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Broadband terahertz tunable metasurface linear polarization converter based on graphene
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摘要:
本文提出了一种基于椭圆形镂空石墨烯的太赫兹宽带可调超表面反射线偏振转换器，通过模拟仿真和法布里-佩罗多重干涉理论进行了验证。设计的超表面模型类似三明治结构，分别由顶层各向异性的椭圆形镂空石墨烯结构、中间介质层和底层金属板组成。仿真结果表明：当给定的石墨烯弛豫时间和费米能级分别为τ=1.0 ps, μc=0.9 eV时，设计的超表面结构偏振转换率(PCR)在0.98 THz~1.34 THz的频率范围内超过90%，相对带宽为36.7%。另外，在谐振频点1.04 THz和1.28 THz，PCR分别高达99.8%和97.7%，这说明设计的超表面可以将入射的垂直(水平)线偏振波转换为反射的水平(垂直)线偏振波。通过法布里-佩罗多重干涉理论进一步验证了该模型，理论预测与数值仿真结果吻合得比较好。此外，设计的超表面反射线偏振转换特性可以通过改变石墨烯的费米能级和电子弛豫时间来动态的调节。因此，设计的基于石墨烯的可调超表面偏振转换器在太赫兹通信、传感以及太赫兹光谱领域具有潜在的应用价值。
Abstract:A terahertz broadband tunable reflective linear polarization converter based on oval-shape-hollowed graphene metasurface is proposed and verified by simulation and Fabry-Perot multiple interference theory in this paper. Our designed metasurface model is similar to a sandwiched structure, which is consisted of the top layer of anisotropic elliptical perforated graphene structure, an intermediate dielectric layer and a metal ground plane. The simulation results show that when the given graphene relaxation time and Fermi energy are τ=1 ps and μc=0.9 eV, respectively, the polarization conversion rate (PCR) of the designed metasurface structure is over 90% in the frequency range of 0.98 THz~1.34 THz, and the relative bandwidth is 36.7%. In addition, at resonance frequencies of 1.04 THz and 1.29 THz, PCR is up to 99.8% and 97.7%, respectively, indicating that the metasurface we designed can convert incident vertical (horizontal) linearly polarized waves into reflected horizontal (vertical) linearly polarized waves. We used the Fabry-Perot multi-interference theory to further verify the metasurface model. The theoretical predictions are in good agreement with the numerical simulation results. In addition, the designed metasurface reflective linear polarization conversion characteristics can be dynamically adjusted by changing the Fermi energy and electron relaxation time of graphene. Therefore, our designed graphene-based tunable metasurface polarization converter is expected to have potential application value in terahertz communication, sensing and terahertz spectroscopy.
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Key words:
- linear polarization converter /
- metasurface /
- graphene /
- interference theory
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Overview:In recent years, terahertz science and related technologies have emerged as one of the rapidly evolving technologies, and they have shown good application prospects in the fields of communication, imaging, sensing and non-destructive testing. These applications require not only efficient terahertz sources, but also high-performance terahertz devices such as modulators, polarization converters, and more. At present, there are relatively few materials in nature that can effectively manipulate terahertz waves, and the corresponding devices are quite scarce. In order to promote the application of terahertz technology in the above related fields, the effective regulation of terahertz waves is particularly important. Since many applications of terahertz waves are related to their polarization states, terahertz polarization converters that control polarization states have become an important research direction. Conventional terahertz polarization converters are usually made of materials based on grating structure and dispersion, and generally have problems such as narrow frequency band and low efficiency, which greatly limits the practical application range. Therefore, it is very important to design and prepare high performance terahertz polarization control devices. Because graphene has very good optical transparency, adjustable