We introduce Plasmene- in analogy to graphene-as free-standing, one-particle-thick, superlattice sheets of nanoparticles ("meta-atoms") from the "plasmonic periodic table", which has implications in many important research disciplines. Here, we report on a general bottom-up self-assembly approach to fabricate giant plasmene nanosheets (i.e., plasmene with nanoscale thickness but with macroscopic lateral dimensions) as thin as ∼40 nm and as wide as ∼3 mm, corresponding to an aspect ratio of ∼75,000. In conjunction with top-down lithography, such robust giant nanosheets could be milled into one-dimensional nanoribbons and folded into three-dimensional origami. Both experimental and theoretical studies reveal that our giant plasmene nanosheets are analogues of graphene from the plasmonic nanoparticle family, simultaneously possessing unique structural features and plasmon propagation functionalities.

Carbon dots (CDots) are a promising biocompatible nanoscale source of light, yet the origin of their emission remains under debate. Here, we show that all the distinctive optical properties of CDots, including the giant Stokes shift of photoluminescence and the strong dependence of emission color on excitation wavelength, can be explained by the linear optical response of the partially sp2-hybridized carbon domains located on the surface of the CDots’ sp3-hybridized amorphous cores. Using a simple quantum chemical approach, we show that the domain hybridization factor determines the localization of electrons and the electronic bandgap inside the domains and analyze how the distribution of this factor affects the emission properties of CDots. Our calculation data fully agree with the experimental optical properties of CDots, confirming the overall theoretical picture underlying the model. It is also demonstrated that fabrication of CDots with large hybridization factors of carbon domains shifts their emission to the red side of the visible spectrum, without a need to modify the size or shape of the CDots. Our theoretical model provides a useful tool for experimentalists and may lead to extending the applications of CDots in biophysics, optoelectronics, and photovoltaics.

Artificially engineered metasurfaces provide extraordinary wave control at the subwavelength scale. However, metasurfaces proposed so far suffer due to limited bandwidths. In this paper, extremely thin metasurfaces made of single metallic layer is experimentally presented for ultra‐wideband operation from 9.3 to 32.5 GHz (with a fractional band of 112%), working at both transmission and reflection modes simultaneously. The phase control is achieved by azimuthally rotating the scatterer based on Pancharatnam–Berry phase principle. Nearly uniform efficiency (≈25%), approaching the theoretical limit of the infinitely thin metasurface, is achieved throughout the operation band. Finally, the proposed design is implemented for applications, e.g., the generation of electromagnetic waves carrying orbital angular momentums as well as anomalous reflections and refractions. The metasurfaces are characterized numerically and experimentally and the results are in good agreements.

introducing abrupt phase changes via an ultrathin sheet of subwavelength resonators (usually known as meta-atoms). The first such metasurface was proposed by Yu and Capasso [2] Afterward, significant number of novel metasurfaces have been proposed in microwave, [3] terahertz, [4] near-infrared, [5] and visible ranges. [6] These metasurfaces are utilized for realizing many applications such as focusing with subwavelength planar lenses, [7] optical cloaking, [8,9] generating nondiffracting beams, [10] manipulating phase profiles having orbital angular momentum (OAM), [11] photon spin Hall effect, [12] and performing computation with the coding metasurfaces. [13,14] Although metasurfaces have proved their unique freedom in wavefront manipulation, most metasurfaces generally suffer from low efficiencies. This is especially critical for transmissive metasurfaces where both the magnetic and electric resonances should be precisely controlled. For example, the efficiency of the first ever demonstrated metasurface was only 5%. [2] A few transmissive metasurfaces have since been introduced with higher efficiencies based on the Huygens' principle [15,16] or meta-transmit arrays. [17,18] However, the operation of these metasurfaces is severely restricted, e.g., may require specific polarization, added complexity in manufacturing, or demand complex, costly computations for each distinct phase.Recently, it has been demonstrated that metasurfaces can be realized based on the photon spin Hall effect. [19][20][21][22] Spin Hall effect is a phenomenon where moving electrons with opposite spins can be transversely separated. Spin Hall effect can be intrinsic based on spin-orbit coupling of electrons [23] or extrinsic due to the spin-dependent scatterings by impurities. [24] Both of these could be utilized to achieve extraordinary wave propagation. It is known that the experimentally observed intrinsic phenomena are usually very weak. [23] On the contrary, extrinsic spin Hall is observed in a special class of metasurfaces, [12] which utilizes subwavelength scatterers to achieve full phase control in the range of 0-2 π. The transmission/ reflection phase control of a cross-polarized wave is achievable through the rotation of the designed scatterers, which in fact make it more promising than the other types of meta-atoms that require phase dispersions to achieve phase control. The maximum achievable efficiency of such metasurfaces was theoretically predicted to be 25% in a single layer structure since only electric responses can be realized in such structures. [25,26] This limitation was overcome by designing reflective metasurfaces with ground planes, [27][28][29] where both the electric and the Metasurfaces offer unprecedented freedom to manipulate electromagnetic waves at deeply subwavelength scales. However, realizing a highly efficient metasurface, yet simple enough to conceptualize, design, and fabricate, is a challenging task. In this paper, a novel approach is proposed for designing meta-atoms which can achieve full phase cont...

Metasurfaces with actively tunable features are highly demanded for advanced applications in electronic and electromagnetic systems. However, realizing independent dual-tunability remains challenging and requires more efforts. In this paper, we present an active metasurface where the magnitude and frequency of the resonant absorption can be continuously and independently tuned through application of voltage biases. Such a dual-tunability is accomplished at microwave frequencies by combining a varactor-loaded high-impedance surface and a graphene-based sandwich structure. By electrically controlling the Fermi energy of graphene and the capacitance of varactor diodes, we experimentally demonstrate the independent shifting of the working frequency from 3.41 to 4.55 GHz and tuning of the reflection amplitude between −3 and −30 dB, which is in excellent agreement with full-wave numerical simulations. We further employed an equivalent lumped circuit model to elucidate the mechanism of the dual-tunability resulting from the graphene-based sandwich structure and the active high-impedance surface. We speculate that such a dual-tunability scheme can be potentially extended to terahertz and optical regimes by employing different active/dynamical tuning methods and materials integration, thereby enabling a variety of practical applications.