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本文提出一种新型超表面结构, 通过改变工作温度能够实现透反双模态极化转换与超宽带吸收等功能切换. 当二氧化钒(VO2)为金属态且碲化锗(GeTe)为晶态时, 太赫兹波沿–z方向入射, 该超表面在7.96—17.76 THz频带表现为超宽带吸收器, 吸收率大于90%. 太赫兹波沿+z方向入射, 该超表面在2.04—4.44 THz频带表现为对x–/y–偏振波的反射极化转换, 极化转换率大于0.9. 当VO2为介质态且GeTe为非晶态时, 该超表面在0.65—5.07 THz频带表现为对x偏振波的透射极化转换, 极化转换率大于0.9. 研究结果表明, 该超表面结构对太赫兹波操控具有双向、可切换和多功能特点, 在太赫兹波传感、成像和通信领域具有广阔的应用前景.In this paper, we propose a vanadium dioxide and germanium telluride composite metasurface. The conductivity of vanadium dioxide and germanium telluride is varied by changing the temperature, which enables the switching of functions such as ultra-broadband absorption, reflective-type polarization, and transmissive-type polarization. When vanadium dioxide is metallic and germanium telluride is crystalline, the terahertz wave is incident along the –z direction, and the metasurface can be used as a broadband absorber, with an absorption rate greater than 90% in a frequency range of 7.96–17.76 THz, and the absorption bandwidth reaches 9.8 THz, with a relative bandwidth of 76.2%. In addition, the designed metasurface absorber is polarization-insensitive and exhibits good absorption performance at large incidence angles. Terahertz waves are incident along the +z direction, and this metasurface can be used as a reflective polarization converter with a polarization conversion ratio greater than 0.9 for x– and y–polarized waves in the frequency band from 2.04 to 4.44 THz. The effects of incidence angle and structural parameters on polarization conversion performance are also investigated. When vanadium dioxide is in the dielectric state and germanium telluride is in the amorphous state, the metasurface can be used as a transmissive polarization converter, with a polarization conversion rate of greater than 0.9 in a frequency band of 0.65–5.07 THz . And the high polarization conversion performance can be maintained in a wide range of incidence angles. Finally, the physical mechanism of polarization conversion is analyzed using surface currents. The results show that the metasurface structure has bi-directional, switchable and multi-functional characteristics for terahertz wave manipulation, and has broad application prospects in terahertz wave sensing, imaging and communication.
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Keywords:
- reflective polarization conversion /
- transmissive polarization conversion /
- ultra-broadband absorption /
- metasurface
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图 1 太赫兹超表面阵列示意和单元结构图 (a)超表面结构示意图; (b)单元结构; (c)顶层图案俯视图; (d)底层图案俯视图
Fig. 1. Schematic of the array and cell structure of terahertz metasurface: (a) Schematic of the terahertz metasurface; (b) cell structure; (c) top view; (d) bottom view.
图 2 太赫兹波沿–z 方向入射时超表面电磁响应曲线 (a)吸收、反射和透射曲线; (b)等效阻抗实部和虚部曲线
Fig. 2. Electromagnetic response curves of the metasurface for terahertz waves incident along the –z direction: (a) Absorption, reflection, and transmission curves; (b) equivalent impedance real and imaginary curves.
图 3 太赫兹波沿–z方向入射时不同组合图案对应的太赫兹波吸收曲线
Fig. 3. Terahertz wave absorption curves corresponding to different combinations of patterns when terahertz waves are incident along the –z direction.
图 4 太赫兹波沿–z方向入射吸收器谐振频点处的电磁场分布 (a)—(c) TE模式和(d)—(f) TM模式下, 谐振频点处的电场分布俯视图; (g)—(i) TE模式下, 谐振频点处的磁场分布侧视图
Fig. 4. Electromagnetic field distribution at the resonant frequency points of the absorber for a terahertz wave incident along the –z direction: Top view of the electric field distribution at the resonant frequency point in (a)–(c) TE mode and (d)–(f) TM mode; (g)–(i) side view of the magnetic field distribution at the resonant frequency point in TE mode.
