Large-Angle, High-Efficiency Beam Deflection Based on Rectangular Dielectric Metagrating

Schematic illustration of the rectangular dielectric metagrating for beam deflection with high diffraction efficiency.

Beam deflection is of fundamental importance in optical beam manipulation. It has potential applications in many fields, such as the LiDAR systems, advanced chips, free-space optical communication, and high-sensitivity sensing. Therefore, it is of great significance to achieve the large-angle and high-efficiency beam deflection for the design of the optical components.

The general ways to deflect the beams are based on the principles of phase accumulation or linear phase gradient, but they are too big in size and hard to integrate, or have low efficiency when the deflection angle is large. Recently, metagrating, one of the emerging sub-wavelength optical structure, has been proposed. It offers a new way to realize high-efficiency and large-angle beam deflection, by combining the Mie scattering behaviors of tailored meta-atoms with the diffraction effect of the grating. However, most of them are constructed by complicated structure with time-consuming design algorithm or extreme nano-parameters for fabrication. Besides, the metagratings based on the metallic rods may suffer from the intrinsic ohmic metallic loss. Consequently, designing metagrating with simple structure, low-loss and high-efficiency is necessary and beneficial to large-scale production and the application of high-performance wavefront shaping.

The research group led by Professor Jianwen Dong from Sun Yat-sen University proposed a rectangular dielectric metagrating to achieve large-angle and high-efficiency beam deflection. The results have been published in Chinese Optics Letters, Vol.18, Issue 7, 2020 (Wei-Yi Shi, Wei-Min Deng, Wei-Nan Liu, et al. Rectangle Dielectric Metagrating for Perfect Diffraction with Large-Angle Deflection[J]. Chinese Optics Letters, 2020, 18(7): 073601).

They concentrated almost all the energy to a specific diffraction order and suppress others by the simple rectangular units. The deflecting angle reaches to 90°, and highest efficiency obtained is 97.55%. Moreover, the efficiency maintains over 90% for a wide range of incident angles (31° to 64°), acting as an element with wide-angle tolerance. This phenomenon is explained by the waveguide-mode expansion and the scattering-matrix method. Besides, they also considered the practicability of the grating and found that the diffraction efficiency could still maintain the high value after considering the influences of the material absorption, material dispersion and the substrate.

"Metagrating is of great significance for the spectrum tailoring or beam manipulation. People can not only substantially decrease the volume and weight of the conventional gratings, but also achieve some novel properties that the conventional elements do not have by utilizing the resonant properties of these sub-wavelength units." says the corresponding author Professor Jian-Wen Dong.

The metagrating in this work has the excellent properties of simple structure consisting, large-angle deflection and high diffraction efficiency. It will be a fundamental optical element and play an important role in the fields of spectral detecting, high-resolution imaging, and the planar optics.

矩形介质超构光栅实现大角度、高效率光束偏折

矩形介质超构光栅实现高衍射效率光束偏折示意图

光束偏折一直是光学领域基础且重要的光束操控行为，在激光雷达、高端芯片、自由空间光通信、高灵敏传感等方面都具有重要潜在应用。因此，实现大角度、高效率的光束偏折对于光学元件的设计具有重要的意义。

通常上使用相位积累或线性相位梯度的光束偏折，具有体积大、不易集成，或是大角度偏折效率低的缺点。近年来，人们提出了超构光栅（一种新型的亚波长光学结构），结合亚波长单元结构的米氏散射特性和光栅衍射效应，可以实现大角度、高效率的光束偏折。

然而，目前超构光栅的单元结构都较为复杂，存在设计单元结构耗时、微纳制备条件极端等劣势；又或是采用金属材料，因而存在不可避免的损耗。因此，设计一种结构简单、损耗低且高效的超构光栅，对于大规模制作及相关的拓展应用都有着积极的推动作用。

近日，中山大学董建文教授领导的研究团队提出了一种矩形介质超构光栅，来实现大角度、高效率光束偏折的方法。相关研究结果发表于Chinese Optics Letters 2020年第7期（Wei-Yi Shi, Wei-Min Deng, Wei-Nan Liu, et al. Rectangle Dielectric Metagrating for Perfect Diffraction with Large-Angle Deflection[J]. Chinese Optics Letters, 2020,18(7): 073601）。

研究人员设计了简单的矩阵单元结构，通过抑制光栅其它所有衍射级次，使得绝大部分的出射能量都集中到某一级。该结构使得偏折角可达90°，衍射效率理论上高达97.55%。同时，该设计具有宽角度容忍特性，入射角在31°到64°范围内，衍射效率均保持在90%以上。研究人员采用波导阵列模式展开与散射矩阵相结合的方法，解释了这一物理光学现象。此外，还对实验可行性进行了讨论，结果表明，在考虑了介质材料吸收、实际色散、以及衬底影响后，衍射效率仍可保持在较高水平。

董建文教授表示："超构光栅在光谱设计、波束整形等方面有着非常重要的意义。通过利用这些亚波长单元的共振特性，不仅可以大幅度减少传统光栅的体积和重量，还能够得到传统光栅不具备的光学特性。"

这种超构光栅集结构简单、偏折角度大、透射效率高等优越性能于一身，将在光谱探测、高分辨成像、平面光学等多个应用领域发挥重要作用，是重要的基础性光学元件。