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Gated Graphene Enabled Tunable Charge-Current Configurations in Hybrid Plasmonic Metamaterials Arash Ahmadivand, Burak Gerislioglu, G. Timothy Noe, and Yogendra Kumar Mishra ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00035 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019
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ACS Applied Electronic Materials
Gated Graphene Enabled Tunable Charge-Current Configurations in Hybrid Plasmonic Metamaterials Arash Ahmadivand,†* Burak Gerislioglu,† G. Timothy Noe,§ and Yogendra Kumar Mishra‡ †Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA §Department of Electrical and Computer Engineering, Rice University, Houston, TX 77005, USA
‡Functional Nanomaterials, Institute for Materials Science, Kiel University, Kaiserstr. 2, D-24143, Kiel, Germany
*
[email protected] Abstract: Here, an active hybrid toroidal metasurface is demonstrated based on artificially engineered plasmonic metamolecules deposited on a gate-controlled graphene layer. Our numerical predictions are confirmed by room-temperature time-domain spectroscopy characterizations in the terahertz (THz) frequency range. We quantitatively and qualitatively show that the gate-controlled active graphene metamaterial allows for an active tuning of the dynamic nonradiating toroidal dipole by tuning the essential mismatch in the spinning magnetic-fields in the strongly coupled resonators. It is demonstrated that this feature can be obtained by varying the resistivity of gate-controlled graphene and tuning the capacitive coupling at the critical openings of the system.
Keywords: Charge-current configurations, toroidal dipoles, plasmonic terahertz metamaterials, gatecontrolled graphene, THz time-domain spectroscopy
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Dynamic charge-current configurations possess detectable signatures from both the microscopic and macroscopic points of view.1 The electromagnetic properties of these multipole excitations have been characterized quantitatively by multipole expansions using modern electrodynamics.1,2 The unconventional charge-current excitations’ fingerprints (i.e. ring currents, poloidal magnetic fields) with concealed far-field radiation patterns have been demonstrated recently on both the atomic as well as extremely large-scale systems (galaxies, neutron starts, black holes, etc.).3 In the subwavelength systems limit, magnetically induced loop-currents have successfully been created in artificial 3D and planar all-dielectric and plasmonic metamolecules.4,5 Despite a dramatic masking by dominant classical electromagnetic far-field radiations patterns, gyrotropic-fashioned toroidal topology provides an additional nonzero contribution to the scattered radiation:6
EsFF 4 k 2 c Ql , m l , m M l , ml , m Tl , m l , m l ,m l ,m l ,m
Q d r. jl dr ; M r J T r . J
(1)
where Q, M, and T correspond to the induced charge, transversal and radial currents, respectively. In terms of the toroidization principle, the optically driven dynamic toroidal dipole can be visualized as magnetically induced currents flowing across the surface of a torus, also known as poloidal currents.1,7 Of particular interest is a toroidal dipole that has been excited in subwavelength metamaterials and utilized extensively for developing advanced technologies.8,9 Recently, novel approaches have been introduced to increase the tunability of charge-current configurations such as using optothermal tuning10 and electro-optical gating.11 In the latter method, numerical studies have shown that the interaction between graphene monolayer and plasmonic unit cells is a reliable technique to actively modulate the amplitude of the toroidal mode by varying the Fermi energy level of the atomic carbon sheet.11 However, experimental studies for electro-optical tuning of the spectral response of toroidal plasmonic metamaterials have not been reported. In photonics, an atomically thin graphene sheet (thinner than
