Graphene as a Tunable Anisotropic or Isotropic Plasmonic Metasurface

Abstract

We demonstrate a tunable plasmonic metasurface by considering a graphene sheet subject to a periodically patterned doping level. The unique optical properties of graphene result in electrically tunable plasmons that allow for extreme confinement of electromagnetic energy in the technologically significant regime of THz frequencies.Here we add an extra degree of freedom by using graphene as a metasurface, proposing to dope it with an electrical gate patterned in the micron or sub-micron scale. By extracting the effective conductivity of the sheet we characterize metasurfaces periodically modulated along one or two directions. In the first case, and making use of the analytical insight provided by transformation optics, we show an efficient control of THz radiation for one polarization. In the second case, we demonstrate a metasurface with an isotropic response that is independent of wave polarization and orientation.Graphene, an atomically-thin layer of carbon atoms arranged in a honeycomb lattice, features outstanding mechanical, thermal and electrical properties. 1 Its characteristic linear * To whom correspondence should be addressed 1 dispersion for electrons close to the Dirac point results in the possibility of tuning the carrier concentration by external means. 2 In particular, graphene can be biased by electrical gating or surface doping, which modifies its Fermi level and permits the existence of surface plasmons propagating along graphene. Because of the two-dimensional (2D) nature of this material, the surface plasmons it supports have very short wavelengths and exhibit extreme out-of-plane confinement to the sheet, as has already been demonstrated in a number of experiments. [3][4][5][6][7][8][9][10][11] Compared to the noble metals commonly employed for plasmonics, 12 graphene has a low carrier concentration, and, for this reason, plasmons in graphene are relatively long-lived and appear at lower frequencies. While the plasmonic response of metals is weak at the infrared or lower frequencies, graphene plasmons exist in the THz regime with relatively low losses. 13-15 These facts, added to the important ingredient of tunability, make graphene a very suitable platform for the design of plasmonic metasurfaces in the THz regime. 16,17 Metasurfaces, the 2D counterpart of metamaterials, 18,19 consist of a planar arrangement of resonant subwavelength-sized building blocks. 20 By appropriately designing the blocks and their arrangement, metasurfaces provide an ultra-thin platform for manipulating electromagnetic (EM) waves. Novel phenomena and applications based on metasurfaces range from broadband light bending and anomalous reflection and refraction 21-23 to strong spin-orbit interactions of light. 24In this work, we study graphene as a plasmonic metasurface that offers the potential to control radiation in the THz regime. Different from previous studies, 16,17 which focused on patterning the graphene, here we consider a continuous graphene sheet with periodically modulated doping. This gives rise ...

Rapid Design of Wide-Area Heterogeneous Electromagnetic Metasurfaces Beyond the Unit-Cell Approximation

“…In the optical and infrared frequency regions, they are of interest in connection with integrated photonics. Although this concept [6] has been initially explored in connection with plasmonic materials [3,4], attention has now turned towards all-dielectric metasurfaces [7][8][9][10]. Such high refractive index dielectric nanoantenna arrays can also be considered as two dimensional high-index contrast subwavelength diffraction gratings; various optical wavefront manipulation possibilities have been demonstrated with these so called HCTA (high contrast transmit arrays) [11][12][13].…”

Tunable plasmonic metasurface for perfect absorption

“…We start by considering a plasmonic metasurface based on a graphene monolayer with spatially periodic conductivity [24,25]. The configuration we propose is sketched in Figure 1a.…”

“…Here, l and s are the chemical potential and the scattering time for electrons, respectively, with s ¼ ml=ev 2 F (m is the mobility and v F % 10 6 m/s the Fermi velocity). In this work, we use the parameters a = 76 GHz/X and c g = 1.5 THz, which correspond to l = 0.65 eV and m = 10 4 cm 2 /V s. The conductivity grating established via periodic doping allows for free space radiation to couple into the surface plasmons sustained by the graphene layer, which have much shorter wavelength [24][25][26][27][28][29][30][31][32][33][34]. Hence, the modulation period needs to be much smaller than the wavelength of incident radiation (k 0 ) 2pc).…”

“…[24,25] for more details). Hence, in the electrostatic limit, both structures have the same plasmon resonance condition [35,36].…”

“…It can be written in terms of the Fourier expansion coefficients of the conformal transformation and the electrostatic potential expansion (explicit expressions can be found in Refs. [24,25]). Then, transmittance and reflectance through the modulated graphene are given by R = |r| 2 and…”