Amplitude Modulation of Anomalously Refracted Terahertz Waves with Gated‐Graphene Metasurfaces

Abstract

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...

Tunable, reconfigurable, and programmable metamaterials

“…Many works have been devoted to graphene‐based reconfigurable metamaterials for dynamic controlling of EM waves from THz to near‐infrared (IR) . Figure A shows a tunable THz metamaterial with gated‐graphene for amplitude modulation in THz . The tunable metamaterial consists of five layers, including a polyimide substrate, a metallic metamaterial particle layer, a graphene layer, a square ring electrode, and an ion‐gel layer as gate‐dielectric material.…”

“…C and D, Simulated y‐component of electric field distributions for left‐handed circular polarization (LCP) incident with two different gate voltages, 2.2 and 0 V. E and F, Energy distributions qt 1.15 THz wtih two different gate voltages, 2.2 and 0 V. Reproduced with permission from Ref. . 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim [Color figure can be viewed at wileyonlinelibrary.com]…”

“…In THz frequencies, transparent conductive oxides, graphene, liquid crystals, and ferroelectric materials are common materials which are used to design these tunable and reconfigurable metamaterials . The characters of these active materials are controlled and tuned through electrical, optical, and thermal methods . Many application and devices have been reported based on these tunable and reconfigurable metamaterials such as tunable absorber and dynamic manipulation of radiation, and amplitude modulations in THz.…”

“…60 There are many tuning mechanisms for controlling the active components, such as electric, [60][61][62][63][64] thermal, [65][66][67][68] and optical. [69][70][71][72] Various types of active or tunable materials have been explored in tunable and reconfigurable metamaterials such as transparent conductive oxides, 73,74 ferroelectrics, 75,76 liquid crystal, 77,78 graphene, 61,79,80 and phase change materials [81][82][83][84] in terahertz. In microwave frequencies, pin-diodes, varactors, and other integrated components are utilized to realize the combination of active characters and metamaterials.…”

Novel Tunable Peace-Logo Planar Metamaterial Unit-Cell for Millimeter-Wave Applications

“…Moreover, reconfigurable unit‐cells may be lossy and very difficult to implement, especially at millimeter‐waves. It should be noted that recently, tunable metasurfaces at different wavelengths, such as encoding graphene metasurfaces and tunable anomalous refraction based on graphene metasurfaces are reported in .…”