He Xiaoluo, Qi Chu, Lei Sheng, Wong Alex M H
Department of Electrical Engineering, City University of Hong Kong, Hong Kong SAR, China.
State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Hong Kong SAR, China.
Nanophotonics. 2023 Aug 18;12(18):3633-3644. doi: 10.1515/nanoph-2023-0331. eCollection 2023 Sep.
Achromatic lenses, which have the same focal length regardless of the illumination frequency, find strong applications in imaging, sensing, and communication systems. Making achromatic lenses with metasurfaces is highly desirable because they are flat, ultrathin, relatively light, and easily fabricable. However, existing metalenses experience combinations of limitations which include single polarization operation, narrow bandwidth, and small numerical aperture (NA). In this work, we propose a dual polarized, broadband and high NA achromatic metalens based on the Huygens' metasurface. We use Huygens' metasurface unit cells with three tunable resonances to realize a stable group delay over a large bandwidth, while also achieving high transparency and large phase tunability. With these cells, we construct a dual-polarized achromatic Huygens' metalens with an NA of 0.64 that works from 22 to 26 GHz. Our achromatic metalens achieves diffraction-limited focusing with 2 % maximum focal length deviation and 70 % average focusing efficiency over a bandwidth of 16.7 %. Most key performance metrics for this lens surpass or are comparable with the best of previous metalenses. An achromatic metalens simultaneously achieving broad bandwidth, large NA, and polarization-independent operation will open wide-ranging opportunities for microwave and mm-wave imaging and communication applications.
消色差透镜，无论照明频率如何，其焦距都相同，在成像、传感和通信系统中有广泛应用。用超表面制作消色差透镜非常可取，因为它们是平面的、超薄的、相对较轻且易于制造。然而，现有的超透镜存在多种局限性，包括单偏振操作、带宽窄和数值孔径（NA）小。在这项工作中，我们提出了一种基于惠更斯超表面的双偏振、宽带和高NA消色差超透镜。我们使用具有三个可调谐共振的惠更斯超表面单元来在大带宽上实现稳定的群延迟，同时还实现高透明度和大相位可调性。利用这些单元，我们构建了一个数值孔径为0.64、工作频率范围为22至26 GHz的双偏振消色差惠更斯超透镜。我们的消色差超透镜在16.7%的带宽上实现了衍射极限聚焦，最大焦距偏差为2%，平均聚焦效率为70%。该透镜的大多数关键性能指标超过或与以往最佳超透镜相当。一种同时实现宽带宽、大NA和偏振无关操作的消色差超透镜将为微波和毫米波成像及通信应用带来广泛机遇。
