Spin-decoupled metalens with intensity-tunable multiple focal points
Fig. 1 Spin-decoupled metalens with intensity-tunable multiple focal points.
Miniaturization and integration are inevitable trends in development of modern communication systems. In addition, with increasing of the communication capacity, the independent manipulation of multiple spin beams with controllable energy allocation enables practical applications in multiple-target detection radar system, multiple-input multiple-output (MIMO) communications, and so on.
The exotic properties of metasurfaces have provided new opportunities to develop various types of ultra-thin and ultra-compact metalenses with unusual functionalities, which are ideal for device miniaturization and system integration (for manipulating spin electromagnetic (EM) waves). However, a geometric-phase-based optical device that can integrate the function of a spin-decoupled lens with intensity-tunable functionality between helicity-dependent focal points hasn't been reported.
In fact, geometric-phase-based (or Pancharatnam-Berry phase-based) metasurfaces (geometric metasurfaces) enable an unprecedented capability to control the phase, polarization, and amplitude of circularly-polarized EM waves by arranging the orientation angle (θ) of each anisotropic antenna. Geometric metasurfaces with predesigned phase profiles are robust against fabrication tolerance (antennas size and roughness) and materials property variations.
However, the inherent conjugated symmetry, i.e. equal and opposite phase distributions under the illumination of left-hand polarized (LCP) and right-hand polarized (RCP) EM waves, inevitably hinders geometric metasurfaces to implement spin-decoupled/switchable functionalities.
To overcome this limitation, the traditional approach is to combine the geometric phase and propagation phase (dynamic phase) together to decouple spin-locking between two helicity components. However, the traditional method that can independently manipulate two orthogonal helicity components, inevitably needs to do a large number of parameters scanning to achieve the desired unit cells, which means that it is very complicated to design a spin-decoupled metalens based on the previous method.
Recently, Prof. Xiaofei Zang and Yiming Zhu from Terahertz Technology Innovation Research Institute, Terahertz Spectrum and Imaging Technology Cooperative Innovation Center, University of shanghai for Science and Technology have proposed a flexible approach that can address the previous issues.
Unlike the previous spin-decoupling approach depending on the joint modulation of geometric phase and propagation phase, an approach based on the pure geometric phase is proposed to design spin-decoupled metalens that can independently modulate each spin state and intensity ratio of the spin EM waves, leading to a spin-decoupled metalens with intensity-tunable multiple focal points (see Fig. 1). The intensity ratio between LCP and RCP multiple focal points can be arbitrarily and continuously controlled with different weights of LCP and RCP incident THz waves.
This method has obvious advantage in the simplicity of designing the spin-decoupled metasurfaces (without a large number of parameters scanning) that can independently manipulate two orthogonal helicity components.
In addition, our unique design will enable metalenses with unusual functionalities that are very difficult or impossible to achieve with conventional lenses. The simplicity and robustness of our design not only provide a platform for simultaneous generating helicity-dependent focal points and controlling the intensity-ratio between them, but also open an avenue for designing multifunctional devices and integrated systems. Related research results are published in Photonics Research, Vol. 9, Issue 6, 2021. (Bingshuang Yao, Xiaofei Zang, Yang Zhu, Dahai Yu, Jingya Xie, Lin Chen, Sen Han, Yiming Zhu, Songlin Zhuang. Spin-decoupled metalens with intensity-tunable multiple focal points[J]. Photonics Research, 2021, 9(6): 06001019)
In summary, the approach demonstrated in this work provides a flexible platform that can independent manipulate the two orthogonal spin states of spin EM waves using the spin-decoupled metalens.
The next thing the team will do is to realize multiplane imaging (e.g. generating multiple images with controllable contrast ratio) based on the proposed spin-decoupled metalens with intensity-tunable multiple focal points. Since the intensity of each focal point can be controlled by changing the ellipticity of incident EM waves, the contrast ratio of multiplane imaging can also be realized by the designed metalens.
