Electric control of coupling in hybrid graphene/metamaterial system enables strong selective modulation of light polarization.

physics, it is still a big challenge to achieve practical metamaterials for real-world applications-primarily due to their bulky size, unavoidable material losses, and fabrication difficulties. Metasurfaces, 2D counterparts of metamaterials that consist of 2D array of planar metallic or dielectric structures, have shown great promise for practical applications owing to their exceptional capability of controlling the wavefront of light. [17][18][19][20] With suitable design of the building blocks, metasurfaces are capable of generating phase discontinuities with in-plane gradient, leading to anomalously refracted beam in transmission and/or reflection. Recent progress in metasurfaces has led to various ultrathin optical devices including flat lenses, [21][22][23][24] vortex beam generators, [23][24][25] broadband quarter wave plates, [26,27] efficient surface plasmon couplers, [28] 3D and high-efficiency holograms. [29][30][31][32] The concept of metasurfaces has also been extended to nonlinear optics for manipulating the nonlinearity phase in harmonic generations. [33,34] Although metasurfaces have offered new degrees of freedom for controlling the propagation of light, the amplitude of anomalous refracted waves in metasurfaces is typically fixed by their structural geometry and dimensions, which limits their potential for various applications that require dynamical control over the electromagnetic waves, such as active focusing for lensing and dynamic holography. Active tuning of metasurface requires incorporation of active media whose electromagnetic properties can be changed in real time under external stimuli. Recently, it was shown that anomalous deflection can be dynamically controlled by means of various tuning schemes based on microelectromechanical system (MEMS) [35] and Schottky diode. [36] One suitable candidate for such purpose is graphene, a 2D form of carbon with the atoms arranged in a honeycomb lattice. Graphene has been studied extensively during the last decade due to its high carrier mobility and unique doping capability originated from its gapless and cone-shaped band structure at the Dirac point. Graphene also shows a gate-controllable lightmatter interaction by the shift of the Fermi level, which can be further enhanced by the electromagnetic resonance provided by suitably designed structures. [37,38] Particularly, in the terahertz (THz) regime, strong modulation has been achieved by electrically tuning the density of states available for intraband transitions. [39] Although significant effort has been devoted to various graphene-based metamaterials for active control of the amplitude and polarization of THz waves in direct transmission, [40][41][42][43] Although recent progress in metasurfaces has shown great promise for applications, optical properties in metasurfaces are typically fixed by their structural geometry and dimensions. Here, an electrically controllable amplitude of anomalously-refracted waves in a hybrid graphene/metasurface system are experimentally demonstrated, which cons...

Memory metamaterials are artificial media that sustain transformed electromagnetic properties without persistent external stimuli. Previous memory metamaterials were realized with phase-change materials, such as vanadium dioxide or chalcogenide glasses, which exhibit memory behaviour with respect to electrically/optically induced thermal stimuli. However, they require a thermally isolated environment for longer retention or strong optical pump for phase-change. Here we demonstrate electrically programmable nonvolatile memory metadevices realised by the hybridization of graphene, a ferroelectric and meta-atoms/meta-molecules, and extend the concept further to establish reconfigurable logic-gate metadevices. For a memory metadevice having a single electrical input, amplitude, phase and even the polarization multi-states were clearly distinguishable with a retention time of over 10 years at room temperature. Furthermore, logic-gate functionalities were demonstrated with reconfigurable logic-gate metadevices having two electrical inputs, with each connected to separate ferroelectric layers that act as the multi-level controller for the doping level of the sandwiched graphene layer.

Extreme optical properties can be realized by the strong resonant response of metamaterials consisting of subwavelength-scale metallic resonators. However, highly dispersive optical properties resulting from strong resonances have impeded the broadband operation required for frequency-independent optical components or devices. Here we demonstrate that strong, flat broadband optical activity with high transparency can be obtained with meshed helical metamaterials in which metallic helical structures are networked and arranged to have fourfold rotational symmetry around the propagation axis. This nondispersive optical activity originates from the Drude-like response as well as the fourfold rotational symmetry of the meshed helical metamaterials. The theoretical concept is validated in a microwave experiment in which flat broadband optical activity with a designed magnitude of 45°per layer of metamaterial is measured. The broadband capabilities of chiral metamaterials may provide opportunities in the design of various broadband optical systems and applications.

However, dislocations, disinclinations, and symmetry-breaking instabilities can frequently prompt the formations of defects and grain boundaries in the synthesis processes [15,16] and crystal growth rate of the 2D nanostructure is very slow. [17,18] In nature system, organic-inorganic hybrid nanostructures with high-crystallinity and diverse morphologies can be constructed by biomineralization process. [19,20] This natural process can efficiently induce the molecular rearrangement for high complexity [21] and many functionalities. [22,23] Therefore, these unique biological phenomena have been applied in synthesis method to control the crystallinity and morphology of 2D nanostructures with superior material properties. Here, we show a solution-based ultrafast 2D growth method for large-scale and high-quality silver nanosheets on air/gel interface. The biological hydrogel polymer forms a multilayered structure by a solvent-induced phase separation process at air/gel interface with trapped silver (Ag) salts between the layers. [24] Furthermore, the trapped Ag salts between each biological hydrogel layers can efficiently reduce into large-scale 2D Ag nanosheets during the annealing process.The mixture of the Ag salts and hydrogel solution is a transparent, light yellow color at room temperature with reduced viscosity as compared with pure gelatin solution (Figure S1a, Supporting Information), while its nominal phase-change temperature is 154 °C based on the thermal gravimetric analysis (TGA) analysis results ( Figure S1b, Supporting Information). The viscosity of the hydrogel mixture decreases further at above 40 °C as the hydrogen bond in the triple-helix gelatin chain [25] degrades by the intercalation of Ag ions between various functional side chains (e.g., amine, carboxyl, hydroxyl, and thiolate groups). [26] Adding methyl alcohol and Ag salts in the solution can partially neutralize and condense the hydrogel polymer chains [24,27] to form large and continuous multilayer membranes via the salting-out effect ( Figure S2, Supporting Information). Conceptually, Ag ions are coordinated on the randomly coiled gelatin chains, which are rapidly accumulated under elevated temperature to form a multi layered structure (Figure 1a). [28] The neutralization and condensation process occurs, and the Ag ions are trapped between the layers of the hydrogel scaffold (Figure 1b). A reducing agent, dopamine, is used to promote the conversion of Ag ions to atoms while large, multilayered gelatin scaffolds constrain the crystallization process laterally to increase the The growth of large and high-quality 2D nanomaterials is challenging due to the formation of defects from dislocations, disinclinations, and symmetrybreaking instabilities. In this study it is demonstrated that biological template can be utilized to align the molecular orientation for large grain size in the synthesis of the high-quality 2D silver nanostructure. The solvent assisted multilayering phenomenon of hydrogel forms biological template at the air/ gel interface...