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任何的偏振光都可以通过Stokes矢量S=[S0, S1, S2, S3]T来描述,其中S0= I0+I90,S1= I0-I90,S2= I45-I135,S3= IR-IL,S0、S1、S2为线偏振分量,S3为圆偏振分量,I0、I90、I45和I135分别表示0°、90°、45°和135°不同偏振方向的偏振光分量,IL和IR分别表示左旋和右旋圆偏振光光强分量。集成偏振HgCdTe探测器采用分焦平面偏振探测成像,将不同偏振方向的偏振片集成在焦平面上,和探测器的像元一一对应,不同像元探测不同的偏振方向。如图1所示,将2×2个像元构成一个超像元,4个子像元同时探测4个不同偏振方向(例如图2所示的0°、45°、90°和135°)上的光强信息,据此可得到S0、S1和S2分量,由公式(1)和公式(2)计算得到偏振度DoLP和偏振角AoLP,进一步处理得到目标的偏振图像。此外,还可利用探测到的偏振信息,如0°和90°的偏振光强信息,进行偏振复原,提高水下及雾霾、沙尘等高浓度散射介质下的图像复原质量[6-7]。需要指出的是,由此超像元结构探测到的光强信息无法得到S3圆偏分量,因此无法实现全Stokes矢量偏振成像。当探测自然目标时,其圆偏振分量很小,可假定为零,但在探测人造圆柱形结构、大气效应扣除等方面圆偏振分量具备特定的应用效果[17]。全Stokes矢量偏振器件工艺复杂,目前还较难实现和焦平面探测器的集成,在文中暂不予以讨论。
图 2 由0°、 45°、 90°、135°偏振方向像元构成的超像元结构
Figure 2. Super-pixel structure composed of pixels with polarization directions of 0°, 45°, 90°, and 135°
$$ \mathrm{D}\mathrm{o}\mathrm{L}\mathrm{P}=\frac{\sqrt{{S1}^{2}+{S2}^{2}}}{S0} $$ (1) $$ \mathrm{A}\mathrm{o}\mathrm{L}\mathrm{P}=\frac{1}{2}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{t}\mathrm{a}\mathrm{n}\left(\frac{S2}{S1}\right) $$ (2) 集成红外偏振成像探测器采用背光照入射结构,偏振滤光微结构阵列集成在红外成像探测器阵列芯片入射面,如图3所示,形成偏振像元单元,用于对目标辐射进行偏振分光。HgCdTe中波红外探测器采用n-on-p器件结构,探测器阵列芯片制备完成后,采用双面光刻技术,在背面光刻出标记,然后利用高精度光刻与刻蚀技术,在芯片背面光刻出与正面像元一一对应的滤光结构阵列;之后与读出电路互连,形成混成芯片,经杜瓦封装后,再进行信号提取和测试评价。集成红外偏振探测器中,偏振滤光微纳结构的性能直接影响到整个探测器的偏振性能,是研究的重点和热点之一。
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金属因其对光的特殊吸收和反射特性,可用来制作偏振光栅。针对不同偏振方向的光栅,偏振方向与光栅方向垂直的入射光可以透过光栅,入射至探测器像元单元;偏振方向与光栅平行的入射光可被金属光栅反射和吸收。基于此,金属偏振光栅可探测出不同偏振方向的光。该现象可用等效介质理论进行解释。
等效介质理论(Equivalent medium theory,EMT)适用于光栅周期远小于入射波长时的情况。将周期结构的光栅看作一层均匀介质,对TE和TM偏振光,光栅的等效折射率如下:
$$ {n}_{TE}=\sqrt{DC{({n}_{1}+\mathrm{i}{\kappa }_{1})}^{2}+(1-DC){({n}_{1}+\mathrm{i}{\kappa }_{1})}^{2}} $$ (3) $$ {n}_{TM}=\sqrt{\frac{{\left({n}_{1}+\mathrm{i}{\kappa }_{1}\right)}^{2}+{\left({n}_{2}+\mathrm{i}{\kappa }_{2}\right)}^{2}}{DC{\left({n}_{2}+\mathrm{i}{\kappa }_{2}\right)}^{2}+\left(1-DC\right){\left({n}_{1}+\mathrm{i}{\kappa }_{1}\right)}^{2}}} $$ (4) 式中:DC为光栅的占空比(光栅宽度与周期的比值);n,κ分别为材料折射率与消光系数,同时也分别为折射率的实部与虚部,下标1、2分别代表光栅材料与光栅缝隙中的材料。当光栅缝隙材料为空气时,κ2=0。同时,为了简化计算,将金属视为完美导体,即κ1趋于无穷。TE和TM偏振光的等效折射率简化结果为:
$$ {n}_{TE}={i\kappa }_{1}\sqrt{DC} $$ (5) $$ {n}_{TM}={n}_{2}{(1-DC)}^{1/2} $$ (6) 由此可以看出,简化后金属光栅的折射率针对TE偏振光来讲只有虚部,针对TM偏振光来讲,只有实部。因此,针对亚波长金属偏振光栅,对于TE偏振光,光栅折射率只有虚部,相当于是金属膜,大部分TE偏振光被反射和吸收,但实际上还会有少量光被透射过去;对于TM偏振光,折射率只有实部,可以把光栅层相当于介质层,光栅对于TM光有微弱的吸收,大部分TM偏振光被透射。
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针对背入射的 HgCdTe FPA探测器,亚波长金属偏振光栅集成在探测器的ZnS增透膜上。非偏振光经过亚波长金属光栅后为TM模式的偏振光,穿过ZnS和CdZnTe介质层,最后偏振光被HgCdTe吸收区吸收。由于HgCdTe吸收层对入射光偏振角度的响应选择性可以忽略不计,所以穿过CdZnTe衬底层的TM波强度大小可以大致表征HgCdTe偏振探测器的响应,因此在模拟仿真时,只需要模拟偏振光栅、ZnS和CdZnTe介质层的偏振光学性能。仿真结构模型如图4所示。
图 4 集成金属偏振光栅的CZT衬底HgCdTe FPA偏振性能仿真模型