In addition, by joining the modulation of helical phase, the spin-decoupled metasurface can be applied to independently control/design the helicity-dependent vortex beams, and even to obtain some special beams by the superposition of helicity-dependent vortex beams.
多焦点强度可调的自旋解耦超构表面透镜
图1. 多焦点能量可调的自旋解耦超构表面透镜示意图
小型化和集成化是现代通信系统发展的必然趋势。随着通信容量需求的增加，对多束自旋相关波束进行独立的操控，在多目标跟踪测雷达、多输入多输出（MIMO）通信等系统中具有实际应用。
超构表面所拥有的独特的波前操控能力为开发各类超薄和超紧凑 (且用于操控自旋电磁波)的超构表面透镜提供了新的机会。然而，基于纯几何位相实现多个自旋相关的聚焦光斑强度连续可调谐功能的自旋解耦的超构表面透镜尚未见诸报道。
众所周知，几何超构表面由亚波长准二维微结构按照特定的排列方式构成，可以灵活地调控电磁波的波前(包括振幅、位相和偏振等)。几何超构表面对制备带来的误差具有较好的宽容性。然而，几何超构表面的内禀对称性--即在左旋圆偏振（LCP）和右旋圆偏振（RCP）的电磁波入射情况下展现出相反的相位分布，阻碍了其实现自旋解耦/独立操控的功能。
为克服以上问题，传统的方法通过几何位相和传输位相（动力学位相）的融合实现两个正交自旋分量之间的解耦。但是，这类方法需要进行大量的参数扫描来获得所需的各种单元结构，这意味着此类自旋解耦功能的超构表面器件的设计工作纷繁复杂。
最近，上海理工大学太赫兹创新研究院的臧小飞教授、朱亦鸣教授团队提出了一种非常简捷的设计方法用于解决上述问题。相关结果发表于Photonics Research 2021年第6期上。 (Bingshuang Yao, Xiaofei Zang, Yang Zhu, Dahai Yu, Jingya Xie, Lin Chen, Sen Han, Yiming Zhu, Songlin Zhuang. Spin-decoupled metalens with intensity-tunable multiple focal points[J]. Photonics Research, 2021, 9(6): 06001019)
与以往融合几何位相和传输位相实现自旋解耦的方法不同的是，他们提出了一种纯几何位相设计自旋解耦超构表面透镜的新方法（图1）。利用该方法可以较为便利地设计出自旋解耦的超构表面透镜，最终实现多个自旋依赖聚焦光斑强度的连续可调谐功能。
在设计上，无需引入大量的参数扫描来获得所需的各种结构不一的单元，只需一种形状一致、旋转方向各异的各项异性“基元”阵列实现自旋解耦功能，这大大简化了设计的复杂性。
该设计方法相对较为简单，同时对样品的制备误差也有较大的宽容性。这不仅提供了一种平台用于同时生成自旋依赖的焦点和焦点能量调控，而且还为设计多功能器件和系统集成开辟了一条新的途径。
综上所述，该研究团队提出了一种非常灵活且简单的方法设计自旋解耦的超构表面，可以利用此自旋解耦的超构表面透镜独立地操纵一对正交自旋电磁波及其聚焦光斑的能量比。
由于每个焦点的强度可以通过改变入射电磁波的椭偏度来控制，利用此平面透镜成像将会呈现多个与入射光的偏振态完全关联的像(多平面成像)，故多平面成像之间的强度比也可以通过该超构表明透镜实现。
因此，该研究团队接下来将基于已提出的自旋解耦超构表面透镜实现强度可控的多平面成像（即：利用可调的强度比产生强度可控的多幅图像）。此外，通过附加螺旋相位调制，自旋解耦超构表面不仅可以被用于独立控制/设计涡旋光束，还可以通过自旋相关涡旋光束的叠加获得一些特殊的光束，如：同时实现聚焦的径向偏振和角向偏振波束的复用。