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超表面在中红外成像的基本原理主要为局域表面等离激元共振、惠更斯原理、传播相位和贝里相位[41-43],利用这些原理可以对电磁波的相位、振幅、偏振等进行调控,从而实现透镜成像、偏振控制、涡旋光束生成等功能。
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表面等离激元是指电磁波入射到介质与金属分界面时,金属表面自由电子与入射电磁波相互作用形成的一种特殊的电磁模式。当金属表面的自由电子的振动频率与入射电磁波的频率相匹配时会发生共振,若电磁波被限制在金属表面很小的区域内如金属纳米颗粒或金属微纳结构内,则称为局域表面等离激元共振(Localized Surface Plasmon Resonance, LSPR)。常通过激发金属天线的局域表面等离激元共振来实现对电磁波的调控,通过改变金属天线的尺寸、形状和空间取向等在亚波长的范围内引入相位突变。
2011年,Yu等[39]通过金纳米棒组成的V形天线阵列在界面处引入相位突变,在8 µm波长实验验证广义折反射定律,在中红外波段约5~10 µm实验观察到异常折反射现象,并使用V形天线产生具有螺旋波前并携带轨道角动量的涡旋光束。2012年,Yu等[44]利用V形天线阵列设计了可在波长5~12 µm范围内工作的四分之一波片超表面,如图1(a)所示,能够将任意方向的入射线偏振光转变为圆偏振光。2018年,Safaei等[45]采用耦合的金纳米盘设计超表面透镜,对于最优设计实验测试在4~10 µm中红外范围内可保持超过70%的透过率。
图 1 基于不同电磁波调控原理的中红外超表面结构。(a)基于V形天线的四分之一波片超表面[44];(b)工作在5.2 µm波长下的介电惠更斯透射超表面,分别为矩形超表面结构倾斜示意图、用于构建超表面光学器件的8个超表面结构单元对应的相移与透过率、制造的超表面结构的扫描电子显微镜图[51];(c)由排列在方形晶格上的全硅纳米圆柱构成的超表面透镜,左图为超透镜单元结构,右图为8个选中的纳米圆柱模拟得到的振幅与相位[31]
Figure 1. Mid-infrared metasurface structures based on different electromagnetic wave control mechanism. (a) Metasurface quarter-wave plate based on the V-shaped antenna[44]; (b) Dielectric Huygens metasurface operating near the mid-IR wavelength of 5.2 µm, schematic tilted view of a rectangular meta-atom structure, the phase shift and transmittance corresponding to the eight meta-atom elements used to construct the meta-optical device, and the scanning electron microscope image of the fabricated metasurface structure, respectively[51]; (c) Long-wavelength infrared metalens composed of silicon nanopillars arranged on a square lattice. The building block of all-silicon metalens (left) and simulated amplitude and phase for eight selected nanopillars (right)[31]
基于金属纳米结构的超表面具有较大的欧姆损耗,难以实现高效率的光场调控,用由介电材料构成的超表面可以有效的解决这一问题。通过全电介质超表面单元调控电磁波的原理可分为三类:惠更斯原理、传播相位原理、贝里相位原理。
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惠更斯原理又称为次级波理论,指任意时刻波前平面上的任意一点,可以视为次级球面波的子波源。2013年,Pfeiffer等[46]解释了如何用表面等效原理[47-48]设计惠更斯超表面,并设计了在微波波段的无反射惠更斯超表面。通过调节超表面的电极化率和磁极化率可以调控超表面结构的透过率。2016年,Epstein等[49]总结了现有的惠更斯超表面设计方法,并提出了一种基于等效原理的惠更斯超表面综合设计方法,包含微观的超表面结构设计和宏观的超表面设计。
2015年,Campione等[50]提出使用PbTe分裂立方体谐振器可以在中红外波段宽光谱范围满足高透过率和360°相移,有望用于惠更斯超表面。2018年,Zhang等[51]设计并实验演示了工作在5.2 µm波长下的介电惠更斯透射超表面,如图1(b)所示,采用具有高折射率且能支持高质量Mie共振的PbTe材料构造H型超表面单元,衬底选用CaF2,对于线偏振光在透射模式下整体光学效率可达75%,厚度仅为自由空间波长的八分之一,同时展示了具有衍射极限聚焦和成像功能的中红外透射超表面透镜,有望用于中红外光学系统的设计。2020年,Leitis等[52]提出了可编程全电介质红外惠更斯超表面,超表面由Ge3Sb2Te6-Ge-Ge3Sb2Te6多层圆盘组成,实验显示在3.75 µm波长GST所有晶态的平均透过率保持在53%以上,且数值数据提取的光相位显示能够进行81%的全2π相移。2020年,Shalaginov等[53]使用工作在5.2 µm波长的惠更斯超表面结构制造单层超表面透镜,视场范围可接近180°,并且实验证明可在整个视场上无像差聚焦和成像。
