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