Electronically Tunable Light Modulation with Graphene and Noble Metal Plasmonics

Abstract

Graphene is a monolayer of carbon atoms constructing a two-dimensional honeycomb structure, and it has an excellent carrier mobility and a very high thermal conductivity. Remarkably, it has been experimentally demonstrated that a monolayer graphene exhibits an exotic optical properties. To be specific, the plasmonic dispersion relation of a transverse magnetic graphene plasmon is electronically tunable by adjusting carrier density in graphene with external gate bias, and graphene plasmonic nano cavities have been utilized to modulate mid-infrared light. In this thesis, we present how to efficiently modulate mid-infrared light by combining graphene plasmonic ribbons with noble metal plasmonic structures. First, we propose and demonstrate electronically tunable resonant perfect absorption in graphene plasmonic metasurface enhanced by noble metal plasmonic effect, which results in modulating reflecting light. In this device, we improve coupling efficiency of free-space photons into graphene plasmons by reducing wavevector mismatching with a low permittivity substrate. In addition, the graphene plasmonic resonance is significantly enhanced by plasmonic light focusing effect of the coupled subwavelength metallic slit structure, which results in strongly fortifying resonance absorption in the graphene plasmonic metasurface. In the proposed device, theoretical calculation expects that perfect absorption in the graphene plasmonic metasurface is achievable with low graphene carrier mobility. We also present an analytical model based on surface admittance in order to fully understand how this enhancement occurs. In the second device, we propose and demonstrate a transmission type light modulator by combining graphene plasmonic ribbons with subwavelength metal slit arrays. In this device, extraordinary optical transmission resonance is coupled to graphene plasmonic ribbons to create electrostatic modulation of mid-infrared light. Absorption in graphene plasmonic ribbons situated inside metallic slits can efficiently block the coupling channel for resonant transmission, leading to a suppression of transmission. This phenomenon is also interpreted by anti-crossing between the graphene plasmonic resonance in the ribbons and the noble metal plasmonic resonance in the subwavelength metal slit arrays. Finally, we devise a platform to demonstrate graphene plasmonic resonance energy transport along graphene plasmonic ribbons. In this device, two metal-insulator-metal waveguides are connected by a subwavelength metal slit, and graphene plasmonic ribbons are located inside this slit. Due to the large impedance mismatch at the junction, light coupling efficiency across the junction is poor. If the graphene plasmonic ribbons are tuned to support strong graphene plasmonic resonances, the light energy can be transferred via graphene plasmons along the ribbons, and it leads to significant improvement in the light coupling efficiency across the junction. In addition to enhanced light coupling efficiency, we also present how to totally suppress the transmission by inducing a Fano resonance between a non-resonant propagation mode across the junction and a resonant graphene plasmonic transport mode, which can be utilized to efficiently modulate light in a noble metal plasmonic waveguide with the graphene plasmon resonance energy transfer.