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摘要:
本文介绍了一种能够产生超强旋光性的弱手性极薄(λ/10)亚波长结构的超表面。该超表面的单元结构由介质层以及设置在介质层上下表面的两层椭圆金属贴片组成，两层椭圆金属贴片之间存在一个扭转角的关系。当扭转角为80°时，超表面在谐振频点11.89 GHz处能将入射线偏振电磁波转换为交叉极化透射波，其透过率超过94%。这种超表面重量轻、体积小，为极化旋转提供了一种可靠的方法。若将其拓展到光波波段，该超表面在弱手性分子的生物学检测中有潜在的应用。
Abstract:A preternatural and extremely thin metasurface with weak asymmetric unit structure is presented here to demonstrate extraordinary strong chirality. The unit cell of metasurface is composed of a double layer of elliptical metal patches with a certain twisted angle and a medium sandwiched between them. When the twisted angle equals to 80°, optical activity can be realized in this metasurface. At the resonant frequency 11.89 GHz, the incident linearly polarized wave is converted into its cross-polarization wave with the transmittance rate higher than 94%. The light weight and miniaturization of this metasurface provide a reliable approach for polarization manipulation. If extended to light waveband, the metasurface may have potentials in biological applications such as detection of weak chiral molecules, etc.
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Key words:
- metasurface /
- chirality /
- extraordinary /
- optical activity
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Overview: A preternatural and extremely thin (λ/10) metasurface with weak asymmetric unit structure is presented here to demonstrate extraordinary strong chirality. The unit cell of metasurface is composed of a double layer of elliptical metal patches with a certain twisted angle and a medium sandwiched between them. The relationship between two elliptical metal structures are not orthogonal, but there is a twisted angle ϕ=90-2θ=80°(θ=5°) around their normal axis in z direction. Therein θ indicates the included angle between the long axis of the metallic elliptic and its adjacent coordinate axis. Optical activity is realized in this metasurface and the incident linearly polarized wave is converted into its cross-polarization wave at the resonant frequency with the cross-polarization transmittance rate higher than 94% at center frequency 11.89 GHz.
Inaddition, the polarization rotation characters of the metasurface under other different included angles θ are studied. When the included angle θ=0°, the transmission of the x- and y-polarized components are both close to 0 at the 11.89 GHz, which proves that the unit cell is achiral and presents giant reflective character in this situation; when the included angle θ varied with the step of 5° from 10° to 25°, the resonance peak of cross polarization transmission wave will split into two, and the two split resonant peaks shift to both sides with the enlargement of the rotation angle.
The chiral characters of the metasurface are studed by observing the surface current distributions of the subwavelength structure. When θ=0°, the subwavelength structure generates symmetrical surface current distribution, so the superimposed field intensity of the induced electric field in the x direction is zero. That is, the structure of subwavelength element is isotropic, and there is no chirality. When θ=5°, due to the asymmetry of the structure and strong coupling between the layers, the y-polarization of the incident electromagnetic wave in the subwavelength structure spark surface current distribution along x direction, so as to realize the transformation of polarization, which demonstrate extraordinary strong chirality. When θ larger than 10°, the surface current distribution modes of unit cells generate corresponding to two different polarization rotation frequencies, which is demonstrate that there is double frequency chirality in the metasurface.
The light weight and miniaturization of this metasurface provide a reliable approach for polarization manipulation. If extended to light waveband, the chiral metasurface may have potentials in biological applications such as detection of weak chiral molecules etc.
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Figure 1. (a) The unit cell of the proposed polarization rotator with weak chirality; (b) Operating principle diagram of the polarization rotator
Figure 2. The transmission curves of the proposed unit cell when θ equals to 5° (a) and 0° (b)
Figure 3. Cross-polarization transmission curves vs. dielectric parameters of the substrate
Figure 6. The changing situation of the transmission with the unit cell at different little rotate angles
Figure 7. The changing situation of the transmission with the unit cell at different large included angles
Figure 8. The surface current distributions of unit cells with the included angle θ=5° ((a), (b)) and θ=0° (c)
Figure 9. The surface current distributions of unit cells with the included angle θ=15° at frequencies of 11.60 GHz (a) and 12.14 GHz (b)
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