基于超构表面的多维光场感知 
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光场包含了振幅、相位、光谱、偏振等多个维度的信息。传统光电探测器一般仅能感知二维光强信息,若想实现对光场其他维度信息的获取,需利用多个光学元件组合,故存在系统复杂、难以集成等问题。超构表面是一种亚波长尺度的新型平面光学元件。通过灵活的平面结构设计,超构表面可以对光场的振幅、相位、光谱、偏振等多维信息进行调控,从而为轻量化、集成化多维光场感知系统的实现提供可能。详细介绍了国内外基于超构表面实现光谱、偏振、深度等多维光场信息感知的最新研究成果,并对目前超构表面在多维光场感知领域面临的挑战以及未来的发展趋势进行了展望。
Significance Light is an electromagnetic wave that carries information of multiple dimensions, such as intensity, phase, frequency, and polarization. The change in the intensity, spectrum, polarization, and other information as a result of the interaction between objects and the light field can reflect the material, morphology, and other characteristics of the objects. However, traditional photodetectors can only detect two-dimensional (2D) light intensity information. To perceive more dimensional optical field information, additional optical components and mechanical devices are required, which will result in issues such as the system’s large volume and complex structure. Many applications today, such as three-dimensional (3D) face recognition, automatic driving, and remote sensing, have an urgent need for miniaturization and lightweight optical systems, presenting a significant opportunity for the development of integrated optical field sensing systems.
A spectrum is a valuable tool for object characterization and analysis, and it is widely used in food safety, environmental monitoring, biological imaging, archaeological exploration, and other fields. Traditional dispersive and interference spectrometers can provide ultra-fine spectral resolution as well as an ultra-wide spectral detection range. Traditional spectrometers, however, have limitations in situations where real-time spectral detection is required due to the presence of optical and mechanical moving parts with large volumes and weight. People anticipate that in the future, spectral sensing devices will be reduced to centimeters or even millimeters in size and will be integrated into smartphones, drones, and other microsystems.
The measurement of light’s polarization state is important in fields, such as remote sensing, medical treatment, and optical communication. Traditional polarization measurement methods are divided into two types: division-of-time and division-of-amplitude. Division-of-time polarimeters measure the intensity of various polarization components by positioning a set of rotating waveplates and polarizers in front of the detector. This method often relies on mechanical rotating structures, resulting in slow measurement speed and reliability. Polarizing beam splitters are used in division-of-amplitude polarimeters to separate different polarization components into different detectors. Both methods have problems, such as large volume and complex measurement system structure.
Many emerging technologies, such as autonomous vehicles, face recognition, robotics, and augmented reality, rely on 3D imaging techniques. There are two types of 3D imaging techniques: active and passive. Active methods typically necessitate structured illumination or scanning, which adds complexity, cost, and power consumption. Passive methods, which typically use multiple views, have limited accuracy and a high computation cost. 3D imaging techniques based on conventional optical elements are limited by high cost, large size, high power consumption, and complex systems for applications requiring compactness, integration, and portability.
Metasurfaces are novel planar optical elements that can control the light field by deploying subwavelength artificial antennas on the surface. Subwavelength structures of metasurfaces, unlike traditional optical elements, can interact with the incident electromagnetic field, causing abrupt changes in optical parameters on the surface and breaking traditional optical elements' dependence on the propagation optical path. Because of this property, metasurfaces can modulate the amplitude, phase, polarization, and other properties of the light field within the subwavelength thickness in a very flexible and powerful way. As a result, metasurfaces open up new avenues for the miniaturization and integration of spectrometers, polarimeters, and depth information perception (
Progress The ability of metasurfaces to flexibly regulate the spectrum opens up a new avenue for the realization of integrated spectrum sensing systems. Metasurface-based spectrometers are classified into two types based on their operating principles: narrowband filtering and computational spectrometers. Narrowband filtering spectrometers use a single tunable narrowband filter or narrowband filter array to achieve spectral sampling (Figs. 2--4). Computational spectrometers do not require narrowband filters. The spectral response of the filters can be wide and random, which makes designing narrowband filter metasurfaces much easier. Computational spectrometers can extract the original spectrum from obtained signals using algorithms (
In recent years, researchers have proposed several types of metasurface-based polarimeters, including division-of-amplitude (Figs. 6--8), division-of-time (
Metasurfaces’ flexible wavefront manipulation enables them to realize 3D imaging systems with a miniaturized form factor and improved performance, for both active and passive methods. Typical active 3D imaging techniques include the structured light method and the beam steering method. Structured illumination achieved by metasurfaces has a simplified optical system and a much larger field of view (
Conclusion and Prospects This paper introduces the sensing of multidimensional light fields, such as spectrum, polarization, and depth information using metasurfaces. Future research into the flexibility of metasurfaces, such as combining metasurface design and reconstruction algorithms with inverse design, end-to-end optimization, deep learning, and other computer technologies, is expected to result in a simultaneous perception of more dimensional light field information. With the in-depth understanding of metamaterial surface, the exploration of new metamaterial surface design, and the improvement of large-scale micro-nano processing technology, metasurface will have a bright application prospect in the field of lightweight integrated multidimensional light field perception.
