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摘要:超构表面是由精心设计和排布的亚波长纳米单元组成的平面元件,其可以在微观尺度下调制电磁场,从而实现波前的任意调控。目前,它已被用来灵活地操纵相位、偏振、振幅等各种光学参数。超构透镜是超构表面中相当重要且非常活跃的一个研究方向,由于其厚度在波长量级,与传统光学透镜相比,能够显著增加光学器件的集成度并且降低结构复杂度。但是,单元结构材料的固有色散以及结构几何形状衍射效应导致的色差会严重影响超构透镜的成像质量,从而限制了其在光电子器件中的潜在应用。本文首先讨论超构透镜控制色差的原理。随后回顾了几种重要的成像应用,包括分立波长消色差,宽带聚焦成像,光场成像等重要的成像系统。最后,本文对超构透镜未来的发展方向和应用前景做出展望。Abstract:Metasurface consists of the arrangement of the specially designed subwavelength nano units, which is the two-dimensional counterpart of metamaterial. Metasurface can modulate the electromagnetic field on a microscopic scale to allow the arbitrary wavefront manipulation. At present, it has been used to flexibly control various optical parameters such as phase, polarization, and amplitude. Among all of the applications based on metasurfaces, metalens is no doubt one of the most important and basic research interset. Because its thickness is on the order of wavelength, compared with traditional optical lenses, it can significantly increase the integration of optical devices and reduce the systematic complexity. However, the chromatic aberration caused by the inherent dispersion of the material of the unit structure and the diffraction effect of the structural geometry will severely influence the imaging quality of the metalens, and hence isolating us from a rich variety of advanced applications. Herein, we firstly discuss the principle of controlling chromatic aberration with metalens. Then we review several important imaging applications, including discrete wavelength achromatic, broadband focus imaging, light field imaging and other important imaging systems. Finally, this article makes some prospects for the incoming development direction and potential applications of metalens.
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Key words:
- metasurface/
- metalens/
- achromatic/
- chromatic aberration control
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图 1分立波长消色差超构透镜。(a) 基于三种不同金属微纳颗粒构成的垂直堆叠超构透镜[69];(b) 由三个分割拼接的复合超构透镜实现三个分立波长的消色差[70];(c) 对偏振不敏感的超构透镜,由多扇区透镜(上图)和交错透镜(下图)中的纳米柱组成[71];(d) 具有双折射椭圆柱超构单元的双波长超构透镜[72];(e) 基于偏振操作的具有双波长超构透镜的双光子显微镜[73];(f) 基于超构表面的菲涅尔波带片[74]
Figure 1.Multiwavelength achromatic metalenses. (a) A vertically stacked metalens based on three different metal nano particles[69]; (b) the achromatic effect of three discrete wavelengths is realized by three segmented composite metalens[70]; (c) a polarization-insensitive metalens is composed of nanoposts in multisector lens (above) and interleaved lens (below)[71]; (d) a dual wavelength metalens with birefringent elliptical cylinder meta-atoms[72]; (e) a two-photon microscope with dual-wavelength metalens based on polarization operation[73]; (f) Fresnel zone plate based on metasurface[74]
图 2算法优化分立波长消色差超构透镜。(a)三波长复消色差衍射透镜[75];(b)基于亚波长等离子体微纳粒子的多波长消色差超构透镜[76];(c)利用全息术原理调控双波长的光场[77]
Figure 2.Algorithm optimization of multiwavelength achromatic metalens. (a) Three-wavelength apochromatic diffractive lens[75]; (b) multiwavelength achromatic metalens based on subwavelength plasma nano particles[76]; (c) the light field which uses the principle of holography to control dual-wavelength[77]
图 3窄带消色差超构透镜。(a)应用了由二氧化钛纳米柱,电介质薄层和金属背反射器制成的超构单元,以实现具有60 nm以上连续带宽的消色差超构透镜[78];(b)能够实现正,零和超负色散的超构表面设计[79]
Figure 3.Narrowband achromatic metalens. (a) Meta-atoms made of titanium dioxide nano-pillars, a dielectric layer and a metal back reflector which is applied to realize an achromatic metalens with continuous bandwidth above 60 nm[78]; (b) a metasurface design capable of realizing positive, zero and super-negative dispersion[79]
图 4宽带消色差超透镜。(a)反射式宽带消色差超构透镜工作原理示意图,其扫描电子显微镜图像以及实验结果[80];(b)由氮化镓微纳柱和反结构制成的超构透镜在400 nm至660 nm范围内的成像效果[81];(c)将所需相位泰勒展开的原理图以及聚焦效果[82]
Figure 4.Broadband achromatic metalenses. (a) Schematic diagram of the working principle of the reflective broadband achromatic metalens, its scanning electron microscope image and experimental results[80]; (b) the imaging effect of the metalens made of gallium nitride nano pillars and reverse structure in the range of 400 nm to 660 nm[81]; (c) schematic diagram of the desired phase Taylor expansion and focusing effect[82]
图 5偏振不敏感的超构透镜。(a)在1200~1650 nm波长范围内偏振不敏感的宽带消色差超构透镜[83];(b) 在可见光波段的偏振不敏感超构透镜[84];(c) 渔网式结构的超构透镜实现宽带高效消色差[85];(d)通过结合超构表面和传统折射光学元件的宽带消色差器件[86]
Figure 5.Polarization-insensitive metalenses. (a) Polarization-insensitive broadband achromatic metalens in the wavelength range of 1200~1650 nm[83]; (b) polarization-insensitive metalens in the visible light band[84]; (c) the fishnet-like structure of the metalens realizes broadband high-efficiency achromatic aberration[85]; (d) broadband achromatic devices by combining metasurfaces and traditional refractive optical elements[86]
图 7(a) 任意调控3个波长的复合式超构透镜[90];(b)构建超构单元色散数据库实现超构透镜任意色差调控[91];(c) 可见光波段多通道寻址[92]
Figure 7.(a) Compound metalens capable of adjusting three wavelengths arbitrarily[90]; (b) constructing a meta-atom dispersion database to realize arbitrary chromatic aberration control of metalens[91]; (c) color router in the visible light band[92]
图 8(a) 任意调控3个波长的复合式超构透镜[93];(b) 宽带等离激元调制实现多色三维全息显示[94];(c) 毫米级高数值孔径超构透镜实现虚拟现实[95]
Figure 8.(a) Compound metalens with arbitrarily adjustable three wavelengths[93]; (b) broadband plasmon modulation realizes multi-color three-dimensional holographic display[94]; (c) millimeter-scale diameter and high numerical aperture metalens for virtual reality[95]
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