Abstract-Graphene aperture-based metalens for dynamic focusing of terahertz waves

Pei Ding, Yan Li, Li Shao, Ximin Tian, Junqiao Wang, and Chunzhen Fan

Fig. 3 (a) Phase distributions 𝑝(𝑥)$p(x)$of the metalens with a focal length of F = 150 um (p1) and 190 um (p2), respectively, at the incident frequency of f0 = 5 THz. The phase difference between the two lenses (𝑝2−𝑝1$p_2-p_1$) along the x axis is also illustrated. (b) E-field intensity distribution of the reflection field on the x-z plane for the metalens with a designed focal length F = 150 um at f0 = 5 THz for Ef = 1.0 eV. The simulated focal length is 140 um. (c) E-field intensity distribution of the reflected wave along the z-axis for different Ef. The simulated focal lengths remain unchanged. (d) The corresponding E-field intensity distributions of the reflected wave on the focal plane (z = 140 um) for different Ef.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-21-28038

We theoretically study a tunable reflective focusing lens, based on graphene metasurface, which consists of rectangle aperture array. Dynamic control of either the focal intensity or focal length for terahertz circular polarized waves can be achieved by uniformly tuning the graphene Fermi energy. We demonstrate the graphene apertures with the same geometry; however, spatially varying orientations can only control the focal intensity. To change the focal length, the spatially varying aperture lengths are also required. A comparative study between the metalenses, which generate only geometric or both gradient and geometric phase changes, has shown that the apertures’ spatially varying length distribution is the key factor for determining the modulation level, rather than the focal length’s modulation range. This kind of metalens provides tunable, high-efficiency, broadband, and wide-angle off-axis focusing, thereby offering great application potential in lightweight and integrated terahertz devices.

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