Abstract
Graphene plasmonics has become a highlighted research area due to the outstanding properties of deep-subwavelength plasmon excitation, long relaxation time, and electro-optical tunability. Although the giant conductivity of a graphene layer enables the low-dimensional confinement of light, the atomic scale of the layer thickness is severely mismatched with optical mode sizes, which impedes the efficient tuning of graphene plasmon modes from the degraded light-graphene overlap. Inspired by gap plasmon modes in noble metals, here we propose low-dimensional hybrid graphene gap plasmon waves for large light-graphene overlap factor. We show that gap plasmon waves exhibit improved in-plane and out-of-plane field concentrations on graphene compared to those of edge or wire-like graphene plasmons. By adjusting the chemical property of the graphene layer, efficient and linear modulation of hybrid graphene gap plasmon modes is also achieved. Our results provide potential opportunities to low-dimensional graphene plasmonic devices with strong tunability.
Highlights
In the context of light-matter interactions, the concentration of electromagnetic fields on materials is a critical issue for the performance of tunable optical devices, such as photodetectors[1], bio-sensors[2], optical modulators[3,4], and lasers[5]
We firstly reveal the existence of hybrid graphene gap plasmon (H-GGP) modes the field profile of which is strongly confined inside the graphene gap between metallic and dielectric graphene layers
We demonstrate that the H-GGP mode has larger field concentration on graphene layers than those of edge[33,34] or wire-like graphene plasmon modes[31]
Summary
We consider the 2D metal-gap-dielectric waveguide system composed of three distinct graphene domains (Fig. 1a); a dielectric gap domain G with the width w (the sheet conductivity σ(G) where Im{σ(G)} < 0) is inserted in-between semi-infinite metallic domain M (Im{σ(M)} > 0) and dielectric domain D (Im{σ(D)} < 0). To demonstrate the distinctive feature of H-GGP modes, we compare with other graphene waveguide modes: graphene edge plasmon (GEP) mode[33,34] (Fig. 2b) and wire-like 1D-SPP mode[31] (Fig. 2c) with same material parameters While both H-GGP and 1D-SPP modes with quasi-antisymmetric potential profiles (σ(x, z)~ −σ(−x, z) for all z) have improved confinement compared to that of the GEP mode with much stronger structural asymmetry (|σ(x,0)| ≪ |σ(−x,0)| and |σ(x, z)| =|σ(−x, z)| for z ≠ 0), H-GGP exhibits more confined transverse (Ex) field on the gap region than that of the 1D-SPP mode, as similar to the difference between gap plasmons and surface plasmons in noble metals[8,9]. The H-GGP mode provides more efficient regime of ΔΩ−1 for controlling effective index compared to 1D-SPP modes (ΔΩ−1 ≤ 0.015) Such improved efficiency is more apparent for the case of the finite modulation region for ΔΩ−1 (dotted lines, for the 3wmax modulation width around the graphene gap), due to the improved transverse localization of the H-GGP mode (Fig. 3c). Because the change of chemical potentials is usually derived by the external electric field, the sensitive and linear modulation of the H-GGP mode demonstrated in Fig. 4 enables the high-speed and low-power realization of tunable graphene devices
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