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摘要:
本文提出了一种基于石墨烯超表面的效率可调太赫兹聚焦透镜。该超表面单元结构由两层对称的圆形镂空石墨烯和中间介质层组成，其中镂空圆形中间由长方形石墨烯片连接。该结构可实现偏振转换，入射到超表面的圆偏振波将以其正交的形式出射，如左旋圆到右旋圆偏振转换。利用几何相位原理，通过旋转长方形条的方向，透射波会携带额外的附加相位并能满足2π范围内覆盖。合适地排列石墨烯超表面的单元结构，以实现太赫兹聚焦透镜。仿真结果表明：通过改变石墨烯的费米能级，可以对超表面圆偏振转换幅度进行调节，进而超透镜的聚焦效率也可以动态调节。因此，这种基于石墨烯超表面的效率可调聚焦透镜不用改变单元结构的尺寸，只需通过改变费米能级便可实现，可以广泛地应用到能量收集、成像等太赫兹应用领域。
Abstract:This paper proposes an efficiency-tunable terahertz focusing lens based on the graphene metasurface. The unit cell is composed of two symmetrical circular graphene hollows and an intermediate dielectric layer, wherein the hollow circular middle is connected by a rectangular graphene sheet. This structure can realize polarization conversion, for example, when an incidence with left-hand circular polarization emitted on the metasurface the polarization of the transmitted light is right-hand circular polarization. According to the principle of geometric phase, by rotating the direction of the rectangular bar, the transmitted wave will carry an additional phase and can cover the range of 2π. An THz focusing lens can be realized by properly arranging the unit structure of the graphene metasurface. The simulation results show that the conversion amplitude of circular polarized light can be adjusted by changing the Fermi level of graphene, and the focusing efficiency of the metalens can also be dynamically adjusted. Therefore, this graphene metasurface-based efficiency-tunable focusing lens can be realized by changing the Fermi level without changing the size of the unit cell, and can be widely used in terahertz applications such as energy harvesting and imaging.
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Key words:
- metasurface /
- focusing lens /
- graphene /
- terahertz
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Overview: An efficiency-adjustable terahertz (THz) focusing lens based on the graphene metasurface is proposed. The unit cell is composed of two symmetrical circular graphene hollows and an intermediate dielectric layer, wherein the middle of the hollow circular is connected by a rectangular graphene sheet. This structure can realize circular polarization conversion, for example, the left-handed circularly polarized wave incident on the metasurface will exude in the right-handed polarized form. According to the principle of geometric phase, the full 2π additional phase-shift of transmitted cross-polarized wave can be obtained by rotating the direction of the rectangular graphene. Thereby, a focusing lens with a good performance can be realized by arranging these unit cells properly. Because of the flexible and controllable optical characteristics, graphene has obvious advantages in the construction of dynamically tunable metasurfaces. By adjusting the voltage, the Fermi level of the graphene can be changed, and the conductivity can also be manipulated artificially. The numerical simulation was carried out based on the time-domain finite-element method. The simulation results show that the conversion amplitude of the circular polarization can be adjusted by changing the Fermi level of the graphene. When the Fermi level is 0.9 eV, the cross-polarization transmission coefficient of the proposed graphene metasurface reaches a maximum of 0.55 at 1.42 THz, and the transmission amplitude of the metasurface increases with the increase of the Fermi level at 1.42 THz. In addition, the resonance frequency of the circular polarization conversion based on the graphene metasurface shows a certain blue shift with the decrease of Fermi level. By arranging the unit cells of the metasurface properly, we can construct an efficiency-adjustable metalens. When the Fermi level is 0.9 eV, the simulate focal length of the proposed metalens is 2.03 mm, which is consistent well with the preset theoretical of 2 mm. However, when the graphene Fermi level is 0.1 eV, the cross-circularly polarized wave passing through the graphene metalens is almost 0, which means the incident THz wave cannot be converted into spherical wave. This adjustable graphene metasurface turns into an on-off focusing lens. Different from other traditional lens, such an efficiency-adjustable THz focusing lens based on graphene metasurface has many advantages, such as simple device structure, adjustable efficiency, reconfigurable, and it has potential application value in THz imaging, high-resolution terahertz displays, communications and so on.
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图 1 (a) 电磁波束垂直入射超表面的聚焦示意图；(b) 超表面单元结构视图
Figure 1. (a) Focusing schematic diagram of the electromagnetic beam perpendicularly incident on the metasurface; (b) The schematic diagram of the unit cell
图 2 弛豫时间τ =1 ps时不同EF下电导率的(a) 实部；(b) 虚部
Figure 2. The (a) real and (b) imaginary parts of the conductivity at different EF when the relaxation time τ is 1 ps
图 3 在固定的旋转角θ=0°时不同EF下的透射幅度
Figure 3. The transmission amplitude at different Fermi levels at a fixed rotation angle θ=0°
图 4 费米能级为0.9 eV时长方形旋转角度不同下的(a)透射幅度和(b)相位图
Figure 4. (a) The transmission amplitude and (b) the phase diagram at different rectangular rotation angles when Fermi level is 0.9 eV
图 5 (a) 费米能级为0.1 eV；(b) 费米能级为0.9 eV入射光频率为1.4 THz下仿真得到的聚焦效果图
Figure 5. The simulation results of Fermi levels are (a) 0.1 eV and (b) 0.9 eV at the working frequency of 1.4 THz
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