1 引言
光是一种电磁波,携带着包括强度、相位、频率、偏振等多个维度的信息。光所携带的丰富信息可以描述为9维全光函数P(x,y,z,θ,φ,λ,t,ψ,χ)[1],其中x、y、z、θ、φ描述了光传播的空间位置和方向,λ表示光波长,t表示时间,ψ和χ表示光的偏振角和椭圆率。物质与光场相互作用后,光场的强度、光谱、偏振等信息的变化可以反映出物质的材料构成、形貌等特性。例如:物体的辐射、反射或吸收光谱可以反映物质的化学组分、分子结构等信息;偏振信息可以用于分析材料的双折射、物体的表面形貌等;光场携带了重要的空间信息(x、y、z、θ、φ),可以反映光场中物体的三维形貌。然而,传统的光电探测器一般只能探测二维光强信息。若想实现对更多维度光场信息的感知,需要借助额外的光学元件和机械装置[2-7],这将导致系统体积较大、结构复杂等问题。如今,例如三维人脸识别、自动驾驶、遥感等众多应用都对光学系统的小型化、轻量化有着迫切的需求,这为发展下一代集成光场感知系统带来了极好的机遇。
光谱是表征和分析物体的重要手段,广泛用于食品安全[8]、环境监测[9-10]、生物成像[11-12]、考古探索[13]等领域。传统色散式和干涉式光谱仪可以提供超精细的光谱分辨率和超宽的光谱探测范围,但是由于它们通常由体积、质量庞大的光学、机械运动部件组成[6-7],其在众多需要实时获取探测目标光谱的场合,例如土壤和作物实时分析、食品工业生产线监控以及海洋/水下科学研究[14-17]等,应用受限。未来人们期待光谱感知设备的尺寸缩小至厘米乃至毫米量级,并希望集成到智能手机[18]、无人机等微小系统中。
对光的偏振态测量在遥感[19]、医疗[20]、光通信[21]等领域也有着重要的应用前景。传统的偏振测量方法主要包括分时型和分振幅型两种。分时型偏振测量是通过在探测器前放置一组旋转的波片和偏振片来测得不同偏振分量的强度[2],这种方法往往依赖于机械旋转结构,并且测量速度较慢,难以满足测量系统可靠性和实时性要求。分振幅型偏振测量是利用偏振分光元件将不同偏振分量的光投射到不同探测器上[3],实现同时测量。这两种方法都存在测量系统体积庞大、结构复杂等问题。
常规相机是从三维场景中捕获光强信息的二维投影,难以感知深度信息。三维成像在许多新兴技术中至关重要,例如自动驾驶、机器视觉、智能制造、人脸识别与检测、增强现实/虚拟现实(AR/VR)等。三维成像技术可粗略地划分为主动式和被动式。主动式三维成像需要主动照明或投射装置,例如激光扫描法[22]、色散共焦法[23]、偏折法[24]等。这些方法精度较高,但具有系统复杂、成本高、耗时长等缺点。目前消费电子领域应用最为广泛的主动式三维成像方法分别是结构光投影法[25]和飞行时间法[26]。这两种技术也都需要较复杂的光学系统,并且对光源和探测器的性能要求较高。被动式三维成像不需要主动投射装置,从而降低了系统复杂度、成本和功耗,其主要包括单目视觉、双目视觉和多目视觉等方式。多目视觉和双目视觉需要多个视点,从而不可避免地增加了系统尺寸和成本;更为复杂的光场相机利用镜头阵列成像,分辨率高,景深大,但商业化的光场相机价格昂贵。单目深度感知理论上是实现紧凑、简单三维成像系统的最佳途径。然而相关具体方法的当前成像效果仍然有限。例如:灰度纹理法[27]为推导法,无法得到真实距离,保真度较低;离焦成像计算法[28]利用在不同离焦程度下的图像序列获得深度信息,该方法通常需要在每次成像中进行物理调整,且成像系统点扩展函数(PSF)的相似性限制了方法的精度;三维点扩展函数的计算法[29]使光学系统的 PSF 随着物体深度的变化产生特定的响应,该方法实现复杂的(例如螺旋形)PSF通常需要用到较为复杂的光学系统。
超构表面(Metasurface)是一种通过在表面上布设亚波长尺寸的人造天线来实现光场调控的新型平面光学元件[30-36]。与传统光学元件不同,超构表面的亚波长结构能与入射的电磁场相互作用,从而在表面引入光学参量的突变,打破了传统光学元件对传播光程的依赖性。因为这一性质,超构表面能够在亚波长的厚度范围内对光场的振幅、相位、偏振等特性施加灵活、大幅度的调制,所以超构表面为光场光谱、偏振、深度信息感知的微型化、集成化提供了新的机会,如
图 1. 超构表面实现集成化的光谱、偏振、深度感知
Fig. 1. Metasurfaces for integrated spectrum, polarization and depth sensing
2 光谱感知
超构表面在亚波长尺度对光谱的灵活调控为微型光谱感知系统的实现提供了新的方式。根据光谱数据采样原理的不同,从以下两个部分进行阐述:第一部分介绍基于窄带滤波原理的集成超构表面微型光谱仪;第二部分介绍近年来兴起的,结合了超构表面滤波和计算光谱重建的计算型超构表面微型光谱仪。
2.1 窄带滤波型超构表面微型光谱仪
现有的窄带滤波型光谱仪有两种形式:一种是可调谐型光谱仪,用滤波波长随时间变化的单个窄带滤波片来实现光谱采样;另一种是阵列型光谱仪,用多个窄带滤波片形成滤波片阵列来实现光谱采样。阵列型滤波片的光谱采样方式通常是等间隔的逐点采样,由采样数和滤波片的带宽决定光谱仪的光谱分辨率。
可调谐型光谱仪通常只有单个滤波片,通过热调谐、电调谐或者光调谐等方式改变滤波片的光谱响应,以实现光谱感知。该类光谱仪通过集成可调谐滤光片和相应波段的面阵相机,可以实现光谱的成像功能。
在具有窄带光谱响应的超构表面中加入可调谐的材料,如液晶[37-39]、相变材料[40-42]等,可以实现波长可调谐的滤波片。滤波片的波长调谐范围、调谐速度一般受可调谐材料性质的制约。利用液晶在电调谐下等效折射率的改变,Chang等[43]通过在亚波长谐振光栅中加入液晶实现了电调谐滤波器。
图 2. 可调谐型微型光谱仪。(a)可调谐液晶-亚波长谐振光栅滤波器的示意图[43];(b)在不同电压下测得的滤波器的光学透射率[43];(c)通过激光脉冲照射GST超构表面,致其发生相变示意图[42];(d)GST超构表面随泵浦光光率变化的透射光谱响应[42]
Fig. 2. Tunable micro spectrometer. (a) Schematic of a tunable liquid crystal-subwavelength resonant grating filter[43]; (b) measured optical transmittance of the tunable filter at different applied voltages to liquid crystal cell[43]; (c) schematic of phase change in GST induced by laser irradiation on the GST-based metasurface[42]; (d) transmittance of a GST metasurface as a function of the pump laser fluence[42]
另一种用窄带滤波片实现光谱探测的方式是阵列型光谱仪,它将一组窄带滤波片阵列与二维光强探测器集成,如
传统的窄带滤波片阵列通常是由基于多层介电材料的渐变法布里-珀罗腔[46-47]制成,通过改变介电层数量、厚度和折射率来改变其光谱响应。但是多层结构需要多次刻蚀,而且在长波红外等波长较长区域,存在介电层较厚、难以加工的问题。超构表面窄带滤波片阵列是由高度一致的光学天线制成,通过调整天线尺寸(不改变高度)就可以改变光谱响应。其平面化的结构给加工带来了便利。根据使用材料的不同,超构表面一般可分为金属超构表面和介质超构表面。