图 5 太赫兹波沿–z方向入射时入射角与极化角对吸收性能的影响 (a) TE模式和(b) TM模式下, 不同入射角对吸收性能的影响; (c)不同极化角对吸收性能的影响
Fig. 5. Effect of incidence angle and polarization angle on absorption performance for terahertz waves incident along –z direction: Effect of different incidence angles on the absorption performance in (a) TE mode and (b) TM mode; (c) effect of different polarization angles on the absorption performance.
图 6 太赫兹波沿+z方向入射下超表面的电磁响应曲线 (a)反射系数; (b)极化转换率PCR
Fig. 6. Electromagnetic response curves of the metasurface under the incidence of terahertz waves along the +z direction: (a) Reflection coefficient; (b) polarization conversion rate PCR.
图 7 太赫兹波沿+z方向入射下, 不同极化角对PCR影响 (a) x偏振波入射下, 不同极化角对PCR影响; (b) y偏振波入射下, 不同极化角对PCR影响
Fig. 7. Effect of different polarization angles on PCR under the incidence of terahertz waves along the +z direction: (a) The effect of different polarization angles on PCR under x–polarized wave incidence; (b) the effect of different polarization angles on PCR under y–polarized wave incidence.
图 8 太赫兹波沿+z方向入射下, 不同入射角对PCR影响 (a) x偏振波入射下, 不同入射角对PCR影响; (b) y偏振波入射下, 不同入射角对PCR影响
Fig. 8. Effect of different incidence angles on PCR under the incidence of terahertz waves along the +z direction: (a) The effect of different incidence angles on PCR under x–polarized wave incidence; (b) the effect of different incidence angles on PCR under y–polarized wave incidence.
图 9 太赫兹波沿+z方向入射下, 不同结构参数对PCRx影响 (a)两边矩形谐振器宽度w3; (b)中间条形谐振器宽度w4; (c)外环半径R1; (d)内环半径R2
Fig. 9. Effect of different structural parameters on PCRx under the incidence of terahertz wave along +z direction: (a) Width of the rectangular resonator on both sides w3; (b) width of the strip resonator in the middle w4; (c) outer ring radius R1; (d) inner ring radius R2.
图 10 太赫兹波沿+z方向入射下超表面的太赫兹电磁响应曲线 (a)透射系数; (b)极化转换率PCR
Fig. 10. Electromagnetic response curves of the metasurface under the incidence of terahertz waves along the +z direction: (a) Transmission coefficient; (b) polarization conversion rate PCR.
图 11 太赫兹波沿+z方向入射下入射角变化对极化转换率PCR影响
Fig. 11. Effect of variation of incidence angle on polarization conversion rate PCR under terahertz wave incidence along +z direction.
图 12 太赫兹波沿+z方向入射下, 顶层、中间和底层结构在谐振频点处的电流分布图, 不同谐振频率下 (a), (d)顶层十字架结构; (b), (e)中间光栅层; (c), (f)底层“工”形结构的电流分布Fig. 12. Current distributions of the top, intermediate and bottom layers of the structure at the resonance frequencies under terahertz wave incidence along the +z direction, current distributions at different resonant frequencies for (a), (d) the top cross structure; (b), (e) the intermediate grating layer; (c), (f) the bottom “工” structure.
表 1 本文提出结构与其他文献报道成果对比
Table 1. Comparison of the proposed structure in this paper with previously reported works.
文献 可调材料 功能 性能 带宽 [18] Graphene 宽带吸收和极化转换 1.74—3.52 THz: A≥90%
1.54—2.55 THz: PCRr≥90%吸收1.78 THz
反射极化转换1.01 THz[19] VO2和Si 宽带吸收和极化转换 0.68—1.60 THz: A≥90%
0.82—1.60 THz: PCRr≥90%吸收0.92 THz
反射极化转换0.78 THz[20] VO2 宽带吸收和极化转换 3.33—5.62 THz: A≥90%
2.54—4.55 THz: PCRr≥90%吸收2.29 THz
反射极化转换2.01 THz[10] VO2 宽带吸收和极化转换 1.49—3.58 THz: A≥90%
1.1—3.2 THz: PCRr≥90%吸收2.09 THz
反射极化转换2.1 THz本文 VO2和GeTe 宽带吸收和极化转换 7.96—17.76 THz: A≥90%
2.04—4.44 THz: PCRr≥90%
0.65—5.07 THz: PCRt≥90%吸收9.8 THz
反射极化转换2.4 THz
透射极化转换4.42 THz
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