Figure 4. Polarization performance simulation model of integrated metal polarization gratings CZT substrated HgCdTe FPA
根据入射波长和光栅尺寸之间的关系,当光栅尺寸接近或者小于入射波长时,需要使用矢量衍射理论进行分析,其中时域有限差分法(Finite-difference time-domain,FDTD)和严格耦合波理论(Rigorous coupled wave approach,RCWA)这两种方法是目前应用最广的计算方法。文中采用FDTD来进行仿真计算,FDTD是电磁场数值计算的重要方法之一,通过对时域的Maxwell方程中心差分方式进行离散化,以此来近似方程的偏导数。采用Yee元胞对空间进行划分,实现磁场和电场的离散化,在给定的初始条件和边界条件下,按照时间顺序来交替计算下一时刻对应的电场和磁场分布[19]。在实际的仿真计算中,需要对计算空间有所限制,采用不同的边界条件来确保在有限空间中进行电磁场的数值计算。
采用FDTD仿真软件Lumerical FDTD进行仿真,入射光波长范围为3~5 μm。入射光正入射时,边界条件设置为周期边界条件,入射光斜入射时,边界条件设置为Bloch边界条件。在光传播的方向上,边界条件采用完美匹配层(PML),用于吸收反射和传输光子。仿真分析中,关注TM光、TE光透过率的变化和消光比变化趋势。采用TM光与TE光透过率的比值来表征消光比。由于TM光波的透过率直接影响到探测器可探测到的光强,因此在保证消光比的同时,TM光波的透过率越大越好。
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金属偏振光栅材料不仅影响其偏振性能,还影响工艺制备的难易。分别选择金、银、铝、铜四种金属材料,仿真分析偏振光栅的性能。仿真的结果如图5所示。
由仿真结果可得,金属Al对应偏振光栅的消光比要远高于其他三种金属,这是因为Al有较大的介电常数虚部,使得TE波的衰减更加明显,从而获得较高的消光比。因此,针对中波红外偏振光栅,通常选择Al来制备光栅。
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金属偏振光栅周期的选择对TM波、TE波透过率及消光比有一定影响。基于如图4建立的仿真模型,对周期的影响进行仿真分析。设定Al光栅的厚度为100 nm,占空比为0.5,周期变化范围为100~1 000 nm,得到的消光比随周期的变化曲线如图6所示。由仿真结果可得,消光比随着周期的减小而增加。进一步分析可得,当光栅周期>600 nm时,消光比很小,几乎无偏振效果;光栅周期<200 nm时,消光比随光栅周期变化剧烈;光栅周期在200~600 nm之间时,消光比变化趋势较缓。这种现象的出现主要是因为光栅周期>600 nm时,TM和TE的透过率都较小,消光比较小;周期在200~600 nm之间时,TM透过率增大,消光比提高;周期<200 nm时,TE的透过率迅速降低,使得消光比提升较大。周期从100~1 000 nm变化的过程中,TM光波的透过率变化~10%。考虑到消光比大小、透过率的大小和工艺制备的难易程度,一般选择光栅周期在200~400 nm。
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金属偏振光栅占空比变化时,其偏振性能也会发生变化。设定金属偏振光栅周期为400 nm,光栅厚度为100 nm,占空比的变化范围为0.2~0.8,得到的消光比随占空比的变化曲线如图7所示。由仿真结果可得,消光比随占空比的增加而增加。这是因为TE波的透过率随占空比的增加而减小,使得消光比随占空比增加而增加。占空比增加时,TM光波的透过率下降,但下降趋势与TE光波透过率下降趋势相比,较缓慢,因此消光比仍随占空比的增加而增加。综合考虑TM波的透过率和偏振光栅的消光比,实际中选择金属偏振光栅的占空比在0.5~0.7。
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仿真分析金属偏振光栅的厚度对偏振性能的影响,设定Al偏振光栅周期为400 nm,占空比为0.5,光栅厚度变化范围为50~150 nm,仿真分析其厚度变化对消光比的影响,结果如图8所示。由仿真结果可得,光栅厚度越大,消光比越高,分析可得厚度的变化对TM波的透过率影响较小,消光比的提升主要取决于TE透过率随光栅厚度的增加而降低。
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本节通过仿真,分析了金属材料、光栅周期、占空比和厚度对光栅偏振性能的影响。由仿真结果可得,选择金属偏振光栅材料为Al时,偏振消光比较高;针对光栅周期、占空比和厚度的选择,要综合考虑待探测偏振方向的光波的透过率、消光比和工艺加工的难度。对于图3中的中波红外集成偏振HgCdTe探测器,偏振光栅结构参数仿真结果如表1所示,Al偏振光栅结构参数可选择周期为200~400 nm,占空比为0.5~0.7,厚度>100 nm。
表 1 中波红外集成偏振探测器偏振光栅参数仿真结果
Table 1. Polarization grating parameters simulation results of mid-wave infrared integrated polarization detector
Parameters Material Pitch/nm Duty cycle Thickness/nm Results Al 200-400 0.5-0.7 >100
Simulation analysis of mid-wave infrared polarization grating performance influenced by the polarizer structural parameters