金属超构表面通过等离激元效应可以实现窄带滤波[48-51]。等离激元效应一般不依赖于导波模的传播,可以用较少的金属天线实现滤波功能[52]。例如,Xu等[49]采用一维金属-绝缘体-金属线栅结构,实现了如
图 3. 阵列型超构表面微型光谱仪。(a)阵列型超构表面微型光谱仪系统示意图;(b)等离激元滤波片阵列的示意图,右下是扫描电子显微镜(SEM)图像和白光照射的光学显微镜图像[49] ;(c)红绿蓝(RGB)滤光片的实验透射光谱[49];(d)具有梯度光栅周期的介质光栅示意图[53];(e)制作的100个宏像素的超构表面阵列的光学图像[54];(f)21个选定宏像素的归一化反射光谱[54]
Fig. 3. Micro spectrometer of metasurfaces array. (a) Schematic diagram of micro spectrometer system of metasurfaces array; (b) schematic diagram of a plasmon filter array, in which the bottom right figure is an SEM image and an optical microscope image illuminated by white light[49]; (c) experimental transmission spectra for the red, green and blue (RGB) filters[49]; (d) schematic of a dielectric grating with gradient grating period[53]; (e) optical images of the fabricated 100-metapixel metasurface[54]; (f) normalized reflectance spectra for selected 21 metapixels[54]
基于低损耗介质超构表面的滤波片是另一种窄带滤波的方案。利用导波模谐振,介质光栅或超构表面可以实现Q值很高的窄带滤波片[55-56]。Lin等[53]采用周期线性变化的TiO2光栅实现了探测范围为506~915 nm的滤波片,结构如
前述滤波片阵列需要额外的光电探测器来得到光谱信号。通过将超构表面直接与探测器集成,可以实现具有窄带滤波响应的探测器,以直接采集光谱信号。在可见光波段,Liu等[58]通过在石墨烯上排布不同尺寸的金纳米颗粒实现了窄带滤波探测器,如
图 4. 窄带滤波响应探测器。(a)等离激元纳米颗粒石墨烯探测器的示意图[58];(b)直径100 nm的纳米盘阵列的SEM图像[58];(c)具有不同尺寸纳米盘阵列的石墨烯探测器的光谱响应[58];(d)在具有2 nm Ti黏附层的n型硅底上的金光栅的示意图[59];(e)金光栅结构的SEM图像[59];(f)响应度峰值随着光栅缝隙距离D的增加而红移(T=200 nm, W=250 nm) [59];(g)等离激元-热释电装置示意图[60];(h)调节金孔尺寸得到的吸收光谱[60];(i)在多晶AlN热释电层上沉积等离激元超表面的垂直探测器结构示意图[61];(j)Au参考探测器和三个超构表面热电感知器的反射光谱和图像[61]
Fig. 4. Narrowband filter response detector. (a) Schematic illustration of the graphene detectors with plasmonic nanoparticles[58]; (b) SEM image of a representative device with the 100-nm diameter nanodisk array[58]; (c) corresponding spectral response of graphene photodetectors with nanodisk arrays of different sizes[58]; (d) schematic of a gold grating on an n-type silicon substrate with a 2-nm Ti adhesion layer[59]; (e) SEM image of gold grating structure[59]; (f) responsivity peaks redshift with increasing grating interslit distance D(T=200 nm, W=250 nm) [59]; (g) schematic diagram of the plasmonic-pyroelectric device[60]; (h) absorption spectrum obtained by adjusting the size of gold pores[60]; (i) schematic of the vertical detector structure with a plasmonic metasurfaces deposited on a polycrystalline AlN pyroelectric layer[61]; (j) reflection spectrum and image of three metasurfaces-pyroelectric detectors and Au reference detector[61]
提高窄带滤波型超构表面微型光谱仪的光谱分辨率需要更高Q值的滤波器和更多光谱信号采样点。滤波器Q值越高,光通量就越小,这会降低光谱仪的信噪比,不利于微弱信号的探测。对于可调谐型微型光谱仪来说,更多的信号采样点意味着需要调谐的波长数量更多,如何快速进行波长调谐是设计中的关键;而对于阵列型光谱成像仪来说,提高光谱分辨率意味着需要更多的滤波片,从而导致单个滤波片阵列尺寸变大,光谱成像的空间分辨率降低。基于阵列型滤光片的光谱成像仪需要在光谱分辨率和空间分辨率之间做出权衡。
2.2 计算型超构表面微型光谱仪
在过去的十年中,利用压缩感知[62-65]和深度学习[66]等数学工具,出现了一种新的滤波型光谱仪设计路线——计算型光谱仪。
计算型光谱仪和窄带滤波型光谱仪之间最显著的区别在于滤光片光谱响应不同。计算型光谱仪的滤波片光谱响应可以是宽谱且随机的,这降低了超构表面滤波片的设计难度。但是计算型光谱仪无法通过采集的信号直接得到原始光谱,需要利用算法辅助恢复原始光谱。根据压缩感知原理,恢复相同精度的光谱时计算型光谱仪需要的滤光片数量小于窄带滤波型光谱仪所需要的滤光片数量[67]。
计算型光谱仪的系统如
式中λmax和λmin分别是测量波段的上下限,n是滤波器总数,S(λ)是相对于波长λ的连续函数,ti(λ)作为已知量通常可在实验中直接测量,η(λ)是探测器的响应度。
图 5. 计算型微型光谱仪。(a)计算型光谱仪重构过程示意图;(b)等离激元超构表面滤光片阵列示意图[68];(c)聚乙烯的重构透射光谱[68];(d)光子晶体滤波片阵列的示意图,右上插图为制成的6×6光子晶体结构的光学图像[69];(e)两个发光二极管(绿色和红色)组合的发射光谱[69];(f)3个选定的超构表面单元的SEM图像和光谱[70];(g)使用普通彩色相机和超光谱相机(黑白)拍摄的水果照片,选择5个单波长光谱图像进行重建[70]
