We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-Q resonances that originate from the physics of bound states in the continuum. We prove rigorously a direct link between the bound states in the continuum and the Fano resonances, and develop a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics.Metasurfaces have attracted a lot of attention in the recent years due to novel ways for wavefront control, advanced light focusing, and ultra-thin optical elements [1]. Recently, metasurfaces based on high-index resonant dielectric materials [2] have emerged as essential building blocks for various functional meta-optics devices [3] due to their low intrinsic loss, with unique capabilities for controlling the propagation and localization of light. A key concept underlying the specific functionalities of many metasurfaces is the use of constituent elements with spatially varying optical properties and optical response characterized by high quality factors (Q factors) of the resonances.Many interesting phenomena have been shown for metasurfaces composed of arrays of meta-atoms with broken inplane inversion symmetry (see Fig. 1), which all demonstrate the excitation of high-Q resonances for the normal incidence of light. The examples are the demonstration of imagingbased molecular barcoding with pixelated dielectric metasurfaces [4] and manifestation of polarization-induced chirality in metamaterials [5], which both are composed of asymmetric pairs of tilted bars [see Fig. 1(a)], observation of trapped modes in arrays of dielectric nanodisks with asymmetric holes [6] [see Fig. 1(b)], sharp trapped-mode resonances in plasmonic and dielectric split-ring structures [7,8] [see, e.g., Fig. 1(c)], broken-symmetry Fano metasurfaces for enhanced nonlinear effects [9, 10] [see Fig. 1(d)], tunable high-Q Fano resonances in plasmonic metasurfaces [11] [see Fig. 1(e)], trapped light and metamaterial-induced transparency in arrays of square split-ring resonators [12,13] presented in Fig. 1(f). Here, we demonstrate that all such seemingly different structures can be unified by a general concept of bound states in the continuum, and we prove rigorously their link to the Fano resonances.Bound states in the continuum (BICs) originated from quantum mechanics as a curiosity [14], but later they were rediscovered as an important physical concept of destructive interference [15] being then extended to other fields of wave physics, including acoustics [16] and optics [17,18]. A true BIC is a mathematical object with an infinite Q factor and vanishing resonance width, it can exist only in ideal lossless infinite structures or for extreme values of parameters [19][20][21]. In practice, BIC can be realized as a quasi-BIC in the form of a supercavity mode [22] when both Q factor and resonance width become finite at the BIC conditions due to ab-d c a...

Metasurfaces provide opportunities for wavefront control, flat optics, and subwavelength light focusing. We developed an imaging-based nanophotonic method for detecting mid-infrared molecular fingerprints and implemented it for the chemical identification and compositional analysis of surface-bound analytes. Our technique features a two-dimensional pixelated dielectric metasurface with a range of ultrasharp resonances, each tuned to a discrete frequency; this enables molecular absorption signatures to be read out at multiple spectral points, and the resulting information is then translated into a barcode-like spatial absorption map for imaging. The signatures of biological, polymer, and pesticide molecules can be detected with high sensitivity, covering applications such as biosensing and environmental monitoring. Our chemically specific technique can resolve absorption fingerprints without the need for spectrometry, frequency scanning, or moving mechanical parts, thereby paving the way toward sensitive and versatile miniaturized mid-infrared spectroscopy devices.

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Infrared spectroscopy resolves the structure of molecules by detecting their characteristic vibrational fingerprints. Subwavelength light confinement and nanophotonic enhancement have extended the scope of this technique for monolayer studies. However, current approaches still require complex spectroscopic equipment or tunable light sources. Here, we introduce a novel metasurface-based method for detecting molecular absorption fingerprints over a broad spectrum, which combines the device-level simplicity of state-of-the-art angle-scanning refractometric sensors with the chemical specificity of infrared spectroscopy. Specifically, we develop germanium-based high-Q metasurfaces capable of delivering a multitude of spectrally selective and surface-sensitive resonances between 1100 and 1800 cm−1. We use this approach to detect distinct absorption signatures of different interacting analytes including proteins, aptamers, and polylysine. In combination with broadband incoherent illumination and detection, our method correlates the total reflectance signal at each incidence angle with the strength of the molecular absorption, enabling spectrometer-less operation in a compact angle-scanning configuration ideally suited for field-deployable applications.