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摘要: 中红外集成偏振焦平面探测技术将偏振探测技术与中波红外焦平面成像探测技术融合,通过异质集成的方式实现偏振光栅和探测器的单片集成,具有体积小、质量轻,机械稳定性高等优势,可实现多偏振方向的同时成像。像元级的亚波长金属光栅可实现不同偏振方向的高消光比,然而金属材料的选择、光栅的周期、占空比、厚度等参数均会影响偏振探测器的偏振性能。给出了亚波长金属光栅的理论分析,建立了中波红外集成偏振HgCdTe探测器的偏振性能仿真模型,对不同光栅参数对探测器偏振性能影响进行了仿真分析,确定了Al光栅周期200~400 nm,占空比0.5~0.7,厚度>100 nm的参数选择。仿真分析得到在±14°入射角范围内,偏振消光比变化较小。同时,引入了Si基HgCdTe探测器,仿真分析了SiO2增透膜厚度对偏振消光比的影响,确定了SiO2最佳厚度在500 nm附近,对Si基和CdZnTe衬底集成偏振HgCdTe探测器的消光比进行比对,得出了Si基探测器偏振性能更优。仿真结果可为中波红外集成偏振HgCdTe探测器偏振光栅的设计提供理论指导和参考。Abstract: Mid-infrared integrated polarization focal plane detection technology, which combines polarization detection technology and mid-wave infrared focal plane imaging detection technology, has realized the monolithic integration of polarization grating and detector through heterogeneous integration, and has the advantages of small size, light weight, high mechanical stability and simultaneous imaging of multiple polarization directions. Pixel-level sub-wavelength metal gratings can achieve high extinction ratios in different polarization directions. However, the selection of metal material and the structural parameters of gratings such as pitch, duty cycle and thickness have a significant impact on the polarization performance. The theoretical analysis of the sub-wavelength metal grating was given, and the polarization performance simulation model of the mid-wave infrared integrated polarized HgCdTe detector was established, and the effects of different grating parameters on the polarization performance were analyzed. The optimal structural parameters of 200-400 nm Al grating pitch, 0.5-0.7 duty cycle, and over 100 nm thickness were determined by simulation. The simulation results show that the range of ±14° incident angle had a small effect on the polarization extinction ratio. Meanwhile, Si-based HgCdTe detector had been introduced. The influence of SiO2 antireflection film thickness on polarization extinction ratio had been simulated to determine the optimal thickness. Compared with Cadmium Zinc Telluride (CdZnTe) substrated polarization HgCdTe detectors, the polarization performance of Si-based detector has been proved better. The simulation results can provide theoretical guidance and reference for the design of the polarization grating of the mid-wave infrared integrated polarization HgCdTe detector.
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Key words:
- mid-wave infrared /
- HgCdTe FPA /
- sub-wavelength metal wire-grid polarizer /
- polarization /
- simulation
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表 1 中波红外集成偏振探测器偏振光栅参数仿真结果
Table 1. Polarization grating parameters simulation results of mid-wave infrared integrated polarization detector
Parameters Material Pitch/nm Duty cycle Thickness/nm Results Al 200-400 0.5-0.7 >100 -
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