Fig. 5. Computational micro spectrometer. (a) Schematic diagram of the reconstruction process of the computational spectrometer; (b) schematic of the plasmonic metasurfaces filter array[68]; (c) reconstructed transmission spectra of polyethylene[68]; (d) schematic of the photonic crystals filer array, in which the upper right figure is the optical image of the fabricated 6 × 6 photonic crystal structures[69]; (e) emission spectrum of a combination of two light-emitting diodes (green and red) [69]; (f)SEM images and spectra of three selected metasurfaces units[70]; (g) photos of fruits captured using an ordinary color camera and our ultraspectral camera (black-and-white), and five single wavelength spectral images are selected to reconstruct[70]
由于实验测量获得的透射率ti(λ)是离散形式,因此可将上述积分近似转换为线性方程组,即
其中I=
ti(λm)],n是滤波器总数,m是实验中测量ti(λ)时的波长采样数,η=diag(η(λ1),η(λ2),…,η(λm))为n×n的对角矩阵,S是稀疏信号S=[S1,S2,…,Sn]T ,Si 为稀疏信号S列向量中的分量。通常将η和t视为一个整体,称为测量矩阵T。(2)式可精简为
在窄带滤波光谱仪中,T是n=m的单位矩阵或对角矩阵。在该情况下,滤波器的数量是限制光谱分辨率的关键因素。与其不同的是,在计算型光谱仪中,n和m通常不等,需要采用优化算法来求解(3)式。当n>m时, 通常利用最小二乘法求解这种超定系统[71-72]。当n<m时,根据压缩感知原理,利用宽带随机的滤波片阵列对稀疏信号S的优化效果很好。稀疏优化非常适用于求解n<m的欠定系统线性方程组。为了求解(3)式,优化l2范数最小化问题,即
实际操作中,考虑到各种噪声、信号的稀疏性等问题,人们会对上述问题进行一定的修正,以得到更好的光谱恢复效果。
基于上述原理,Craig等[68]开发了如
3 偏振感知
超构表面由于其具有超薄的结构和灵活的设计方法,为实现轻量、集成的偏振测量系统提供了新的可能[73-74]。本节回顾了近年来基于超构表面的偏振测量方法,按照偏振测量原理的不同,主要分为分振幅型、分时型、探测器集成型和其他方法。
3.1 分振幅型偏振测量
基于超构表面的分振幅型偏振测量原理与传统方法类似,先将不同偏振分量的光偏折或散射到不同方向上,再利用多个探测器分别测量对应偏振分量的光强。目前,分振幅型的偏振测量超构表面主要可以分为光栅型[75-83]、波导型[84-88]和超透镜型[89-94]。
3.1.1 光栅型
光栅型偏振测量超构表面通过合理设计对入射光偏振敏感的超构表面单元的排布方式,使超构表面对不同偏振分量的光具有不同的相位梯度,从而使特定偏振分量的光以不同的角度出射。一种实现偏振敏感的相位梯度设计方法是几何相位[也称作Pancharatnam-Berry(PB)相位]法。几何相位法是利用具有相同形状的超构表面单元,通过在空间上改变超构表面单元的旋向,并对正交的圆偏振光施加相反的相位控制。普渡大学的Shaltout等[75]在2015年提出了基于几何相位法的间隙等离激元超构表面(GSPM),实现了偏振测量,如
图 6. 光栅型偏振测量超构表面。(a)基于几何相位的GSPM示意图[75];(b)不同波长的LCP和RCP光束入射GSPM后的反射功率随反射角变化的实验结果[75];(c)全Stokes参量测量GSPM光栅的工作原理示意图[77];(d)实验测得的偏振态(实心圆标示)与在庞加莱球主轴上的偏振态(星号标示)对比[77];(e)二维光栅型偏振成像超构表面的一个结构单元[81];(f)光栅型偏振成像超构表面获得的偏振图像[81]
Fig. 6. Grating metasurface polarimeters. (a) Illustration of the geometric phase GSPM[75]; (b) reflected power for LCP and RCP incident beams at different wavelengths as a function of reflected angle after passing through GSPM in the experiment[75]; (c) illustration of the full-Stokes GSPM grating’s measurement principle[77]; (d) measured diffraction contrasts (denoted by filled circles) for polarization states along the main axes of the Poincaré sphere (indicated by asterisks)[77]; (e) a unit cell of the 2D grating metasurface for polarization imaging[81]; (f) polarization images required by the grating metasurface for polarization imaging[81]
3.1.2 波导型
波导型偏振测量超构表面通常利用超构表面结构单元将入射光的不同偏振分量耦合到面内的不同方向,从而实现偏振态的测量。Pors等[84]发现入射光的偏振可以控制“超散射体”激发的表面等离激元(SPP)的方向,从而提出利用单个超散射体或散射体阵列,根据不同方向的SPP的强度信息,推测入射光偏振态的想法。基于这种想法,他们设计了三种GSPM结构[85],覆盖GSPM结构的入射光可以激发起SPP,并且入射光的(
图 7. 波导型偏振测量超构表面。(a)波导GSPM结构单元示意图(左)和三种波导GSPM组合的俯视图[85];(b)不同偏振态入射光激发的SPP强度分布[85];(c)散射天线阵列示意图(左),天线阵列偏振测试光路(中),耦合光栅的相机图像显示四个偏振分量被耦合光栅散射(右)[86];(d)基于硅波导和耦合天线的偏振测量示意图以及测量偏振分量在庞加莱球上的表示(左),偏振测量结果(右)[87]
