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Design of long-wavelength infrared polarizer based on sub-wavelength aluminum-ZnSe grating
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摘要
为提高长波红外偏振成像系统中偏振器件性能，本文通过分析光栅材料及结构参数对光栅偏振性能的影响，设计并优化了一种双层材料构成的亚波长光栅。该光栅为矩形形貌，光栅区由铝与硒化锌构成，两种材料的厚度分别为0.6 μm和0.4 μm，光栅周期1 μm，占空比50%。利用严格耦合波理论分析并计算该结构光栅的衍射效率，7~15 μm波段的光以0~60°入射后其0级横磁模透射率达到87.54%以上，消光比超过47 dB。该光栅在10.6 μm的测试波长下，TM透射率高达90.80%且具有50 dB以上的消光比，相比槽深相同的单层铝光栅，偏振透过率明显提高。仿真结果显示，该光栅在整个宽长波红外波段具有良好的偏振性能。
Abstract
A dual-layered sub-wavelength grating consisting of two kinds of materials, aluminum and ZnSe, is developed to improve the performance of polarimetric elements in long-wavelength infrared (LWIR) polarization imaging system. Parameters of the designed grating's morphological structure are optimized on the basis of analyzing the effects on the polarization performance through the rigorous coupled wave theory, which helpfully calculates the diffraction efficiency. With a rectangular profile, the grating designed for applications in LWIR band has periods of 1 μm and 50%-fill-factor. The depths of aluminum and ZnSe in the grating region are 0.6 μm and 0.4 μm respectively. A TM transmission greater than 87.54% with an extinction ratio exceeding 47 dB is achieved in the 7~15 μm band when the angle of incidence is from zero to sixty degree. The grating maintains an extinction ratio better than 50 dB and TM transmission over 90.80% above 10.6 μm incident wavelength, which is superior to single-layered aluminum gratings with the same depth in the transmission performance in comparison. The simulation results show that this grating has excellent polarization performance in the broad LWIR band.
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Overview
Abstract: Polarization measurement is able to effectively solve the problems that are beyond the reach of conventional photometry. When it comes to long-wavelength infrared (LWIR) polarization imaging system, polarization device plays a vital role in measuring the targets’ radiation and reflection and distinguishing them from busy background, which compensates for the lack of traditional thermal imaging if the difference in temperature is unobservable. Sub-wavelength wire-grid polarizer (WGP) is characterized by small volume and compact structure with the micro- or nano-manufacturing technology. It is a grating structure whose period is smaller than the incident wavelength and when that is smaller than the critical one, the grating will only have zero-ordered diffraction, which helps improve the utilization ratio of polarization information. A dual-layered sub-wavelength grating consisting of two kinds of materials, aluminum and ZnSe, is developed to improve the performance of polarimetric elements in LWIR polarization imaging system. Parameters of the designed grating’s morphological structure are optimized on the basis of analyzing the effects on the polarization performances through the rigorous coupled wave theory, which helps describe the diffraction of electromagnetic waves by periodic grating structures and calculate diffractive efficiencies of different orders. With a rectangular profile, the grating designed for applications in LWIR band has a structure of 1μm-period and 50%-fill-factor. The depths of aluminum and ZnSe in the grating region are 0.6 μm and 0.4 μm respectively. A TM transmission greater than 87.54% with an extinction ratio exceeding 47 dB is achieved in the 7 μm ~15 μm band when the angle of incidence is from zero to sixty degree. The grating maintains an extinction ratio better than 50 dB and TM transmission over 90.80% above 10.6 μm incident wavelength, which is superior to single-layered aluminum gratings with the same depth in the transmission performance in comparison. The structure is featured for the excessive etching on substrate, resulting in a series of air grooves. Therefore, the dielectric grating layer beneath the metal wire grid is formed. This method for improving polarization performances is easier to implement than coating anti-reflective films. It is investigated that the TM transmission increases with the depths of both metal and dielectric layers when the extinction ratio is dominated by the depth of metal layer, while the single-layered ZnSe grating shows little potential in extinction ability for the lack of metal component. Compared with the existed designs of WGP, the simulation results show that the TM transmission and extinction ratio are effectively improved in broad LWIR band with the proposed structure. Besides, the angle-tolerance indicates that the design has great capability in applications with wide field angle.
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图 1 矩形面型光栅几何结构图.
Figure 1. An illustration of the geometry for the rectangulargroove grating.
图 2 10 mm厚ZnSe透光率.
Figure 2. Transmission data measured through a 10 mm thick ZnSe substrate from Thorlabs.
图 3 光栅结构剖面图. (a)铝层不完全刻蚀. (b)铝层完美刻蚀. (c)硒化锌层过度刻蚀.
Figure 3. Profile of the designed grating. (a) Incomplete etching of aluminum layer. (b) Fine etching of aluminum layer. (c) Excessive etching of zinc selenide layer.
图 4 四种金属材料对偏振性能的影响. (a)金属材料对TM透射率的影响. (b)金属材料对消光比的影响.
Figure 4. Influences on polarization performance from four different metal materials. (a) TM transmission results for various metal materials. (b) Extinction ratio results for various metal materials.
图 7 偏振性能与栅槽深度关系. (a)铝层深度对双层光栅性能影响. (b)硒化锌深度对双层光栅性能影响.
Figure 7. Relationship of polarization performances and grating depths. (a) Results of varying aluminum depth on dual-layered grating. (b) Results of varying ZnSe depth on dual-layered grating.
图 8 入射角度对单层光栅与设计光栅透光性能影响.
Figure 8. Transmission performances of monolayer gratings and designed grating with light incoming from varying angles.
图 9 单层光栅与设计光栅在LWIR透光性能比较.
Figure 9. Comparison diagram of transmission performances between monolayer gratings and designed grating in the LWIR.
图 10 铝/硒化锌双层光栅TM波透射衍射效率.
Figure 10. TM transmission results of the designed bi-layered grating.
表 1 不同槽深对偏振性能的影响结果.
Table 1. Results of polarization performances in different layer depths.
h1/μm h2/μm TTM /% ER/dB 0.4 0.2 87.62 29 0.6 0.4 92.84 41 0.4 0.4 89.57 40 0.8 0.2 92.85 31 0.4 0.6 90.80 50 0.8 0.4 95.62 41 0.6 0.2 90.26 30 0.9 0.8 98.64 62 
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