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Register a new userNear-zero-sidelobe optical subwavelength asymmetric focusing lens with dual-layer metasurfaces
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The existing metasurfaces with ultrathin volume for asymmetric transmission were often constructed by metal with low efficiency in optical frequency, and could not realize the optical asymmetric transmission and focusing simultaneously. Although the acoustic asymmetric focusing in asymmetric focusing lens (AFL) was realized, the obtained focal point was accompanied by large sidelobe. To solve those problems, an AFL with dual-layer metasurfaces designed by using dielectric materials can realize optical asymmetric transmission and focusing simultaneously in this paper. Furtherly, through optimizing the design theory of AFL, the near-zero-sidelobe (NZS) focusing can be realized on the subwavelength scale. The NZS asymmetric focusing of the optimized AFL is effective in broadband, which is expected to possess tremendous potential in the optical micro-nano processing, quantum communication, optical micromanipulation, etc.
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Metasurfaces with tunable spatial phase functions could benefit numerous applications. Currently, most approaches to tuning rely on mechanical stretching which cannot control phase locally, or by modulating the refractive index to exploit rapid phase changes with the drawback of also modulating amplitude. Here, we propose a method to realize phase modulation at subwavelength length scales while maintaining unity amplitude. Our device is inspired by an asymmetric Fabry-Perot resonator, with pixels comprising a scattering nanopost on top of a distributed Bragg reflector, capable of providing a nearly 2{pi} nonlinear phase shift with less than 2% refractive index modulation. Using the designed pixels, we simulate a tunable metasurface composed of an array of moderately coupled nanopost resonators, realizing axicons, vortex beam generators, and aspherical lenses with both variable focal length and in-plane scanning capability, achieving nearly diffraction-limited performance. The experimental feasibility of the proposed method is also discussed.
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