Terahertz Wavefront Control by Graphene Metasurface
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Terahertz Wavefront Control by Graphene Metasurface Takumi Yatooshi, Atsushi Ishikawa, and Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushimanaka, Kitaku, Okayama 700-8530, Japan ABSTRACT We propose and numerically investigate a tunable metasurface made of an array of graphene ribbons to dynamically control terahertz (THz) wavefront. The metasurface consists of graphene micro ribbons on a silver mirror with a SiO2 gap layer. The graphene ribbons are designed to exhibit localized plasmon resonances depending on their Fermi levels to introduce abrupt phase shifts along the metasurface. With interference of the Fabry-Perot resonances in the SiO2 layer, phase shift through the system is largely accumulated, covering up to 2π range for full control of the THz wavefront. Numerical simulations prove that wide-angle reflected THz beam steering from -53° to +53° with a high reflection efficiency as high as 60% is achieved at 5 THz while the propagation direction of THz beam could be switched within 0.6 ps. INTRODUCTION Metamaterials have been developed for advanced light control, by composing subwavelength arrays expanding a degree of freedom of material resonances and dispersions [14]. These capabilities achieved by plasmonic nanostructures realize a variety of fascinating applications, such as optical magnetism, negative refractive index, and cloaking [5-9]. More recently, metasurface, two-dimensional (2D) counterpart of metamaterials, has been proposed as a simple yet powerful concept to mold the flow of light [10-11]. Metasurface relies on gradual phase shift by strong light-matter interaction of plasmonic resonators at the interface instead of using phase accumulation along the optical path like conventional optical components. Then arbitrary light wavefront is formed by deep modulating properties of the resonator array and their planar arrangements on the metasurface. A wide variety of novel functionalities on the metasurfaces have been explored and demonstrated, such as lenses, holograms, and beam shaping, leading to high-performance light control technologies by flat optics [12-16]. Although metasurfaces have offered new way for controlling light propagation, their operations are naturally static depending on their structural parameters, so dynamic response control has been anticipated. One of ways to meet this requirement is graphene, densely-packed carbon atoms in a 2D honeycomb lattice, that gains much attention over recent years for its unique properties [17,18]. Graphene exhibits an ultra-high mobility and a remarkable high light absorption despite its atomically thinness [19,20]. In addition, 2D plasmons in graphene offer strong field confinement, relatively long lifetimes, and tunability by electrical or chemical stimulus [21-23]. The graphene plasmonics has a variety of potential applications, such as subwavelength waveguides, high-sensitive detectors, and tunable metamaterials for efficiently controlling terahertz (THz) and infrared radiation [24-29]. In thi
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