches have been reported to create metasurfaces featuring nonreciprocal transmission, such as plasmonic and dielectric metasurfaces that enhance Faraday rotation, nonlinear metasurfaces with intensity-dependent refractive indices, and implementing spatiotemporal modulation in a metasurface. On the other hand, AT has also been reported in reciprocal structures by creating multiple paths for the transmission of EM waves by changing the polarization of light or redirecting light to higher-order diffraction orders. Here, we present a review of various approaches implemented for realizing AT in subwavelength structures in both reciprocal and nonreciprocal systems. We also discuss the main design principles and limitations of AT achieved in various approaches.

中文翻译：

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纳米光子学中的不对称传输

在互易介质中，电磁 (EM) 波的传输沿相反方向对称，这限制了光学和光子学中各种系统的设计和实现。非对称传输 (AT) 对于设计光学和光子学中的隔离器和环行器至关重要，它也有利于光伏系统、激光、隐身和 EM 屏蔽等其他应用。虽然基于磁场偏置的笨重的不可逆装置已广为人知，但在亚波长结构中创建 AT 更具挑战性，在过去十年中，具有显示 AT 的亚波长厚度的结构引起了很多关注。已经报道了各种方法来创建具有非互易传输的超表面，例如增强法拉第旋转的等离子体和介电超表面，具有强度相关折射率的非线性超表面，并在超表面中实现时空调制。另一方面，通过改变光的偏振或将光重定向到更高阶的衍射级，通过为 EM 波的传输创建多条路径，在倒易结构中也报道了 AT。在这里，我们回顾了在互易系统和非互易系统中实现亚波长结构中的 AT 的各种方法。我们还讨论了在各种方法中实现的 AT 的主要设计原则和局限性。通过改变光的偏振或将光重定向到更高阶的衍射级，AT 也被报道为通过创建用于 EM 波传输的多条路径的倒易结构。在这里，我们回顾了在互易系统和非互易系统中实现亚波长结构中的 AT 的各种方法。我们还讨论了在各种方法中实现的 AT 的主要设计原则和局限性。通过改变光的偏振或将光重定向到更高阶的衍射级，AT 也被报道为通过创建用于 EM 波传输的多条路径的倒易结构。在这里，我们回顾了在互易系统和非互易系统中实现亚波长结构中的 AT 的各种方法。我们还讨论了在各种方法中实现的 AT 的主要设计原则和局限性。