Sheikh Ansari Abbas, Iyer Ashwin K, Gholipour Behrad
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Canada.
Nanophotonics. 2023 Apr 10;12(14):2639-2667. doi: 10.1515/nanoph-2022-0820. eCollection 2023 Jul.
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.
在互易介质中，电磁波的传播沿相反方向是对称的，这限制了光学和光子学中各种系统的设计与实现。非对称传输（AT）对于光学和光子学中隔离器和环行器的设计至关重要，并且它还惠及其他应用，如光伏系统、激光器、隐身和电磁屏蔽。虽然基于磁场偏置的大型非互易器件已广为人知，但在亚波长结构中实现非对称传输更具挑战性，并且具有亚波长厚度且呈现非对称传输的结构在过去十年中引起了广泛关注。已经报道了各种用于创建具有非互易传输特性的超表面的方法，例如增强法拉第旋转的等离子体和介质超表面、具有强度依赖折射率的非线性超表面，以及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高阶衍射级来为电磁波的传输创建多条路径，在互易结构中也报道了非对称传输。在此，我们对在互易和非互易系统的亚波长结构中实现非对称传输所采用的各种方法进行综述。我们还讨论了通过各种方法实现的非对称传输的主要设计原理和局限性。
