Spin-controlled wavefront shaping with plasmonic chiral geometric metasurfaces

Yang Chen,Xiaodong Yang,Jie Gao

doi:10.1038/s41377-018-0086-x

Yang Chen, Xiaodong Yang + Show 1 more

Open Access

https://doi.org/10.1038/s41377-018-0086-x

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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.

Highlights

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 functionalities[10,20]
To simultaneously achieve high circular dichroism and a large cross-polarization ratio in transmitted light, plasmonic stepped nanoapertures etched in an optically thick gold film are proposed as unit cells (Fig. 1a)
In summary, we have proposed and experimentally demonstrated one unique type of chiral geometric metasurface based on plasmonic stepped nanoapertures to achieve spin-controlled wavefront shaping functionalities, including the realization of both chiral metasurface holograms and the spin-controlled generation of hybridorder Poincaré sphere beams

Summary

Introduction

Metasurfaces composed of ultrathin metallic or dielectric nanostructures with subwavelength size and spacing[1,2,3,4] that are able to fully control the electromagnetic wavefront have recently been developed for many applications, such as flat optical elements[5,6,7,8,9], holograms[10,11,12,13,14], and vortex beam generation[15,16,17,18,19]. 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 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. 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 functionalities[10,20]. 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 distributions[22]. Several approaches employing an off-axis design have presented spin-dependent performance, but complicated optical setups and metasurface

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