Spin-controlled wavefront shaping with plasmonic chiral geometric metasurfaces

Chen et al. Light: Science & Applications (2018)7:84 DOI 10.1038/s41377-018-0086-x ARTICLE Official journal of the CIOMP 2047-7538 www.nature.com/lsa Open Access Spin-controlled wavefront shaping with plasmonic chiral geometric metasurfaces 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Yang Chen1, Xiaodong Yang 1 and Jie Gao1 Abstract Metasurfaces, as a two-dimensional (2D) version of metamaterials, have drawn considerable attention for their revolutionary capability in manipulating the amplitude, phase, and polarization of light. As one of the most important types of metasurfaces, geometric metasurfaces provide a versatile platform for controlling optical phase distributions due to the geometric nature of the generated phase profile. However, it remains a great challenge to design geometric metasurfaces for realizing spin-switchable functionalities because the generated phase profile with the converted spin is reversed once the handedness of the incident beam is switched. Here, we propose and experimentally demonstrate chiral geometric metasurfaces based on intrinsically chiral plasmonic stepped nanoapertures with a simultaneously high circular dichroism in transmission (CDT) and large cross-polarization ratio (CPR) in transmitted light to exhibit spin-controlled wavefront shaping capabilities. The chiral geometric metasurfaces are constructed by merging two independently designed subarrays of the two enantiomers for the stepped nanoaperture. Under a certain incident handedness, the transmission from one subarray is allowed, while the transmission from the other subarray is strongly prohibited. The merged metasurface then only exhibits the transmitted signal with the phase profile of one subarray, which can be switched by changing the incident handedness. Based on the chiral geometric metasurface, both chiral metasurface holograms and the spin-dependent generation of hybrid-order Poincaré sphere beams are experimentally realized. Our approach promises further applications in spin-controlled metasurface devices for complex beam conversion, image processing, optical trapping, and optical communications. Introduction Metasurfaces composed of ultrathin metallic or dielectric nanostructures with subwavelength size and spacing1–4 that are able to fully control the electromagnetic wavefront have recently been developed for many applications, such as flat optical elements5–9, holograms10–14, and vortex beam generation15–19. Among the various types of metasurfaces, geometric metasurfaces have drawn the greatest attention for their superior capability in optical phase manipulation20,21. The geometric phase or Pancharatnam–Berry phase is introduced by rotating the metallic or dielectric nanostructure in the unit cell when the circularly polarized incident beam is converted to the Correspondence: Xiaodong Yang () or Jie Gao (gaojie@mst. edu) 1 Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA output beam with the opposite handedness. Compared with other types of metasurfaces, geometric metasurfaces can operate over a broad spectrum with generated phase distributions that are robust against fabrication tolerance and material property variations. However, when the incident beam and the converted output beam change their handedness simultaneously, the sign of the geometric phase produced by the metasurface is reversed, which has limited the ability of geometric metasurfaces to implement spin-switchable functionalities10,20. Combining the geometric phase with the propagation phase can overcome this problem, but at the cost of losing the broadband and robust phase properties since the shapes of the nanostructures start to influence the generated phase distributions22. Several approaches employing an off-axis design have also presented spin-dependent performance, but complicated optical setups and metasurface © The Author(s) 2018 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Chen et al. Light: Science & Applications (2018)7:84 designs are required, and the phase reversal issue remains unsolved23,24. In addition, chiral supercells designed through collective spin-selective destructive or constructive interference have also been realized. However, supercell-based metasurfaces are inherently sophisticated in both design and fabrication, and the pixel size is usually larger than the wavelength25,26. An alternative scheme to realize spin-controllable geometric metasurfaces is to consider chiral nanostructures as unit cells. By constructing two independently designed subarrays of the two enantiomers of chiral nanostructures and then combining the subarrays into one metasurface, spin-controlled wavefront shaping can be enabled. When a circularly polarized wave with a certain handedness, say right-handed circularly polarized (RCP), is incident on the merged metasurface, the transmission from one subarray, say subarray A, is allowed, while the transmission from the other subarray (subarray B) is strongly suppressed due to the unit-cell chirality. Then, the merged metasurface only shows the transmitted signal with the phase distribution of subarray A. Once the incident handedness is switched from RCP to left-handed circularly polarized (LCP), the generated phase profile of subarray A is still reversed, as in ordinary geometric metasurfaces, but the transmission through subarray A is substantially prohibited, and thus, the merged metasurface only exhibits the phase distribution of subarray B in the transmission. However, the main obstacle for this scheme lies in the design of the chiral nanostructure unit cells. Twodimensional chiral nanostructures are not truly chiral and thus suffer from weak chiroptical responses27–30, while three-dimensional (3D) chiral nanostructures are difficult to fabricate with tailored orientation angles31–34. Moreover, the low cross-polarization ratio in the transmitted light, as a common issue for geometric metasurfaces, must also be solved. In this work, we report a new type of chiral geometric metasurface based on plasmonic stepped nanoapertures to demonstrate spin-controlled wavef (...truncated)