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摘要: 随着新兴光学设备对微型化、一体化、智能化光学变焦系统的需求与日俱增，大大促进了纳米光电子学的迅猛发展。超构透镜是由具有特殊电磁属性的人造元素按照一定的排列方式组成的具有透镜功能的二维平面结构，其最大优点就是：轻薄和易于集成。然而，集成在超构透镜上的微纳结构一旦制备完成，便难以再改变其形貌或者尺寸，因而无法对其聚焦性能进行实时调控，限制了其功能及应用范围的进一步扩展。近年来，科学家们探索了实现超构透镜聚焦性能实时调控的多种途径，其中最引人注目的是将智能材料与超构透镜相结合。本文首先回顾了可调谐超构透镜的最新进展，分别详细阐述和分析了它们的调节原理和器件性能。最后，归纳分析了当前阻碍可调谐超构透镜发展的主要问题，并进一步对未来可调谐超构透镜的发展趋势做出了展望。Abstract: Emerging optical devices demand miniaturized, integrated and intelligent optical zoom systems, thus stimulating development in nano-optoelectronics. Metalenses are two-dimensional planar structures with lens function composed of arrays arranged specifically to equally focus wavelengths of light. Due to their ultrathin and lightweight properties and their ease of integration, it is expected that they will revolutionize optics by replacing the conventional bulky, curved lenses used that pervade optical devices. However, once the micro/nano-structures of a metalens are fabricated, their shape and size cannot be modified, which can not realize the real-time adjustment of focusing and will limit the further development of metalenses' functions and applications. Currently, substantial effort is being devoted to solving this problem. One of the most attractive aspects of metalenses is in the way they combine metasurface lenses with smart materials. In this article, we first provide an overview of novel tunable metalenses. Then, we elaborate and analyze their regulatory principles and device performance, respectively. Finally, we summarize the current problems and difficulties facing the development of tunable metalenses and describe the direction of their future development.
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Key words:
- nano-optoelectronics /
- tunable metalens /
- smart materials /
- focusing performance
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图 1 (a) 可调谐超透镜实现电磁波的动态聚焦和多点聚焦切换示意图; (b)极化电磁波相位和幅度随电容变化曲线[23]
Figure 1. (a)Schematic diagram of dynamic focusing and multi-point focus switching of electromagnetic waves by reconfigurable metalens; (b)capacitance-dependent phase and amplitude responses of polarized electromagnetic waves, respectively[23]
图 2 (a) 太赫兹可调谐超构透镜示意图; (b)太赫兹频率中石墨烯介电常数的实部随费米能级EF的变化曲线。矩形孔长度和旋转角度分别为L=160 μm和φ=0;(c)右旋圆偏振波通过具有不同费米能级EF的矩形孔单元时，透射波相位随矩形孔长度L变化曲线; (d)当费米能级EF从0.1 eV增加到0.3 eV时，经过所设计的超构透镜的透射右旋圆偏振太赫兹波的电场强度分布[24]
Figure 2. (a)Schematic of the THz reconfigurable metalens; (b) real part of the graphene permittivity in THz frequency as a function of EF. The aperture length and rotation angle are L=160 μm, φ=0; (c)phase of RCP scattering THz wave through one rectangular aperture versus different aperture lengths L for different EF; (d)electric field intensity distributions of the transmitted RCP THz wave at different EF[24]
图 4 基于介电弹性体的可调谐超构透镜的(a)实物图; (b)原理图；(c)对中心电极V5加电时，焦距的调谐能力；(d)两个不同的电压下扫描聚焦强度分布图[27]
Figure 4. (a)Physical photo and (b) principle of dielectric elastomer actuator based tunable metalens; (c)tuning ability of focal length when power is applied to the center electrode V5; (d) Z-scan of the focus intensity profile was scanned at two different voltages[27].
