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1. Design of Planar Penetrable Metasurfaces on an Irregular Grid Using Point-Dipole Approximation

   Kwon, Do-Hoon ( July 2020 , International symposium digest antennas and propagation)

   A design technique for free-standing planar metasurfaces comprising an array of subwavelength resonant inclusions on an irregular grid is presented. The local E-field is evaluated as a sum of discrete and continuous contributions from neighboring and far-separated elements, respectively. The dimension of each resonator is determined from the polarizability relation. Free from the limitations associated with unit-cell analysis and design under periodic boundary conditions, the new design technique allows use of irregular grids for functional electromagnetic surfaces. 

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2. Compounding Meta‐Atoms into Metamolecules with Hybrid Artificial Intelligence Techniques

   https://doi.org/10.1002/adma.201904790  

   Liu, Zhaocheng ; Zhu, Dayu ; Lee, Kyu‐Tae ; Kim, Andrew S. ; Raju, Lakshmi ; Cai, Wenshan ( December 2019 , Advanced Materials)

   Molecules composed of atoms exhibit properties not inherent to their constituent atoms. Similarly, metamolecules consisting of multiple meta‐atoms possess emerging features that the meta‐atoms themselves do not possess. Metasurfaces composed of metamolecules with spatially variant building blocks, such as gradient metasurfaces, are drawing substantial attention due to their unconventional controllability of the amplitude, phase, and frequency of light. However, the intricate mechanisms and the large degrees of freedom of the multielement systems impede an effective strategy for the design and optimization of metamolecules. Here, a hybrid artificial‐intelligence‐based framework consolidating compositional pattern‐producing networks and cooperative coevolution to resolve the inverse design of metamolecules in metasurfaces is proposed. The framework breaks the design of the metamolecules into separate designs of meta‐atoms, and independently solves the smaller design tasks of the meta‐atoms through deep learning and evolutionary algorithms. The proposed framework is leveraged to design metallic metamolecules for arbitrary manipulation of the polarization and wavefront of light. Moreover, the efficacy and reliability of the design strategy are confirmed through experimental validations. This framework reveals a promising candidate approach to expedite the design of large‐scale metasurfaces in a labor‐saving, systematic manner.

    

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3. High‐Efficiency Ultrathin Dual‐Wavelength Pancharatnam–Berry Metasurfaces with Complete Independent Phase Control

   https://doi.org/10.1002/adom.201900594  

   Xie, Rensheng ; Zhai, Guohua ; Wang, Xiong ; Zhang, Dajun ; Si, Liming ; Zhang, Hualiang ; Ding, Jun ( July 2019 , Advanced Optical Materials)

   Metasurfaces are planar structures that can offer unprecedented freedoms to manipulate electromagnetic wavefronts at deep‐subwavelength scale. The wavelength‐dependent behavior of the metasurface could severely reduce the design freedom. Besides, realizing high‐efficiency metasurfaces with a simple design procedure and easy fabrication is of great interest. Here, a novel approach to design highly efficient meta‐atoms that can achieve full 2π phase coverage at two wavelengths independently in the transmission mode is proposed. More specifically, a bilayer meta‐atom is designed to operate at two wavelengths, the cross‐polarized transmission efficiencies of which reach more than 70% at both wavelengths. The 2π phase modulations at two wavelengths under the circularly polarized incidence can be achieved independently by varying the orientations of the two resonators constructing the meta‐atom based on Pancharatnam–Berry phase principle. As proof‐of‐concept demonstrations, three dual‐wavelength meta‐devices employing the proposed meta‐atom are numerically investigated and experimentally verified, including two metalenses (1D and 2D) with the same focusing length and a vortex beam generator carrying different orbital angular momentum modes at two operation wavelengths. Both the simulation and experimental results satisfy the design goals, which validate the proposed approach.

    

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4. Spin‐Selective Second‐Harmonic Vortex Beam Generation with Babinet‐Inverted Plasmonic Metasurfaces

   https://doi.org/10.1002/adom.201800646  

   Chen, Yang ; Yang, Xiaodong ; Gao, Jie ( July 2018 , Advanced Optical Materials)

   Metasurfaces have drawn considerable attentions for their revolutionary capability of tailoring the amplitude, phase, and polarization of light. By integrating the nonlinear optical processes into metasurfaces, new wavelengths are introduced as an extra degree of freedom for further advancing the device performance. However, most of the existing nonlinear plasmonic metasurfaces are based on metallic nanoantennas as meta‐atoms, suffering from strong background transmission, low laser damage threshold and small nonlinear conversion efficiency. Here, Babinet‐inverted plasmonic metasurfaces made of C‐shaped nanoapertures as meta‐atoms are designed and demonstrated to solve these issues. Rotation‐gradient nonlinear metasurfaces are further constructed for producing spin‐selective second‐harmonic vortex beams with the orbital angular momentum (OAM) and beam diffraction angle determined by both the spin states of the fundamental wave and second‐harmonic emission. The results enable new types of functional metasurface chips for applications in spin, OAM, and wavelength multiplexed optical trapping, all‐optical communication, and optical data storage.

    

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5. Active terahertz spin Hall effect in vanadium dioxide metasurfaces

   https://doi.org/10.1364/OE.421283  

   Kang, Lei ; Wu, Yuhao ; Werner, Douglas_H ( March 2021 , Optics Express)

   Insulator-to-metal transition induces large material property variations in vanadium dioxide (VO₂) over a broad frequency band. VO₂, therefore, has been introduced into metallic resonating structures to realize reconfigurable metadevices from microwave to optical wavelengths. Beyond enabling metal/VO₂hybrid meta-atoms, in the THz regime metallic-phase VO₂micro-structures can support strong electromagnetic resonances, offering great potential in active manipulation of THz radiation. In this paper, we show that VO₂dipole antennas can be used to realize geometric phase coded metasurfaces for wave-front shaping and polarization rotation of THz waves. Moreover, we demonstrate that the corresponding efficiency of the THz spin Hall effect is closely related to VO₂’s THz electrical conductivity. In light of the dispersionless nature of the geometric phase, our study indicates that metasurfaces constructed by VO₂subwavelength resonators are good candidates for active control of broadband THz radiation.

    

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