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1. Arbitrary aperture synthesis with nonlocal leaky-wave metasurface antennas

   https://doi.org/10.1038/s41467-023-39818-2  

   Xu, Gengyu; Overvig, Adam; Kasahara, Yoshiaki; Martini, Enrica; Maci, Stefano; Alù, Andrea (December 2023, Nature Communications)

   Abstract The emergence of new technological needs in 5 G/6 G networking and broadband satellite internet access amplifies the demand for innovative wireless communication hardware, including high-performance low-profile transceivers. In this context, antennas based on metasurfaces – artificial surfaces engineered to manipulate electromagnetic waves at will – represent highly promising solutions. In this article, we introduce leaky-wave metasurface antennas operating at micro/millimeter-wave frequencies that are designed using the principles of quasi-bound states in the continuum, exploiting judiciously tailored spatial symmetries that enable fully customized radiation. Specifically, we unveil additional degrees of control over leaky-wave radiation by demonstrating pointwise control of the amplitude, phase and polarization state of the metasurface aperture fields by carefully breaking relevant symmetries with tailored perturbations. We design and experimentally demonstrate metasurface antenna prototypes showcasing a variety of functionalities advancing capabilities in wireless communications, including single-input multi-output and multi-input multi-output near-field focusing, as well as far-field beam shaping. 

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2. Leaky-wave metasurfaces for integrated photonics

   Huang, Heqing; Overvig, Adam C.; Xu, Yuan; Malek, Stephanie C.; Tsai, Cheng-Chia; Alù, Andrea; Yu, Nanfang (January 2023, Nature nanotechnology)

   Metasurfaces have been rapidly advancing our command over the many degrees of freedom of light within compact, lightweight devices. However, so far, they have mostly been limited to manipulating light in free space. Grating couplers provide the opportunity of bridging far-field optical radiation and in-plane guided waves, and thus have become fundamental building blocks in photonic integrated circuits. However, their operation and degree of light control is much more limited than metasurfaces. Metasurfaces integrated on top of guided wave photonic systems have been explored to control the scattering of light off-chip with enhanced functionalities – namely, point-by-point manipulation of amplitude, phase or polarization. However, these efforts have so far been limited to controlling one or two optical degrees of freedom at best, and to device configurations much more complex compared to conventional grating couplers. Here, we introduce leaky-wave metasurfaces, which are based on symmetry-broken photonic crystal slabs that support quasi-bound states in the continuum. This platform has a compact form factor equivalent to the one of conventional grating couplers, but it provides full command over amplitude, phase and polarization (four optical degrees of freedom) across large apertures. We present experimental demonstrations of various functionalities for operation at λ= 1.55 μm based on leaky-wave metasurfaces, including devices for phase and amplitude control at a fixed polarization state, and devices controlling all four optical degrees of freedom. Our results merge the fields of guided and free-space optics under the umbrella of metasurfaces, exploiting the hybrid nature of quasi-bound states in the continuum, for opportunities to advance in disruptive ways imaging, communications, augmented reality, quantum optics, LIDAR, and integrated photonic systems. 

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3. Single atom trapping in a metasurface lens optical tweezer

   https://doi.org/10.48550/arXiv.2110.11559  

   Hsu, T-W; Zhu, W.; Thiele, T.; Brown, M.O.; Papp, S.B.; Agrawal, A.; Regal, C.A. (July 2022, ArXivorg)

   Optical metasurfaces of sub-wavelength pillars have provided new capabilities for the versatile definition of the amplitude, phase, and polarization of light. In this work we demonstrate that an efficient dielectric metasurface lens can be used to trap and image single neutral atoms. We characterize the high numerical aperture optical tweezers using the trapped atoms and compare to numerical computations of the metasurface lens performance. We predict future metasurfaces for atom trapping can leverage multiple ongoing developments in metasurface design and enable multifunctional control in complex experiments with neutral-atoms arrays. 

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4. Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

   https://doi.org/10.1088/1361-6633/ac2aaf  

   Yang, Jingyi; Gurung, Sudip; Bej, Subhajit; Ni, Peinan; Howard Lee, Ho Wai (March 2022, Reports on Progress in Physics)

   Abstract Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years. 

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5. 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)

   Abstract 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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