图 5 (a) 多栅极偏置技术示意图；(b)在工作频率fm=218.5 THz下，施加不同偏压时的反射相位变化曲线；(c)使焦点偏离y轴±3λ时所需的偏压分布；(d)离轴聚焦时归一化电场强度分布图[28]
Figure 5. (a)The schematic overview of multigate biasing technique; (b)the reflection phase varies with different applied biases at the operating frequency of fm = 218.5 THz; (c) the required biasing distributions when focal point deviating away from y-axis with amount of ±3λ; (d) the square normalized amplitude of the electric fields for off-axis focusing[28]
图 8 (a) 硅纳米天线单元结构示意图；(b)构成超构透镜的天线排布方式；(c)液晶-超表面混合结构侧视图；(d)常温及(e)35°下，超构透镜聚焦焦点区域的强度分布[34]
Figure 8. (a) Schematic of silicon nanoantennas unit cell; (b) antennas arrangement of flat cylindrical lens; (c) side view of liquid crystal-metasurface hybrid structure. The intensity distributions in the focal region for the metalens at room temperature (d) and 35° (e)[34]
图 9 (a) 反射型超构透镜聚焦特性示意图；(b)集成化可调谐超构透镜实物图；(c) 3种实验配置的示意图(上)，反射光束焦线处的光学轮廓(下)[37]
Figure 9. (a) Schematic of focusing characteristics of reflective metalens; (b) photograph of the integrated tunable metalens; (c) schematic of the three experimental configurations(top), experimentally measured optical profiles at focal lines for the three configurations (down)[37]
图 10 (a) 可调焦超构透镜结构示意图；(b)最终设备的显微镜图像；(c)实验测量得到的前焦距d和两个超构透镜之间距离f随施加的直流电压变化曲线[38]
Figure 10. (a) Schematic illustration of the proposed tunable metalens; (b) microscope image captured by device; (c) measured front focal length d and the separation values between the moving and stationary lenses vary with the applied DC voltage[38]
图 11 (a) 可调谐超构透镜系统示意图；(b)经过完整蚀刻工艺和切割后的超表面立方相位板；(c)可调谐超构透镜在红外和(d)可见光波段的焦距随超表面相位板的横向位移的变化曲线[39]
Figure 11. (a) Schematic representation of tunable metalens system; (b) a fully etched and cleaved metasurface cubic phase plate; focal length as a function of lateral displacement for the infrared (c) and visible (d) for tunable metalens[39]
图 12 (a) 可拉伸PDMS上的超构透镜示意图；(b)当PDMS拉伸率s分别为100%，115%和130%时，变焦超构镜头的透射侧产生的纵向光束轮廓[40]
Figure 12. (a) Schematic illustration of a metalens on stretched PDMS; (b) measured longitudinal beam profiles generated on the transmission side of the zoom metalens with stretch rate of PDMS(s) of 100%, 115%, and 130%[40]
图 13 (a) 可调弹性超构透镜的示意图；(b)测量和分析预测的对于不同应变值的焦距；(c)实验测量得到的轴向平面(左)和焦平面(右)中的径向应变超构透镜(ε= 0%至50%)的光强度分布[41]
Figure 13. (a) Schematic diagram of tunable elastic metasurface lenses; (b) measured and analytically predicted focal distances at different strain values; (c) measured optical intensity profiles of a radially strained metasurface microlens (ε=0% to 50%) in the axial plane (left) and the focal plane (right)[41]
图 14 (a) 利用3D打印技术制备的高抗冲聚苯乙烯(HIPS)超构透镜照片。(b)不同频率电磁波入射时，实验(球体)和理论(实线)的聚焦焦距随拉伸因子的变化曲线。虚线表示近轴近似中的理论距离。沿着器件轴向平面的模拟(左)和实验(右)光功率分布图，对应拉伸因子s=0.8(c)和(d)，1.0(e)和(f)，以及1.4(g)和(h)[42]
Figure 14. (a) Photograph of the 3D-printed HIPS lens. (b) Experimental (spheres) and theoretical (solid lines) focal distances as a function of the stretching factor. The dashed line represents the theoretical distance in the paraxial approximation. Simulated (left) and experimental (right) optical power profiles along the axial plane of the device with stretching factors s of 0.8 (c) and (d), 1.0 (e) and (f), and 1.4 (g) and (h)[42]
图 15 (a) 可调谐平面超构透镜结构示意图；(b), (c), (d)调控过程示意图；(e)-(g)当设计焦距分别为5λ，10λ和15λ时，不同相位梯度分布的实验结果[43]
Figure 15. (a) Schematic of tunable flat metalens. (b), (c), (d) Schematic diagram of the regulation process; (e)-(g) experimental results of different phase gradient distributions when the designed focal length is 5λ, 10λ, and 15λ, respectively[43]
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