Fig. 7. Waveguide metasurface polarimeters. (a) Sketch of the unit cell of a waveguide GSPM (left) and top view of three combined waveguide GSPMs (right)[85]; (b) color map of the SPP intensity distribution excited by the incident light with different states of polarization[85]; (c) illustration of scattering antenna arrays (left), setup for characterizing the antenna array polarimeter (middle), and camera image of the outcoupling gratings, showing polarization-dependent intensities scattered by the four outcoupling gratings[86]; (d) illustration of polarimeter based on silicon waveguide and the coupling antenna, the measured polariztion states shown in the Poincaré sphere (left), the retrieved polarization states (right)[87]
3.1.3 超透镜型
超透镜型偏振测量超构表面往往利用偏振敏感的超透镜结构,将不同的偏振分量聚焦到不同的焦点处。一个典型的结构如
图 8. 超透镜型偏振测量超构表面[90,93-94]。(a)DoFP超透镜单元的SEM图(上),单元每一部分的偏振基矢用彩色箭头表示(比例尺:1 μm);DoFP超透镜的工作原理示意图(下);(b)目标偏振掩模版(比例尺:100 μm);(c)利用传统方法获得的掩模版的偏振图像(比例尺:100 μm);(d)利用DoFP超透镜获得的相同掩模版的偏振图像(比例尺:100 μm);(e)广义Hartmann-Shack超透镜阵列的系统示意图;(f)径向偏振光束的焦斑强度分布,箭头定性表示局部的偏振态(比例尺:50 μm);(g)超透镜阵列获取的径向偏振光束的焦斑图像;(h)利用超透镜阵列获取的偏振态分布图;(i)PMT超构表面实现矢量光束的自旋、轨道角动量的同时测量
Fig. 8. Metalens metasurface polarimeters[90,93-94]. (a) SEM image of a fabricated DoFP metalens unit cell (upper), in which the polarization basis for each part is shown with the colored arrows and scale bar is 1 μm, and illustration of the DoFP metalens’ working principle (lower); (b) target polarization mask (scale bars:100 μm); (c) fabricated mask imaged using conventional polarimetry(scale bars:100 μm); (d) the same mask imaged using the DoFP metalens(scale bars:100 μm); (e) scheme of the generailzed Hartmann-Shack metalens array; (f) intensity distribution of the focal spot for a radially polarized incident beam, in which the arrows qualitatively indicate the local polarization states and scale bar is 50 μm; (g) image of the focal spot from the metalens array for the radially polarized beam; (h) polarization profile obtained from the metalens array; (i) metasurface-based PMT for simultaneous detection of spin and orbital angular momenta
总而言之,基于超构表面的分振幅型偏振测量方法可以同时获得多个偏振分量的强度信息,偏振测量速度较快。光栅型超构表面可以通过超构表面单元的合理设计,使不同波长的入射光具有不同的衍射角度,实现偏振、光谱的同时测量。超透镜型超构表面可以在偏振分光的同时实现聚焦、甚至成像功能,有望实现集成化的偏振成像。然而,分振幅型偏振测量也存在一定的局限性。首先,分振幅型偏振测量是基于偏振分光各通道的强度信息,偏振分光的效率以及各通道之间的串扰很大程度上会影响偏振测量的准确性。其次,分振幅型偏振测量往往需要多个光电探测器或二维相机才能实现偏振测量,器件成本相对较高。
3.2 分时型偏振测量
分时型偏振测量超构表面是将传统的偏振测量系统中的波片、偏振片等元件集成于一体,并且能够起到类似于旋转波片、偏振片等调制作用的超构表面。一个典型的例子是Jung等[95]提出的石墨烯集成的各向异性超构表面(GIAM),在中红外波段实现了全Stokes偏振测量,基本结构如
图 9. 分时型偏振测量超构表面[95]。(a)GIAM偏振测量的原理图,以及GIAM的扫描电镜显微图;(b)电压值Vg为0时入射光沿x轴偏振(方块),以及三个电压值下入射光沿y轴(圆)偏振的反射谱(Rxx(yy) = |rxx(yy)|2),垂直虚线对应工作波长为6.7 μm;(c)四种偏振态入射下测得的反射光强度(左;绿:RCP;蓝:LCP;紫:x偏振;红:椭圆偏振);GIAM测得的偏振态(虚线)与旋转分析仪测得的偏振态(实线)的对比(右)
Fig. 9. Division-of-time metasurface polarimeters[95]. (a) Schematic of the GIAM-based polarimeter and the SEM image of the fabricated GIAM; (b) reflectivity spectra (Rxx(yy) = |rxx(yy)|2) for the incident light polarized along the y-axis (circles) at three values of Vg, and along the x-axis (squares) at Vg=0 V, in which vertical dotted line corresponds to wavelength of operation λ0=6.7 μm; (c) measured intensities (left) of the reflected light at the detector measured for the four incident polarization states (Green, near-RCP; blue, near-LCP; purple, near-x-polarized; red, elliptical polarization), and comparison (right) between the polarization states obtained from the GIAM polarimetry (dashed lines) and those obtained from the rotating analyzer polarimetry (solid lines)