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传播相位指的是电磁波在传播的过程中会产生光程差,利用这一光程差来实现对相位的调控[54]。波长为λ的电磁波在有效折射率为n的均匀介质中传播距离d,电磁波累积的传播相位是
$ \phi = n{k_0}d$ ,${k_0} = 2\text{π}/ \lambda $ 是自由空间波矢。根据等式,可以通过改变厚度d来调节相位,或者在保持d不变时基于介质等效折射率理论,对折射率n进行空间调制。即按不同的占空比排列介质柱或孔等结构以改变其等效折射率,从而实现对相位的调控。2017年,Zuo等[30]采用MgF2衬底和氢化非晶硅纳米柱(α-Si:H)实现工作波长在4 µm的中红外偏振不敏感、高效全介电超表面透镜,能够形成接近衍射极限聚焦光斑并且可以达到78%的聚焦效率。2018年,Fan等[31]设计了工作在长波红外10.6 µm、数值孔径0.6、偏振无关、由全硅纳米圆柱构成的超表面透镜,如图1(c)所示,能够将入射光会聚成约1.08倍波长大小的光斑且能够高分辨率成像,角分辨率可达2.2 mrad。上述工作均在特定波长下设计、在激光照射下实验。2021年,Huang等[28]设计、制造、表征了数值孔径0.45、直径2 cm、工作波长10 µm的全硅纳米方柱超表面透镜,并在混合温度环境(模拟不同波长)中进行测试,例如正常人体温度,高温65 ℃、320 ℃以及低温0 ℃下,虽然存在色差且具有较低的空间分辨率,但仍然能够对不同温度下发射黑体辐射的物体成像,说明了中红外超表面透镜在环境热辐射应用中的可行性。2021年,Leitis等[12]设计了锗纳米圆柱排列在六边形Al2O3晶格薄膜上的直径700 µm、数值孔径0.36的超表面透镜,实验测量该透镜可以对6.5 µm波长的光达到90.3%的透过率、70.4%的聚焦效率、直径22.2 µm的衍射极限光斑尺寸。
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Pancharatnam-Berry(PB)贝里相位或者几何相位,1956年印度拉曼研究所Pancharatnam教授[55]发现电磁波在偏振态转化过程中会产生一个额外相位。1984年,几何相位的概念由英国Berry教授[56]首次提出。电磁波在超表面结构中的几何相位可以用琼斯矩阵来说明[54],对于圆偏振光,经过各向异性超表面后透射电场表达式为:
$$ \left[ \begin{array}{l} {E_{xout}}\\ {E_{yout}} \end{array} \right] = \frac{1}{{2\sqrt 2 }}\left( {({t_u} + {t_v})\left[ \begin{array}{l} 1\\ i\sigma \end{array} \right] + ({t_u} - {t_v}){{\rm{e}}^{2i\sigma \zeta }}\left[ \begin{array}{l} 1\\ - i\sigma \end{array} \right]} \right)$$ (1) 式中:
${t_u} $ 和${t_v} $ 为各向异性超表面两个主轴方向的透射复振幅;$ \zeta $ 为主轴u与x轴夹角,σ=±1,对应于右旋和左旋偏振态;${E_{xout}} $ 为出射电磁波的x偏振分量;${E_{yout}} $ 为出射电磁波的y偏振分量。从表达式可以观察到,透射场包含两部分:一部分是与入射电磁波旋向相同的电磁波;一部分是与入射电磁波旋向相反的透射电磁波,且附加了$2 \sigma \zeta $ 的相位延迟,附加的相位延迟即为几何相位,因此可以通过改变各向异性超表面结构的旋转角度来调控相位。2019年,Yan等[33]提出一种用于中红外10.6 µm主动照明偏振成像的硅矩形柱超表面,通过改变矩形柱的长宽与角度实现相位0~2π的覆盖,实验中可在实时条件下同时在两个正交偏振态成像,该项工作揭示了紧凑中红外偏振检测超表面系统的潜力。
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中红外超表面光谱检测主要基于表面增强红外吸收原理,使用等离子体平台[4]实现对分子振动信号的显著增强,在红外光谱上观察到对应于分子指纹的共振峰,从而实现对物质的检测。等离子体具有的较大欧姆损耗将导致低品质因数Q谐振,使检测性能不理想,近年来,有研究提出基于准连续体束缚态原理、使用高折射率介电材料产生高Q共振的方法,利用成像实现分子指纹检测[57-58]。
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1980年,Hartstein等[9]发现使用衰减全反射技术薄金属覆盖层或底层可将分子单层的红外吸收提高20倍,这一现象被称为表面增强红外吸收(Surface-Enhanced Infrared Absorption, SEIRA)。目前普遍认为关于表面增强红外吸收至少有两种不同机理:电磁效应和化学效应[3, 59-62]。
