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Efficiency-tunable terahertz focusing lens based on graphene metasurface
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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.-
Keywords:
- metasurface /
- focusing lens /
- graphene /
- terahertz
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References
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Overview
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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Figure 1.
(a) Focusing schematic diagram of the electromagnetic beam perpendicularly incident on the metasurface; (b) The schematic diagram of the unit cell
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Figure 2.
The (a) real and (b) imaginary parts of the conductivity at different EF when the relaxation time τ is 1 ps
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Figure 3.
The transmission amplitude at different Fermi levels at a fixed rotation angle θ=0°
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Figure 4.
(a) The transmission amplitude and (b) the phase diagram at different rectangular rotation angles when Fermi level is 0.9 eV
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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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Figure 6.
The profile distributions of the intensity at EF=0.1 eV, 0.9 eV in the x-o-z plane, when the cross polarized light is along (a) x and (b) z axes