由于石墨烯的调制速率比较快,因此GIAM偏振测量系统的测量速率有望达到10 MHz量级,这在一定程度上解决了分时型偏振测量速度慢的问题。此外,这类基于可调谐超构表面的偏振测量系统只需探测单点的光强,因此单个探测器即可满足偏振感知的要求,极大地降低了应用的成本。由于分时型偏振测量超构表面只调制了入射光的偏振态和强度,不破坏入射光的波前信息,因此将超构表面置于成像光路中,根据相机各像素接收的强度信息,即可还原每个像素对应的偏振态,实现偏振成像。
3.3 探测器集成型偏振测量
无论是分振幅型还是分时型偏振测量超构表面,都依靠额外的光电探测器或相机来测量光强信号。近年来,研究人员利用超构表面设计灵活的特性,提出了集成光电探测器的超构表面设计[96-101],通过将不同偏振分量的光信号转换成电信号,实现了偏振态信息的获取。
2012年,Afsinmanesh等[96]设计了集成金属纳米天线的硅基光电探测器,实现了全Stokes参量的偏振测量,如
近年来,研究人员提出了集成金属超构表面的石墨烯-硅基光电探测器设计[99],可以在不需要扫描光束的情况下实现全Stokes参量的偏振测量。其基本结构如
图 10. 探测器集成型偏振测量超构表面。(a)集成6组不同等离激元狭缝结构的偏振探测器示意图[96];(b)基于等离激元超构表面的四像素硅-石墨烯偏振探测器示意图[99];(c)硅-石墨烯偏振探测器的扫描电子显微图[99];(d)四像素偏振探测器测得的偏振态在庞加莱球上的表示,输入光为波长1550 nm的椭圆偏振光[99]
Fig. 10. Photodetector integrated metasurface polarimeters. (a) Schematic diagram of the photodetector integrated polarimeter that contains six differently shaped plasmonic slit structures[96]; (b) schematic diagram of the four-pixel silicon/graphene hybrid detector integrated polarimeter containing differently orientated plasmonic metasurfaces[99]; (c) SEM images of the silicon/graphene polarimeter[99]; (d) measured states of polarization on a Poincaré sphere from the four-pixel polarimeter with elliptically polarized inputs at the wavelength of 1550 nm[99]
用于偏振测量的探测器集成型超构表面无需复杂的光路和探测元件,照射在超构表面上的待测光根据偏振态的不同,被集成的探测器转化为不同强度的光电流,通过分析电流的强度就可以分辨待测光的偏振态,为偏振测量的集成化、低成本化提供了新的解决方案。此外,将探测器集成型超构表面作为一个像素,并做成阵列化结构替代成像光路中的相机,有望实现集成化的偏振成像。
3.4 其他方法
除了上述分振幅型、分时型和探测器集成型外,研究人员还提出了多种用于偏振测量的超构表面设计[102-106]。
Basiri等[105]从口足类动物的复眼得到启发,设计了一种类小眼型双层超材料(ODLM)结构,如
Zhang等[106]通过设计一种基于几何相位的复合全息超构表面,实现了偏振态的直接测量。研究人员对LCP光和RCP光分别设计了两种全息图,如
图 11. 其他偏振测量超构表面。(a)ODLM的结构示意图[105];(b)用于偏振态测量的ODLM和金线栅阵列超构表面结构示意图,所需要的结构单元为P0-P6,两个额外的阵列P'5和P'6用于在不同工作波长测量偏振态[105];(c)利用ODLM超构表面和偏振分析仪测得的8个随机偏振态的入射光的Stokes参量(S0-S3)[105];(d)圆偏振相关的超构表面全息示意图,用于分别在LCP和RCP照明下产生RCP和LCP全息图像[106];(e)入射光偏振态为RCP、45°线偏振、左、右旋椭圆偏振时,超构表面全息图的x偏振分量的理想分布[106];(f)HWP快轴旋转角度为αHWP=0°, 30°以及QWP快轴旋转αQWP=-45°, 15°时CCD相机获取的全息图像,箭头标示对应的理论偏振态[106];(g)当旋转HWP和QWP时,理论、实验和仿真的手性相位差和Stokes参量S1、S2和S3 [106]
Fig. 11. Other polarimeter metasurfaces. (a) Schematic of ODLM design[105]; (b) schematic of metasurface design consisting of ODLM and gold nanowire arrays to fully characterize the polarization state of the incident light. The required units are marked by P0 through P6. The two additional units labeled P'5 and P'6 provide the capability to identify polarization states at different working wavelengths[105]; (c) Stokes parameters (S0-S3) extracted for eigth different random input polarization state, using a polarization analyzer and the metasruface[105]; (d) schematic of circular-polarization-dependent metahologram for generating RCP and LCP images under LCP and RCP illumination, respectively[106]; (e) ideal calculated x-polarized image components for illumination of the metahologram by waves with RCP, +45° linear, left-handed elliptical and right-handed ellipical polarization[106]; (f) holographic images captured by the CCD camera for orientation angles of the HWP’s fast axis of αHWP=0°, 30° and the QWP’s of αQWP=-45°, 15°. The inset arrows shcematically indicate the corresponding theoretical polarization state[106]; (g) Theoretical, experimental and simulated chiral phase difference and Stokes parameters S1, S2 and S3 when rotating the HWP and QWP, respectively[106]
4 深度信息感知