electromagnetic properties and high electron mobility, it can be widely used in the design of optoelectronic devices. In addition, the addition of a bias voltage to the graphene can change its Fermi level and electron relaxation time, thereby achieving dynamic adjustment of its electromagnetic properties. In this paper, a terahertz broadband tunable reflective linear polarization converter based on oval-shape-hollowed graphene metasurface is proposed and verified by simulation and Fabry-Perot multiple interference theory. Our designed metasurface model is similar to a sandwiched structure, which is consisted of the top layer of anisotropic elliptical perforated graphene structure, an intermediate dielectric layer and a metal groundplane. The simulation results show that when the given graphene relaxation time and Fermi energy are τ=1 ps and μc=0.9 eV, respectively, the polarization conversion rate (PCR) of the designed metasurface structure is over 90% in the frequency range of 0.98 THz~1.34 THz, and the relative bandwidth is 36.7%. In addition, at resonance frequencies of 1.04 THz and 1.29 THz, PCR is up to 99.8% and 97.7%, respectively, indicating that the metasurface we designed can convert incident vertical (horizontal) linearly polarized waves into reflected horizontal (vertical) linearly polarized waves. We used the Fabry-Perot multi-interference theory to further verify the metasurface model. The theoretical predictions are in good agreement with the numerical simulation results. In addition, the designed metasurface reflective linear polarization conversion characteristics can be dynamically adjusted by changing the Fermi energy and electron relaxation time of graphene. Therefore, our designed graphene-based tunable metasurface polarization converter is expected to have potential application value in terahertz communication, sensing and terahertz spectroscopy.
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图 1 超表面的设计方案。(a)，(b)单元结构的正视图和立体视图；(c)三维(3D)阵列结构
Figure 1. The design scheme of the metasurface. (a), (b) The front and perspective views of the unit-cell structure; (c) Three dimen-sional (3D) array structure
图 2 在固定的驰豫时间τ=1.0 ps时不同μc下电导率的(a)实部和(b)虚部
Figure 2. The (a) real part and (b) imaginary part of the conductivity with fixed relaxation time τ=1.0 ps under different μc
图 3 在固定的化学势μc=0.9 eV时不同τ下电导率的(a)实部和(b)虚部
Figure 3. The (a) real part and (b) imaginary part of the conductivity with fixed μc=0.9 eV under different τ
图 4 设计的超表面在τ=1.0 ps, μc=0.9 eV下仿真的(a)反射系数和(b)偏振转换率
Figure 4. The simulated reflection coefficients (a) and γx(y) (b) of the designed metasurface with τ =1.0 ps, μc=0.9 eV
图 5 (a) 线偏振波与超表面单元结构相互作用后的电场矢量分解示意图；(b), (c)分别为表层结构在谐振频点1.04 THz和1.28 THz处的表面电流密度分布。其中黑色的粗线箭头表示电流流动方向
Figure 5. (a) Schematic diagram of electric field vector decomposition after interaction of linearly polarized waves and unit-cell structure of metasurface; (b), (c) are the surface current density distributions of front layer surface structures at resonance frequencies of 1.04 THz and 1.28 THz, respectively. Where the thick black arrow indicates the direction of current flow
图 6 在法布里-佩罗谐振腔中x轴偏振波传播的示意图
Figure 6. Schematic sketch of the x-pol. wave propagation in a Fabry-Perot like resonance cavity
图 7 设计的超表面在τ=1.0 ps, μc=0.9 eV时正入射x轴偏振波的仿真和计算得到的(a)反射系数和(b)偏振转换效率(γx)
Figure 7. The simulated and calculated (a) reflection coefficients and (b) γx of the designed metasurface with τ=1.0 ps, μc=0.9 eV under normal incident x-pol. wave
图 8 设计的超表面在固定的驰豫时间τ =1.0 ps时正入射x轴偏振波下不同μc时的(a)仿真和(b)计算得到的偏振转换效率(γx)
Figure 8. The (a) simulated and (b) calculated γx of the designed metasurface with fixed relaxation time τ =1.0 psand different μc under normal incident -pol. wave
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