Electrically tunable polarizer based on graphene-loaded plasmonic cross antenna

Abstract

Dynamic manipulation of light at nanoscale is highly important for many applications, including optical security systems, optical tweezers and compact optoelectronic devices. A number of tuning methods using thermal, mechanical, optical, and electrical mechanisms have been reported. Among all, electrical tuning, based on incorporation of phase-change media, is the most prevailing thanks to on-chip integration of subwavelength photonic components with electronics. Of all media candidates, graphene stands out due to its superior electrical and thermal conductivity, widely tunable electro-optical properties, material abundance, and good chemical resistance. The conductivity of grapheneis strongly dependent on the Fermi level, which can be dynamically controlled by a gate voltage via electrostatic doping. Moreover, due to the forbidden interband transitions by Pauli blocking, the doped graphene supports strong plasmonic effects typically at the terahertz or mid-infrared regime. Hence, graphene is often used as an electrically tunable plasmonic material. In this work, by combining plasmonic structures with electro-statically tunable graphene, we propose an in situ control of polarizations of nanoantennas. The tunable polarizer is designed based on an asymmetric cross nanoantenna comprising two orthogonal metallic dipoles antennas sharing the same feed gap. Graphene nanoribbon is inserted beneath the dipole antennas. The strong coupling between the graphene and metallic structure enables broad tuning of the antenna resonance, thus changing the phase and polarization of the reflected wave. Our simulation results demonstrate a broad tuning range of 660nm for the graphene-loaded dipole in the mid-infrared regime by electrically doping the graphene while keeping the resonance of the other dipole unchanged. As a result, by measuring the axial ratio at far field, we elucidate that the reflected wave can be tuned from circular to linear polarization. This study confirms the strong coupling between graphene and metallic nanostructures, which illuminates promises for high-speed electrically-controllable optoelectronic devices.