随着新一代信息技术的不断发展,尤其是消费电子、自动驾驶等领域的蓬勃发展,研究具有体积小、轻量化、易于集成、系统复杂度低等特性的三维感知系统愈发重要。现有的三维成像技术普遍存在系统复杂、成本高、体积大等问题。超构表面为微型化、易于集成的三维成像系统提供了一条新的途径。本节从主动式和被动式两个方面回顾了近年来利用超构表面的三维成像研究。
4.1 主动式
基于超构表面对光波前的灵活调控能力,其可用于主动式三维成像系统。其中结构光方法使光源发射的光按照一定的规则和模式编码,投影于物体上。经物体表面反射回来的图案随着物体距离和三维形貌的不同会发生不同的形变,通过计算图像传感器所得形变图案中每个像素的变形量,得到相应的视差,从而可以得到物体的深度信息。目前已有不少基于超构表面实现结构照明(点云投射)的工作。Li等[108]利用基于几何相位的硅超构表面,仿真实现了视场角为59°×59°的4×4均匀点阵投射,如
图 12. 超构表面实现结构光投射。(a)基于几何相位的超构表面仿真实现4×4点阵投射[108];(b)二元相位超构表面实现偏振不敏感的5×5点阵投射[109];(c)扰动超构表面生成4π全空间随机点云[110];(d)基于几何相位的复振幅调制超构表面实现衍射级次选择性出射[111];(e)超大视场角结构光点阵投射超构表面[112];(f)VCSEL集成超构表面,可实现宽范围的动态光束转向[113]
Fig. 12. Realization of structured light projection by metasurfaces. (a) Geometric phase metasurface for 4×4 spots array projection on the simulation[108]; (b) binary phase metasurface for polarization-independent 5×5 spots array projection[109]; (c) scrambling metasurfaces generating random point cloud covering 4π space[110]; (d) selective diffraction with complex amplitude modulation by geometric phase metasurface[111]; (e) metasurface for structured light projection with a large field of view[112]; (f) VCSEL-integrated metasurfaces, realizing wide-ranging dynamic beam steering[113]
另一种被广泛使用的主动三维成像方法是飞行时间法。该方法是向被测物体发射激光脉冲,光脉冲被物体反射回去后被探测器接收,通过计算光脉冲的飞行时间来计算被测物体与相机的距离。为保证较长的探测距离与较高的精度,通常需要利用光束进行扫描,主动超构表面可通过电压调控光束偏转,从而避免机械转动。Park等[114]实现了反射光振幅与0至2π相位可独立调控的超构表面反射阵列,并制成了固态空间光调制器(SLM),演示实现了三维光束扫描功能,如
图 13. 主动超构表面实现光束方向动态调控。(a)超构表面反射阵列及单个散射体结构的示意图(上)[114]、演示实现Lidar获取的深度图(下);(b)非晶态(左)和晶态(右)主动切换的等离激元超构表面示意图[115];(c)利用MEMS实现主动超构表面动态光束调控[116];(d)基于超构表面和液晶的SLM的示意图[117]
Fig. 13. Realization of active beam steering by metasurfaces. (a) Schematic of metasurface reflecting array and the unit-cell (top)[114], and experimental demonstration of LiDAR and the acquired depth map (bottom); (b) schematic of actively switchable plasmonic metasurfaces in amorphous (left) and crystalline (right) states[115]; (c) metasurface active beam steering using MEMS[116]; (d) schematic of metasurfaces and liquid crystal based SLM[117]
4.2 被动式
被动式三维成像不需要特殊的结构光照明或光束扫描,可以极大地降低三维成像系统的复杂度、成本与功耗。然而基于传统光学元件的被动式三维成像方案在系统体积、成像精度等领域仍然存在挑战。利用超构表面实现被动式三维成像有望解决这些问题。
4.2.1 多目视觉
多目视觉利用多个镜头的视差计算深度,即不同镜头像面上物体的平移关系。光场相机利用镜头阵列成像,分辨率高,景深大,但商业化的光场相机价格昂贵,体积较大。微超透镜阵列作为微型平面光学元件,可以显著提高系统的集成度。相较于传统微透镜阵列,其还具有更多种的偏振和光谱的控制能力,并且与现有半导体工艺具有更好的兼容性。Lin等[118]利用60×60的超透镜阵列实现了宽带消色差的光场成像。利用相位补偿可以在宽光谱范围内消除色差[119],使该超透镜阵列可以实现彩色图像的重聚焦与深度感知,如
图 14. 基于多目视觉原理的超构表面三维成像。(a)用于全色光场成像的消色散超透镜阵列[118](上:全色成像与重聚焦示意图;下:场景的深度感知);(b)利用超透镜阵列快照式3D定位,物体位置与3个像的平移的对应关系[120];(c)像差的校正[120];(d)恢复的垂直距离S⊥和水平距离S∥与实验设置(红线)的比较[120]
Fig. 14. Metasurface 3D imaging based on multiview stereo. (a) A chromatic metalens array for full-color light-field imaging[118](top: schematic of full-color imaging and refocusing; bottom: depth perception of scenes); (b) three-dimensional positioning with a single-shot metalens array, correspondence between the position of the object and the translation of the three images[120]; (c) correction of aberrations[120]; (d) recovered vertical distance S⊥ and horizontal distance S∥ compared with the experimental setup (red lines) for different object distances[120]
4.2.2 单目视觉
与多目视觉相比,单目视觉进一步降低了系统复杂度,不需要不同镜头间具有较大的基线距离即可保证深度测量精度,易于集成,同时也不需要不同角度的镜头所成的图像相匹配,有望以更高效率的算法得到深度信息。
不同于主动式三维成像与多目视觉,在基于灰度纹理的推断法之外,单目视觉主要通过成像系统中PSF随物方深度的变化获取深度信息。然而,使用传统折射透镜的离焦法需要对镜头进行物理移动且测量精度有限。