Huygens' metasurfaces have demonstrated almost arbitrary control over the shape of a scattered beam, however its spatial profile is typically fixed at fabrication time. Dynamic reconfiguration of this beam profile with tunable elements remains challenging, due to the need to maintain the Huygens' condition across the tuning range. In this work, we experimentally demonstrate that a time-varying meta-device which performs frequency conversion, can steer transmitted or reflected beams in an almost arbitrary manner, with fully dynamic control. Our time-varying Huygens' metadevice is made of both electric and magnetic meta-atoms with independently controlled modulation, and the phase of this modulation is imprinted on the scattered parametric waves, controlling their shapes and directions. We develop a theory which shows how the scattering directionality, phase and conversion efficiency of sidebands can be manipulated almost arbitrarily. We demonstrate novel effects including all-angle beam steering and frequency-multiplexed functionalities at microwave frequencies around 4 GHz, using varactor diodes as tunable elements. We believe that the concept can be extended to other frequency bands, enabling metasurfaces with arbitrary phase pattern that can be dynamically tuned over the complete 2π range. arXiv:1807.08873v3 [physics.app-ph] 2 Dec 2018 AUTHOR CONTRIBUTION M. Liu conceived the idea, performed the theoretical, numerical and experimental studies, with support from

Long-term stability and high-rate capability have been the major challenges of sodium-ion batteries. Layered electroactive materials with mechanically robust, chemically stable, electrically and ironically conductive networks can effectively address these issues. Herein we have successfully directed carbon nanofibers to vertically penetrate through graphene sheets, constructing robust carbon nanofiber interpenetrated graphene architecture. Molybdenum disulfide nanoflakes are then grown in situ alongside the entire framework, yielding molybdenum disulfide@carbon nanofiber interpenetrated graphene structure. In such a design, carbon nanofibers prevent the restacking of graphene sheets and provide ample space between graphene sheets, enabling a strong structure that maintains exceptional mechanical integrity and excellent electrical conductivity. The as-prepared sodium ion battery delivers outstanding electrochemical performance and ultrahigh stability, achieving a remarkable specific capacity of 598 mAh g −1 , long-term cycling stability up to 1000 cycles, and an excellent rate performance even at a high current density up to 10 A g −1 .

We introduce the concept and a generic approach to realize Extreme Huygens' Metasurfaces by bridging the concepts of Huygens' conditions and optical bound states in the continuum. This novel paradigm allows creating Huygens' metasurfaces whose quality factors can be tuned over orders of magnitudes, generating extremely dispersive phase modulation. We validate this concept with a proof-of-concept experiment at the near-infrared wavelengths, demonstrating all-dielectric Huygens' metasurfaces with different quality factors. Our study points out a practical route for controlling the radiative decay rate while maintaining the Huygens' condition, complementing existing Huygens' metasurfaces whose bandwidths are relatively broad and complicated to tune. This novel feature can provide new insight for various applications, including optical sensing, dispersion engineering and pulse-shaping, tunable metasurfaces, meta-devices with high spectral selectivity, and nonlinear meta-optics.

Three-dimensional (3D) hierarchical hybrid nanomaterials (GNR-MnO₂) of graphene nanoribbons (GNR) and MnO₂ nanoparticles have been prepared via a one-step method. GNR, with unique features such as high aspect ratio and plane integrity, has been obtained by longitudinal unzipping of multi-walled carbon nanotubes (CNTs). By tuning the amount of oxidant used, different mass loadings of MnO₂ nanoparticles have been uniformly deposited on the surface of GNRs. Asymmetric supercapacitors have been fabricated with the GNR-MnO₂ hybrid as the positive electrode and GNR sheets as the negative electrode. Due to the desirable porous structure, excellent electrical conductivity, as well as high rate capability and specific capacitances of both the GNR and GNR-MnO₂ hybrid, the optimized GNR//GNR-MnO₂ asymmetric supercapacitor can be cycled reversibly in an enlarged potential window of 0-2.0 V. In addition, the fabricated GNR//GNR-MnO₂ asymmetric supercapacitor exhibits a significantly enhanced maximum energy density of 29.4 W h kg(-1) (at a power density of 12.1 kW kg(-1)), compared with that of the symmetric cells based on GNR-MnO₂ hybrids or GNR sheets. This greatly enhanced energy storage ability and high rate capability can be attributed to the homogeneous dispersion and excellent pseudocapacitive performance of MnO₂ nanoparticles and the high electrical conductivity of the GNRs.