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1. Non-local and non-Hermitian acoustic metasurfaces

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

   Wang, Xu; Dong, Ruizhi; Li, Yong; Jing, Yun (October 2023, Reports on Progress in Physics)

   Abstract Acoustic metasurfaces are at the frontier of acoustic functional material research owing to their advanced capabilities of wave manipulation at an acoustically vanishing size. Despite significant progress in the last decade, conventional acoustic metasurfaces are still fundamentally limited by their underlying physics and design principles. First, conventional metasurfaces assume that unit cells are decoupled and therefore treat them individually during the design process. Owing to diffraction, however, the non-locality of the wave field could strongly affect the efficiency and even alter the behavior of acoustic metasurfaces. Additionally, conventional acoustic metasurfaces operate by modulating the phase and are typically treated as lossless systems. Due to the narrow regions in acoustic metasurfaces’ subwavelength unit cells, however, losses are naturally present and could compromise the performance of acoustic metasurfaces. While the conventional wisdom is to minimize these effects, a counter-intuitive way of thinking has emerged, which is to harness the non-locality as well as loss for enhanced acoustic metasurface functionality. This has led to a new generation of acoustic metasurface design paradigm that is empowered by non-locality and non-Hermicity, providing new routes for controlling sound using the acoustic version of 2D materials. This review details the progress of non-local and non-Hermitian acoustic metasurfaces, providing an overview of the recent acoustic metasurface designs and discussing the critical role of non-locality and loss in acoustic metasurfaces. We further outline the synergy between non-locality and non-Hermiticity, and delineate the potential of using non-local and non-Hermitian acoustic metasurfaces as a new platform for investigating exceptional points, the hallmark of non-Hermitian physics. Finally, the current challenges and future outlook for this burgeoning field are discussed. 

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2. 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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3. Dispersion engineered metasurfaces for broadband, high-NA, high-efficiency, dual-polarization analog image processing

   https://doi.org/10.1038/s41467-023-42921-z  

   Cotrufo, Michele; Arora, Akshaj; Singh, Sahitya; Alù, Andrea (December 2023, Nature Communications)

   Abstract Optical metasurfaces performing analog image processing – such as spatial differentiation and edge detection – hold the potential to reduce processing times and power consumption, while avoiding bulky 4 F lens systems. However, current designs have been suffering from trade-offs between spatial resolution, throughput, polarization asymmetry, operational bandwidth, and isotropy. Here, we show that dispersion engineering provides an elegant way to design metasurfaces where all these critical metrics are simultaneously optimized. We experimentally demonstrate silicon metasurfaces performing isotropic and dual-polarization edge detection, with numerical apertures above 0.35 and spectral bandwidths of 35 nm around 1500 nm. Moreover, we introduce quantitative metrics to assess the efficiency of these devices. Thanks to the low loss nature and dual-polarization response, our metasurfaces feature large throughput efficiencies, approaching the theoretical maximum for a given NA. Our results pave the way for low-loss, high-efficiency and broadband optical computing and image processing with free-space metasurfaces. 

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4. Spin-controlled wavefront shaping with plasmonic chiral geometric metasurfaces

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

   Chen, Yang; Yang, Xiaodong; Gao, Jie (October 2018, Light: Science & Applications)

   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. 

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5. Terahertz Metasurfaces Exploiting the Phase Transition of Vanadium Dioxide

   https://doi.org/10.3390/ma16227106  

   Liu, Meng; Wei, Ruxue; Taplin, Jasmine; Zhang, Weili (November 2023, Materials)

   Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator–metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures’ design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes. 

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