与标准透镜相比,衍射光学元件具有更特异的PSF,从而为实现更高精度的单目深度识别提供了新的路径,即只需在标准透镜的相位之外增加一个额外的相位。典型例子便是双螺旋点扩展函数(DH-PSF),设计的相位掩模版使成像系统产生随深度旋转的PSF,使所成图像携带目标的深度信息,通过分析图像的功率倒谱,最终得到深度和强度信息。传统的DH-PSF通过SLM[121]或计算机制全息图[122](CGH)实现,这些方法通过光传播过程中累积的相位来调控波面,所用元件的厚度和尺寸远大于波长,系统复杂度相对较高。应用超构表面有望克服这些缺点,并进一步具有偏振和光谱复用的潜力。
Jin等[123]使用等离子超构表面实现了DH-PSF,提出了集成化双螺旋-超透镜结构,将双螺旋相位掩模版与会聚透镜集成,使得元件的体积大幅减小,如
图 15. 利用DH-PSF的超构表面三维成像。(a)等离子超构表面集成DH-PSF和超透镜用于3D成像[123]。(左:PSF随深度变化的旋转;右:轴外双点在不同深度的成像);(b)基于介质超构表面的DH-PSF三维成像[124](左:成像原理示意图;右:3D场景成像结果及获取的深度信息);(c)超构表面场景重建与测距[125]
Fig. 15. Metasurface 3D imaging using DH-PSF. (a) Metasurface integrated with DH-PSF and metalens for three-dimensional imaging[123](left: rotation of PSF with the change of depth; right: imaging of two off-axis point sources at different depths); (b) 3D imaging by DH-PSF based on dielectric metasurfaces[124](left: schematic of imaging principle; right: imaging results of a 3D scene and the retrieved depth information); (c) ranging and scene reconstruction using a metasurface[125]
为了高度集成且同时获取较清晰的二维强度图和深度图,Colburn等[125]将三次方相位掩模版与双螺旋相位掩模版并列排放于单一超构表面,其工作于可见光波段。通过将三次方相位掩模所成像解卷积恢复二维强度图,再根据二维强度图对双螺旋相位掩模版所成像解卷积,得到成像时的点扩散函数,从而得到相应物体的深度,如
利用超构表面还有望实现快照式精度更高的离焦法三维成像。Guo等利用了超构表面单元的空间复用[126]在单个超构表面上实现了2种相位分布[127],使用3 mm孔径超透镜在同一像面上实现了两个不同焦距的交错离轴聚焦,从而实现快照式离焦法深度感知,如
图 16. 改进离焦法的超构表面三维成像。(a)超构表面基于超透镜空间复用的交错离轴聚焦实现快照式离焦三维成像的原理图[127];(b)随物体深度的深度传感结果概率分布[127];(c)利用超构表面极端色散的离焦3D成像,超透镜将不同色光聚焦于不同深度[128];(d)不同场景的重建结果[128];(e)利用分立U-Net和端对端优化的3D(深度和RGB图像)重建算法[128];(f)三维重建结果分析[128](左:随物体深度的深度感知结果概率分布;右:所使用超透镜与传统透镜的成像结果对比)
Fig. 16. Metasurface 3D imaging with improved defocus method. (a) Schematic of single-shot defocus 3D imaging metasurface realized by interleaved off-axis focusing based on spatial multiplexing of metalens[127]; (b) probability distribution of depth sensing results depending on object depth[127]; (c) defocus 3D imaging using extreme dispersion in metasurfaces, in which metalens focuses different colors at different depths[128]; (d) reconstruction results of different scenes[128]; (e) schematic of 3D (depth and RGB image) reconstruction algorithm using separate U-Net and end-to-end optimization[128]; (f) analysis of 3D reconstruction results[128] (left: probability distribution of depth sensing results depending on object depth; right: comparison between reconstruction results of conventional lens and the applied metalens)
另一个可以利用的性质是超构表面的极大色散。色散在超透镜相关的研究中一般被认为是限制其应用的重要问题,但Tan等[128]利用超构表面的极端色散性质作为计算三维成像的额外自由度,利用色散使超透镜对RGB三色光具有不同的光焦度,如
5 结束语
重点介绍了超构表面在光谱、偏振、深度等多维光场感知中的应用。当前,研究人员提出的超构表面一般仅能实现对较少维度光场信息的感知,部分器件虽然能同时获取光谱、偏振等信息,但会破坏光的波前,难以实现成像功能[75, 78-80]。未来人们有望进一步利用超构表面设计的灵活性,实现对更多维度光场信息的同时感知。其中一种可能的方案是利用可重构式超构表面。例如,通过在超构表面上同时集成可调谐型光谱滤波片和偏振滤波片,结合计算重建,有望实现对光谱、偏振信息的同时测量。此外,进一步结合超构表面微纳结构设计的极大自由度以及不断发展的计算机技术,在超构表面设计及重建算法中利用逆向设计、端对端优化、深度学习等方法,以极小尺寸的微纳结构结合计算后端,有望实现光谱、偏振、深度等多维光信息的同时感知。随着人们对超构表面认识的深入,对新型超构表面设计的探索,以及对大规模微纳加工技术的完善,超构表面将在轻量化、集成化的多维光场感知领域有着光明的应用前景。
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Article Outline
倪一博, 闻顺, 沈子程, 杨原牧. 基于超构表面的多维光场感知[J]. 中国激光, 2021, 48(19): 1918003. Yibo Ni, Shun Wen, Zicheng Shen, Yuanmu Yang. Multidimensional Light Field Sensing Based on Metasurfaces[J]. Chinese Journal of Lasers, 2021, 48(19): 1918003.
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