电磁效应机理,分子的增强吸收强度正比于入射光的局域场增强[6]。表面等离激元共振,尤其是纳米结构的局域表面等离激元共振(LSPR)[63],以及避雷针效应(Lightning rod effect, LRE)[64]都可以增强局域电场强度。对于棒状结构,棒两端的电场增强最大[10]。
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连续体中的束缚态(Bound states in the continuum, BIC)是可以与带走能量的辐射波的连续光谱共存的、仍保持局域化的波[65]。真正的BIC在理论上具有无限大的品质因数Q和零线宽。在实际中,当Q和谐振宽度变得有限时,可以实现准BIC,也称为超腔模式[66-67]。准BIC模式的利用为调整超表面结构的共振带宽、光谱位置、品质因数Q以及电磁场分布提供了极大的灵活性[12]。当超表面单元结构的面内对称性被破环,真正BIC转变为准BIC,产生高Q共振[57, 68]。
Principles and application progress of mid-infrared metasurfaces in imaging and detection (Invited)
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摘要: 中红外波段包含两个大气窗口及分子指纹区,在红外成像与物质检测方面具有重要应用。传统中红外光学器件在成像方面受材料、加工等限制成本昂贵、加工复杂;在检测方面,受分子吸收截面小的限制,检测灵敏度低,对微量化学物质检测具有较大挑战。超表面是由亚波长尺度的人造单元构成的二维结构阵列,具有体积小、易集成、调控自由度高等特点,能够为制造低成本、轻型化、集成化的中红外光学器件提供一种新的实现方案。表面增强红外吸收能够有效增强分子振动信号,提高检测灵敏度。文章介绍了中红外超表面在电磁波调控方面的机理及其中红外检测应用的原理。着重整理了超表面结构在中红外波段的成像与检测领域的研究进展,包括偏振成像、可调及可重构超表面、其他特殊功能以及用于检测的基于等离子体激元或连续体束缚态原理的使用金、银、铝、石墨烯、硅、锗等材料的超表面结构。Abstract: The mid-infrared band contains two atmospheric windows as well as the molecular fingerprint region, and therefore has important applications in infrared imaging and detection. Conventional mid-infrared optics are expensive and need complicated fabrications limited by the material and processing technology in imaging. In terms of the detection, limited by the small molecular absorption cross-section, the sensitivity is extremely low and there is a great challenge for the trace chemical detection. Metasurfaces are two-dimensional arrays composed of artificial building blocks at the subwavelength scale. They have the characteristics of small size, easy integration and high degree of freedom, which may provide a new implementation scheme for manufacturing the low-cost, light-weight and integrated mid-infrared optical devices. Surface-enhanced infrared absorption can effectively enhance molecular vibration signals and improve the detection sensitivity. In this review, the mechanism of mid-infrared metasurfaces in electromagnetic wave regulation and the principals of mid-infrared detection applications are introduced. The research progress in the imaging and detection of mid-infrared metasurfaces is sorted out, including the polarization imaging, tunable and reconfigurable metasurfaces, other special functions and metasurface structures using gold, silver, aluminum, graphene, silicon, germanium and other materials based on plasmon or bound states in the continuum principles for the detection.
