Optical beam splitting and asymmetric transmission in bi-layer metagratings

Magnetic field diagram of asymmetric transmission.

The asymmetric transmission of electromagnetic waves refers to the difference in transmittance or polarization state when they pass through media from two opposite directions. This asymmetric transmission effect has potential applications in the integrated photonic system of optical communication, information processing and all-optical computing, and has been one of the important research topics. The traditional devices with asymmetric response are too bulky to meet the needs of the integration and miniaturization of modern optical devices. In recent years, much effort has been devoted to metasurfaces used to the design of asymmetric transmission devices. Due to its flat and ultra-thin features, metasurfaces have unique application prospects in miniaturized and integrated devices compared with traditional optical components.

At present, most of metasurface-based asymmetric transmission devices work in the low-frequency regime, such as the terahertz band. There are few studies on asymmetric transmission in visible light. Although some studies have proposed conically structured optical metasurfaces that can achieve asymmetric transmission and have good broadband response, but their asymmetric response is weak. Therefore, how to design a high-efficiency asymmetric transmission device with a simple structure is still an open problem to be solved urgently.

Recently, the Prof. Yadong Xu's group from Soochow University proposed a device based on bi-layer meta-gratings that can enable efficiently beam splitting and asymmetric transmission, with obtained results published in Chinese Optics Letters Volume 19, Issue 4 (Shi Qiangshi, et al, Optical beam splitting and asymmetric transmission in bi-layer metagratings).

While there are numerous researchers that developed technologies to improve the time resolution, all such research were only focused on converting the required temporal shots into spatially or polarization multiplexed single shot. As a consequence, the temporal resolution was increased at the cost of either the field of view or signal to noise ratio. In all the previous studies, the inline FINCH hologram was reconstructed by a Fresnel back propagation which generated the twin images and bias terms. At least three camera shots with relative phase-shifts were needed to synthesize a complex hologram which, when reconstructed by Fresnel propagation, generates the image without the twin image and bias terms. A game changing approach to FINCH was introduced by the Swinburne’s nanotechnology and nanophotonics research team in 2020, in which a modified reconstruction mechanism was applied which simultaneously increased the temporal as well as axial resolution. In this study, the point spread function of FINCH is recorded and is used as the reconstructing function with a cross-correlation by a non-linear filter. Thus, the problem was converted into a pattern recognition problem. The FINCH system was realized using a spatially random multiplexed single diffractive optical element fabricated using electron beam lithography. The above procedure resulted in a compact, light-weight version of FINCH with a single camera shot and improved axial resolution.

The Swinburne’s nanotechnology and nanophotonics research team has recently investigated a fundamental problem, the transfer of characteristics from modulation function to the reconstructed image in FINCH, with two types of numerical reconstructions: Fresnel propagation and non-linear correlation in both spatial as well as polarization multiplexing modes. The results of the study are published in Chinese Optics Letters, Volume 19, No. 2, 2021 (V. Anand, et al., Review of Fresnel incoherent correlation holography with linear and non-linear correlations [Invited]). The investigation revealed interesting characteristics of FINCH in non-linear correlation mode. FINCH in Fresnel propagation mode faithfully transfers the characteristics of the modulation function to the reconstructed image. FINCH in non-linear correlation mode offers a platform to manipulate the degree by which the beam characteristics are transferred to the reconstructed image by controlling the degree of chaos in the spatially multiplexed diffractive element. Consequently, the above mode of operation improves the tolerance of FINCH to noise and aberrations. The findings of the study are expected to not only open new pathways to introduce special imaging characteristics but also guide in building versatile FINCH scopes in the future.

A bi-layer meta-grating with an operating wavelength at near 650 nm is designed and studied. The designed metagrating is relatively simple, with each period containing only two unit cells. They showed that the interlayer interaction can be controlled by changing the layer gap of the bi-layer meta-grating, which can lead to a transition from nearly perfect beam splitting to high-efficiency asymmetric transmission. Specifically, when the gap size is zero, the normally incident light from both sides will almost perfectly transmit through the bi-layer meta-grating, generating spatial beam splitting. When the gap size meets certain values (i.e., ), for positive normal incidence (PI), the transmission is about 2%, while for negative normal incidence (NI), the transmission is about 92.5%.

The researchers also made an optimization by considering the real material parameters, which further revealed the good asymmetric transmission effect and verified the feasibility of the bi-layer metagrating in experiment. This work provides a way for the design of high-efficiency asymmetric transmission devices and multifunctional devices in the visible light.

可见光波段不对称传输，照亮多功能光学器件前路

不对称传输磁场图

电磁波的不对称传输指的是电磁波从两个相反的方向通过某种介质时透射率或者偏振状态有所不同。这种不对称传输效应在光通信、信息处理以及全光计算的集成光子系统中具有潜在的应用，一直是备受关注的重要研究课题之一。

传统的不对称传输器件体积较大，不利于片上集成，难以满足现代光学器件集成化和小型化的需求。近年来人们考虑利用超构表面来设计不对称传输器件。和传统光学元件相比，超构表面具有平面、超薄等优点，在小型化和集成化器件设计方面具有独特的应用前景。

目前基于超构表面的不对称传输器件大部分工作在低频波段，比如太赫兹波段，而可见光波段的不对称传输研究较少。尽管有研究证明锥形结构超构表面实现了可见光波段的不对称传输且具有较好的宽频响应,但其不对称性能较差。因此，如何设计效率高、结构简单的不对称传输器件仍然是目前亟待解决的问题。

近来苏州大学的徐亚东教授课题组将相位梯度超构表面与亚波长金属光栅相结合，提出利用相位梯度超构光栅来设计不对称传输器件的方法，为实现高效率不对称传输提供了新的思路。相关研究成果发表在Chinese Optics Letters第19卷第4期 (Shi Qiangshi, et. al., Optical beam splitting and asymmetric transmission in bi-layer metagratings)。

研究人员设计并研究了一种工作波长在650 nm附近的双层相位梯度超构光栅，不仅结构相对简单，且每个周期仅包含两种不同的结构单元。研究发现：通过改变双层超构光栅的层间隙可以调控层间相互作用，从而实现光束分裂到高效率不对称传输的转变。具体来说，当双层超构光栅的间隔为0时，光从超构光栅前后两侧入射时，都将几乎完美地透射过超构光栅并产生空间光束分裂。当层间间隔满足一定条件时（例如 ），光从一侧入射超构光栅结构时，发生几乎完美反射，透射率只有2%；光从另一侧入射则全部透过超构光栅并呈现光束分裂现象，透射率高达92.5%。

该工作还考虑了真实材料参数，对结构进行了优化设计，进一步揭示了良好的不对称传输效果，验证了双层超构光栅实验上的可行性。该成果为设计可见光波段的高效率不对称传输器件、多功能光学器件提供了新的思路。