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Key words:
- mid-infrared /
- metasurface /
- imaging /
- spectrum detection
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图 1 基于不同电磁波调控原理的中红外超表面结构。(a)基于V形天线的四分之一波片超表面[44];(b)工作在5.2 µm波长下的介电惠更斯透射超表面,分别为矩形超表面结构倾斜示意图、用于构建超表面光学器件的8个超表面结构单元对应的相移与透过率、制造的超表面结构的扫描电子显微镜图[51];(c)由排列在方形晶格上的全硅纳米圆柱构成的超表面透镜,左图为超透镜单元结构,右图为8个选中的纳米圆柱模拟得到的振幅与相位[31]
Figure 1. Mid-infrared metasurface structures based on different electromagnetic wave control mechanism. (a) Metasurface quarter-wave plate based on the V-shaped antenna[44]; (b) Dielectric Huygens metasurface operating near the mid-IR wavelength of 5.2 µm, schematic tilted view of a rectangular meta-atom structure, the phase shift and transmittance corresponding to the eight meta-atom elements used to construct the meta-optical device, and the scanning electron microscope image of the fabricated metasurface structure, respectively[51]; (c) Long-wavelength infrared metalens composed of silicon nanopillars arranged on a square lattice. The building block of all-silicon metalens (left) and simulated amplitude and phase for eight selected nanopillars (right)[31]
图 2 中红外偏振超表面设计。(a)同轴宽带消色差聚焦涡旋光束发生器(BAFOV)的原理图(左),由单晶硅构成的双折射超表面单元(右)[69];(b)基于GIAM的偏振检测仪示意图[71];(c)中红外全斯托克斯偏振检测装置示意图,由七个超表面单元组成[73]
Figure 2. Mid-infrared polarization devices. (a) Schematic illustration of the BAFOV generation with polarization-dependent functions (left). The birefringent meta-atoms are made of monocrystalline Si (right)[69]; (b) Schematic of the GIAM-based polarimeter[71]; (c) Schematic of the Mid-IR full-Stokes polarization detection device design with seven cells for direct Stokes parameter measurement[73]
图 3 GST相变材料超表面。(a)可切换完美吸收器示意图(左)和实验测试不同天线尺寸的非晶态与晶态条件下的反射率(右)[36];(b)用于可调光束偏转器的等离子体超表面(左)和双焦点柱透镜实验结果(右)[84];(c)可重构变焦超透镜示意图(左)和USAF-1951分辨率图成像(右)[90]
Figure 3. GST phase-change material metasurfaces. (a) Sketch of the switchable perfect absorber device (left) and measured reflection spectra in amorphous and crystalline conditions for different antenna sizes (right)[36]; (b) Active plasmonic metasurface for beam switching (left) and experimental results for the cylindrical bifocal lens (right)[84]; (c) Artistic rendering of a reconfigurable varifocal metalens (left) and well-resolved lines of USAF-1951 resolution charts (right)[90]
图 4 石墨烯电可调超表面。(a)基于石墨烯电可调超表面吸收器的中红外光学调制器[35],基于可调超表面吸收器的中红外光学调制器示意图及石墨烯超表面扫描电子显微镜图像(左),不同栅极电压下的超表面吸收体测得的反射谱(右);(b)栅极电可调石墨烯-金超表面[92],用于控制反射相位的栅极可调装置示意图,金天线阵列位于石墨烯上,以及在石墨烯上的金天线的电子显微镜图像(左),比例尺为1 µm,在8.2、8.5、8.7 µm下的相位调制(圆-实验,线-仿真)(右);(c)电可调谐振吸收的混合石墨烯超表面[95],混合石墨烯超表面示意图(左)和施加不同栅极电压下的测量反射光谱(右)
Figure 4. Graphene electrically tunable metasurfaces. (a) Mid-infrared optical modulator based on an electrically tunable metasurface absorber[35]. Schematic of the ultrathin optical modulator based on a tunable metasurface absorber and a scanning electron microscope (SEM) image of the metasurface on graphene (left). Measured reflection spectra from the metasurface absorber for different gate voltages (right); (b) The gate-tunable graphene-gold reconfigurable mid-infrared metasurface[92]. Schematic of a gate-tunable device for control of reflected phase and SEM image of gold resonators on graphene (left). The scale bar indicates 1 μm. Phase modulation at wavelengths of 8.2 µm, 8.5 µm, and 8.7 µm (circles-experiment, line-simulation) (right); (c) Hybrid graphene metasurface allows for electrically tunable resonant absorption[95]. Schematic of the hybrid graphene metasurface (left) and measured reflection spectra when applying different gate voltages (right)
图 5 拉伸前后开环谐振器的示意图(上);不同应变程度的双开环谐振器结构阵列的实验测量反射光谱和代表性环境扫描电子显微镜图像(下)[77]
Figure 5. A schematic of the substrate prior to stretching with Au split ring resonators attached and a schematic of a stretched array (Top); The measured reflectance spectra and representative environmental scanning electron microscope (ESEM) images for the double SRR array for various degrees of strain (bottom)[77]
图 6 基于金属材料的用于表面增强红外吸收的超表面结构。(a)具有化学特异性、无标记的中红外纳米光子生物传感器[16];(b)用于同时检测CO2和CH4气体的MOF-SEIRA平台[21]
Figure 6. Metal-based metasurfaces for surface enhanced infrared absorption. (a) Chemically specific, label-free nanophotonic biosensor in the mid-infrared[16]; (b) MOF-SEIRA platform for simultaneous sensing of CO2 and CH4 gases[21]
图 7 用于表面增强红外吸收的石墨烯超表面。(a)石墨烯等离子增强分子指纹传感器示意图[117];(b)用于气体识别的石墨烯等离子体装置的实验方案[20]
Figure 7. Graphene metasurfaces for surface-enhanced infrared absorption. (a) Schematic of graphene plasmon enhanced molecular fingerprint sensor[117]; (b) Experimental scheme of the graphene plasmon device for gas identification[20]
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(in Chinese) doi: 10.3788/IRLA20201035